152 GRB Organic Chemistry for Medical Entrance Exanis

Dry ether (i) St•ttc : Due to weak forces, the alkanes upto four
(i) CH 3 Br +2Li - - - - , CH 3 Li + Li Br carbon atoms, i.e., methane, ethane, pr?pane and butane are
l\kth:1-I lithium

l
colourless, odourless gases, the next thirteen members (frolll
(ii) 2CH 3Li +Cul ~ Li(CH 3 ) 2 Cu + Lil c to C 17 ) are colourless, odourless liquids. Alkanes from C
Lithium dimethyl
5
onwards arc colourless and odourIess so i I·d s. 18

cuprate (ii) Density : The density of alkanes i?c~eases very

slowly with the rise of molecular mass until Jt becorn
Dry ether . h ter than
(iii) C 2 H 5 Br + Li(CH3 )zCu ---➔ C2H5 - CH constant at about 0.8. Thus, all alkanes are I1g water.es
3
Bromoethanc - Propane CH) Solubility : They are ge~erally insoluble in polar
solvents such as water but soluble m non-polar solvents like
+CH3Cu + LiBr
For the third step to give a good yield of alkane, the alkyl ether, carbon tetrachloride, benzene, etc. The solubility
decreases with increase in molecular mass.
halidl! must be either CH 3 X, or a primary alkyl halide or a
secondary cycloaU,.-yl halide. The alkyl group of the lithium However, the liquid hydrocarbons the~selves are good
diaU,-yl cuprate (also called Gilman reagent) may be methyl, non-polar solvents for other non-pol~~ orgam_c compounds.
1°, 2° or 3°. Moreover, the two alkyl groups being coupled (iv) Boiling points : The boilmg po1~ts ~f straight
may be same or different. chain or n-alkanes increase regularly with mcreasing
This method is particularly suitable for the preparation of number of carbon atoms. In general, the boiling point
unsymmetrical alkanes of the type, R - R', which cannot be difference between two successive members of the series
prepared by Wurtz reaction. (except for the first few members) is about 20 -3 0°C. Among
(i) Li the isomeric alkanes, the normal isomer has a higher
(CH3)i CHBr [(CH 3 )zCH]i LiCu boiling point than the branched chain isomer. The greater
lsopropyl bromide (ii) Cul
Lithium isopropyl cuprate the branching of the chain, the lower is the boiling point.
Physical Constants of Some Alkanes
(CH 3 )zCH-CH 2CH 3
2-Methylbutane Density
Name Formula B.pt. (K) M.pt. (K)
CH 3 (as liquids)
I Methane CH4 111 .5 90.5 0.4240
[(CH 3) 3 Ch LiCu CH3-C-CH2CH3 Ethane CH3CH3 184.4 101.0 0.5462
Lithium ten-butyl cuprate I Propane CH3CH2CH3 230.9 85.3
CH 3 0.5824
2,2-Dimethylbutane n-Butane CH3(CH2)iCH3 272.5 134.6 0.5788
11. By the action of water (or dil. HCI) an aluminium n-Pentane CH3(CH2hCH3 309. 1 143.3 0.6264
a.:..1d Let)llium carbide : Both these carbides on treatment n-Hexane CH3(CH2)4CH3 341.7 177.7 0.6594
with water or dilute HCl at room temperature yield methane.
Al4C3 + 12H2O ~ 3CH4 + 4Al(OH)] For example, the following boiling point values of various
Be 2C + 4H2O~ CH4 +2Be(OH)i isomers of butane and pentane justify the above statement.
Beryllium carbide Methane n-Butane > Isobutane
12. Jndu,trial preparation : Methane is formed by b.pt. 272.5K 261.0K
passing a mixture of hydrogen sulphide and carbon disulphide
vapours through red hot copper tube. . n-Pentane > Isopentane > Neopentane
b.pl. 309.JK 301.0K 282.5K
High tel'J1)Cfature
CS 2 +2H 2 S +8Cu - - -------, CH4 +4Cu 2S . The' trend in boiling points can be explained in terms of
mtennolecular (dipole-dipole) forces of attraction. These
2.6 GENERAL CHARACTERISTICS OF forces act along the surface of the molecules and their
ALKANES magnitude increases with increase in surface area. As the
Physical : The phy~ical properties (boiling point, melting ~olccular size increases in the series, the surface area
point, solubility, density, etc.) of hydrocarbons depend upon increases and with that the boiling points also increase. This
the intermolecular forces of attraction. Since alkanes are bas been depicted in Fig. 2.2.
almost non-polar and therefore, their molecules are held only
by weak van der Waals 'forces which mainly depend upon the
size (surface area) and the structure of the molecule.
 153

-i -
·1
()
100

-1; 7
-
......
~
/
~
.,,,,,., ...
~
_. _.. :-
_..
-
__. __,

~
cond 1\1Cln~. At hi~:h tcmpc r:iturc , e nergi es
suffi cient f,,r hrca\ dng C 11 amt C - C bond
f,,rma110n of free radic als.
R - Cll 2- ll _ _. R-< 11 2 + ll
of co\fo ,ions arc
s resul ting the

and Sll\h1li1y ,1f free rad1c a\ .. , .. m the order ·

. .
I
-200 - I I H 2C- t' ll < C:ll 1 < CII ,t 11 2 < (CH 3 >i H ..... (CH 3 )JC
0
2 S 4 S 6
7 8 9 10 11 12 13 14 1S 18
1
N unlbef of carbon atoms
< C. 11 2 ~ H .C = Cll- ( H ~
,00 - Howe ver, alkan cs do sh ow some rc<1c tiufh untl..:
9 200 - condi tio ns. Some of the impo rtant react ion-. of
r ... p.:~iul
a!kan es are
i 100 I.--
descr ibed below :
1. s lJ . •
""' :. Thes e are most c.hara cten-
1_1; ., ... 1 • ...
~ / stic react ions of the alkan es.
...
... .- (a) I.Al .,'- ,a Whe n the alkan es are tuate d v..1th
halog ens (parti cu larly chlor ine or brom ine) in
-4001 2 3 4 S 6 7 8 9 10 11 12 1 3 14 1S 16 pre-,ence of
Nwnber of carbon atoms light (sunl ight or UV) o r at elevated temp eratu
res 5:!J-6 73 K.
Fig. 2.2 the hydro gen atom s of alkanes are succe ssive
ly replac..:d by
halog en atom s. This process is knov..'ll as balog
Toe rt-all;aneS have most exten ded structure and enati on. TI:e
large r exten t of halog enation depe nds on the amou
~ area in comparison to branc hed chain isome rs havin nt o f halrn! en
g used. For exam ple, meth ane under goes chlor inatio
· ,mpact structure (as the shape appro aches that of a n in exc-ess
spher e in of chlor ine in prese nce of UV light givin g the
: branch ed chain isome rs). Thus, interm olecu mi.""ttu re of
lar force s are meth yl chlor ide, methylene chlor ide, chlor oform
weaker in branc hed chain isome rs, there fore, they and cano n
have lowe r tetrac hlori de.
boiling points in comp ariso n to straig ht chain isome
rs. Cl 2 Cl-,
(v) Melting point s ~ The melti ng point s of C H4 ~ CH 3 Cl
alkan es do ~
not follow a very smoo th grada tion with the incre Metha ne hv Mono chloro metba ne
ase of hv Dichloromethane
molecular size. Alkanes with even numb er of carbo (Meth yl chlori de)
n atom s (;-,.!ethylene chlorid e)
u,·c biglaer melti ng point than the next lowe r and next Cl2
higher alkanes havin g odd num ber of carb ~
Cl~
on atom s CHCl 3 ~ CCl-t
(Fig. 2.2). hv
Trichl orome thane hv Tetra.:hloronxthan.!
It is expla ined by the fact that alkan es with an odd numb (Chlo roform) (Carbon tetr.1..:hl,m,k)
er For chlor inatio n reacti vity of 1°, 2° and 3° bvdro
of carbon atoms have their end-c arbon atom s on the 2.en is in
same side the ratio l O : 2° : 3° : : l : 3 .8 : 5
of the molecule and the even-numb ered carbo n atom · -
alkan es
ba\'c their ~ n atom s on the oppo site
sides of the
H H
molecules. Thus, alkan es with even numb er of carbo
n atom s H-
I I
pack closely in such a mann er as to perm C- H --+ H - c • + H• · ~ = 105 kcal mol
mt~ l ~ attrac tion and there fore have sligh
it great er I I '
meltmg pomt. tly highe r H H
·
/c, /c, /c, /c, ;,,c, /c CH3 - CH3 --+C H 3- CH 2 + 1-t ;
CCC C CCC
Odd number of carbo n atoms CH 3
Even numb er of carbo n atoms
Chemical : Alka nes are · quite inert towa rds comm
on
I CH
CH 3- C- H --+ CH 3)c
~ent s such_as acids , a~ie s, oxidi sing and reduc ing 3 + H• ~
c~ ordin ary cond itions becau se of their satur
agent s, I H
ated H
( )
26 and
The electr onega tivity diffe rence betw een carbo
n CH 3
ahno hydro gen (2.1) is smal l and thus, C- H bond
Th ~ non-p olar. The C-C bond is comp letely non-p is I CH r,,;.,
C:
do ~the polar reage nts (elec troph iles and
cent!
olar.
nucle ophil es)
CH - C- H --+ CH 3/
3
\ CH 3
+ H• ; Af/ =93 kcal . mol
react ion sites (elec tron- rich or electr on-de ficien
t CH 3
strnn- ) '!8 alkane mole cules . The C---C and C- H bond
--. llama .bond s and cann ot break unde r ordin s are Bond energ y for extra ction of hydro gen lies in follow
ary ing
seque nce:
 ◄
154 GRfl Organic Chemistry for Medical Entrance Exa
- - ~
iodina tion can occur in presence of ~n oxidising agent such aa
('11 > \ 0 >20> 3°, hcm:c, the reactivit y 1mkr wi ll he
3 iodic ucid or nitric ac_id, etc., which decomposes BI into
Cll 4 < 1°<2" < 3". . . . . .. . iod ine.
On the basis of rdati,·l· t'l':tl'tivity nl dilk_rl'lll ty p1:s 1 ° C l 14 + 12 ~ CH 3 I+ HI (reducing agent)
hydrogen, the pl'rt'l'lltagc o I• ti 1c p II• 1·u'11 l•t •s' in llllXtUl'e
· Ca ll 1)C
calculated. e.g., Heat 312 + 3H2O
(.4) CII , - Cl l~- Cll 2 - C ll2C I 5111 + Hl03
-. - ~ Rd:'11\'C 111111111n1 -. No. ofcq11iv11 lc111 or 2HI +2HNO 3 ~ 2NO2 + 2H2O+I 2
<12 111
, hydrogen x n.:t11.:tivity
=6x l =6 1lalogcnation of alkanes proceeds through free radicai
Cl 1,- Cll~--Cll~-('111~ mechanism.
{It h:is 6 pnmary hydn)gen /n• Cl
anJ 4 """''nd,11') h~ Jn,~cn} Cl: I Free radical substitution reactions :
(8) Cl l,1- Cl 1--CI l 2- CI l3 (a) Chlorination of metha~e : _The _chlorination of
Rl.'lati vc mnounl = No. of equivalent methane in the presence of ultravwlet hght 1s an example of
hydrogen x reactivity
free radical substitution (HOMOLYSIS).
= 4 X 3.8 = 15.2
Mechanism:
Total amount= 6 + 15.2 = 21.2 Initiation step: The reacti?n is initiat~d b)'. the breaking
6 of chlorine molecule into chlorine-free radicals m presence of
% of A = - X l00 = 28.3
21.2 UV light.
15 2
%of B = · x 100= 71.7
21.2 c1 2 Homolytic fission Cl• + Cl• (Chain initiation) .. . (i)
!Nott hv or heat
The relative reactivi1y of I O , 2°, 3° hydrogen to bromination is
I : 82 : 1600. Propagation step: The chlorine-free radicals attack
Ethane and higher alkanes react with chlorine in a similar methane molecule (Cl• is a substituent).
way and all possible substitution products are obtained. •
However, propane and higher members contain hydrogen CH 4 +Ct ~ CH 3 + HCI (Chain propagation) .. . (ii)
atoms of two or three types, i.e., primary, secondary or tertiary,
Each of the methyl-free radicals, in turn, reacts with
which differ in reactivity. The order of reactivity is:
chlorine molecule to form methyl chloride and at the same
Tcrt. hydrogen > Sec. hydrogen > Primary hydrogen
Propane gives two monochloro substituted products on
chlorination.
.
time chlorine-free radical is produced.

CH3 +CI 2 ~ CH 3 CI +c1•

.

(Chain propagation)
CH 3 CH2 CH 2CI
1-Chloropropanc • The chlorine-free radical can react with fresh methane
(primary hydrogen is substituted) 45% molecule as in step (ii) or methyl chloride.
CH3CH2CH3 Cli • •
Propane Cl +CH3Cl ~ CH 2CJ + HCI {Chain propagation)
Cl
I •
CH 3-CH-CH 3 The CH 2CI free radical would react with another molecule
2-Ch loropropane of chlori~e to form dichloro methane. This process may extend
(secondary hydrogen is substituted) 55% further till all the replaceable hydrogen atoms in the methane
2-Methyl butane gives six monobromo structural have been substituted by chlorine atoms.
substituted isomeric products on bromination. •
CH2CI + Clz ~ CH 2CJ 2 +Ct(Chain propagation)
~ 2
).<H/ ~
2-Mcthyl
butane
~:
r •. _,. .
Br
+
ci• +CH2Cl2 ~ CHCI 2 + HCJ (Chain propagation)
•
Enantiomcrs CHClz +CI2 ~ CHC1 3 +c1• (Chain propagation) .

+
•/ l.,
Anr + ~Br ci• +CHCl3 ~ CCJ 3 + HCI (Chain propagation)_
Enantiomcrs •
The bromination proceeds in almost identical manner but CCl3 +Cl2 ~ CCl 4 +ct• (Chain propagation)
the reaction is Jess vigorous than chlorination. Reaction with Termination step:
fluorine is highly exothermic and very violent. Iodine does b The chain of reactions initiated and propagated as shoWll
substitute as the reaction is very slow and reversible but :hove :nay
b~ terminated if free radicals combine amongsl
emse ves w1tho4t giving rise to any new radicals.
 155
Hydrocarbons
• •• 673K
Cl +Cl ~ CI2 (Chain tcnnination) .. . (iii) C6 II 13- Jl + II0N0 2 C6 ll 11N02 + 11 20
I lcxnnc (Fuming) N1trnhcxane
(Chain termination) However, when a mixture of vapours of an alkane and
nitric acid is heated at 673-773K, nitroalk ane is formed
• •
cH 3 + CH3 ~ CH 3 -CH 3 (Chain tennination) readily. This is known as vapour phase nitrati on. By this
....t (H F!u(•nnation can be achieve d without viole nce when alkane process lower as well as higher alkane s can be converted into
Note ;
1~ treated W"lth fluonne diluted ~ ith Bil mert gas or by the nitroalkanes.
act10n ofmorg amc fluonde s such as AsF3, SbF3, AgF, HgF2 723K
or Hg1 F~ etc on bromo or iodo derivatives. CH3 - H + H0N0 2 --➔ CH 3- N02 + H20
Nitromcthane
. 2C 2Hsl + H&fi ~ 2C2H5F + Hgl2
2CH3 CH1CI + H g 2F2 ~ 2CH3CH2F + Hg2Cl2
Vapour state
In alkanes having two or more carbon atoms, there is
Fluoroe thane
always a possibility that the C-C bonds may break at high
nus reaction ts called Swarts reaction. temperatures and a mixture of nitro alkane s is formed . For
(ii) Iodo compou nds are obtained by treating chloro or bromo example, in the nitration of propan e, the followi ng produc ts
denvativ cs with sodium iodide (concen trated solution in are obtained:
acetone). It is a halide exchang e reaction (Finkelstein N0 2
Raction ).
HNO 3 (f) I
C2H 5Br + Nal
Acetone C 3 H 8 _ ____:;_____,. CH 3 CH2C H2N02 + CH3- CH-G I3
(excess) Propane 723K 1-Nitropropane 2-Nitrop ropane
(25%) (40%)
Points to remember : (i) Relative reactivities of
halogens on alkanes follow the following order:
F2 >Cl 2 >Br2 >1 2 +C 2H 5 N0 2 + CH3N 0 2
The reaelivity decreases with decrease in the electro- Nitroethane Nitromethane
(10%)
negativity of the halogens.
(ii} The replacement of hydrog en atom by halogen Higher alkanes yield even more comple x mixtures.
follows the following order: Alkane s containing neo-skeleton are easily oxidised with
Tertiary hydrogen > Secondary hydrogen > Primary hydrog en HN0 3 to form carboxylic acids and no nitro compo unds.
> Methan e H
(::H3
(iii) Halogena; ion takes place in presen ce of sunlight, TJV I HN03
light or in dark at high temperatures (523-6 73K). The reaction CH 3 -C-C H 3 +40 2 --➔ 2CH 3 C00H + C0 2 + 2H20
may be initiated by peroxi de (dibenz oyl peroxide). I
(iv) Laboratory chlorination of alkane s is often done with CH3
S02CI 2 (Sulphuryl chloride), instead of Cl 2 and an organic Neopen tane
peroxide, ROOR is used as an initiator. Nitration like haloge nation also fo llows frre radica l
(v) Tetra-ethyl lead, Pb(C 2H 5 ) 4 , initiates the
mecha nism.
chlorination of methane in the dark at 423K. 423-673 K • •
. (vi) Small amount of oxygen slow down the reaction for a H0N0 2 110 + N 0 2
penod of time, which depends upon the amoun t of oxygen . • •
C 3 H7 - H + II0 ~ C3 1I 7 + H20
~xygen is believed to react with methyl free radical to form
ess reactive free radical. Propane

• • • •
CH3 +0 2 ~ CH3 - 0-0 C3H 7 + N0 2 ~ C 3ll 7 N02
Here, oxygen acts as inhibitor. . (,) '-;ulphon \Cion : The replace ment of hydrog en atom
Wh. ~b) Nitration : Nitration is a su~titu tion reaction in by sulpho nic acid group (- S0 3 H) is known as sulpho nation .
gro:p. a hydroge
n atom efalka ne is replace d by nitro (- N02) Lower alkane s do not underg o su lphona tion but higher
membe rs (from hexane onward s) are sulpho nated slowly when
R High treated with fuming sulphu ric acid (oleum ) at about 673K.
-H+H 0N0 2 --➔ R-N0 2 +Hz0
S03
Lo Alkane (Fwning) te111>, Nitroalkane R-H+ H0S0 3H - - - - - ~
at ordi~ Dletnbers do not react with concen trated nitric acid Alkane (Prolong ed heating) Alkane
With fiunin ~ratu res but long chain membe rs on heating sulphonic acid
I llltric acid yield nitroalkanes.
 1
- ~~~ - - - - - - - - - - -----------~G'.!t,~B~
O::..:
rg~a=n.:..ic_ C_h_e_
m_is_t_ry_f_o=r_M~e~d-ica
_ 1 Entrance ~ ,
Cu -tube -......_~ I
503 2 CH 4 + [Oh 10 0 atmt 413i7 2CB30Ji f
C6H13-H+HOS0 3 H C'6lluSO3ll + ll 2O
Hexane 6 7JK I le-.anc sulphonic nciJ 9 I
1
Methano !
However, lo\\er members such as p,,,pnnc, butan~. pcr_ilunc, (Tl11•.S 1·5. the industrial method for the Illanuf~ :I
etc., react with SO~ in vapour phase to fonn sulpl10111c m:,ds. methyl alcohol.) re QI ti
(ii) When a mixture. of methane and oxygen is Pas ·
Ease of replacement of It atoms is: heated molybdenum oxide under pressure or in air at ~ °'•1i :
3° >2'-' >1° is oxidised to fonnaldchyde. ll,
Sulphonation also follows free rndical mcd1a11ism.
tin K • •
M oO l
HOSO,H . HO + SO3H CJJ4 + O i 543 K. 100 atm FHCBo + li o )
., tH,,nll.,tyric fission) Methane . . 0 rrnaldehydc 2 j
• • Higher alkanes are oxidised . to carboxyJic aci ,
C 6 H 13 - H + OH ~ C 6 H13 + H2~ ds ~ I
presence O f manganese stearate or silver oxide ·
C6Hl3 +S03H ~ C6H13SO3H 02
:.:!. fuidation : Oxidation of alkanes gives different 373-433K
products under different conditions. Ag2 0
{a) Combustion or complete o~idation: A~anes 2R-CH3 +3[Oh
readily bum vvith non-luminous flame in excess of ~tr 0 ~ 2R -COOH + 2t1
11:rJ I
oxygen to form carbon dioxide and water. The co:1llbustwn ~ The controlled partial oxidation of methane at 1 , t
.
alkanes . an exothermic
1s . reaction,
• ,.· e., large quantity
. of heat 1s 73
yields acetylene.
evolved and heat of combustion increases with mol~cu1ar
weight. It forms the basis of the use of alkanes as. fue! m the 6CH4 +202 1773 K 2HC==CH+2C02 +JOH.' •
internal combustion engines. The cooking gas, which is often Ethyne ·
called LPG (Liquefied Petroleum Gas) is a mixture of propane
and butane. _ (d) Chemical o:xii... .. tion : . Alk.anes are usually ra:r j
affected by oxidising agents hke KMnO4 or K Cr~O- I
c n H ln+ 2 +(Jn+l)o
2 2 ~ nCO
. .2
+(n+I)HzO+Q However, alkanes having tertiary hydrogen .
2
are oxidised ~. II
these oxidising agents to the correspondrng alcohols. 1
CH4 (g)+202 (g) ~ CO 2 (g)+2H 2 O(/)+213kcal/mol, · ,
KMn04
2C H +70 2 ~ 4CO 2 +6H 2 O+746kcal/mol "' . (CH 3 ))CH + [OJ - - - (CH3 )J C-OH
(b)2 Incomplete
6
combustion• or ox1'd a tion : 2-Methylpropane (alk. ) 2-Methylpropan-~-d
(i) When alkane is burnt in insufficient supply of oxyge~, · {lsobutane) (tert-Butyl alcoooO
carbon monoxide or carbon black may be formed. - ·' · · 3. Thermal decor1 1 ::.· - • ._, -r-- • g ' ·
or cracking) : The decomposition of a compound by hem f
. l
2CH4 + 302 _B_um_ 2CO + 4HzO
called pyrolysis (Greek: pyro = fire; lysis = loosening). \\~a t
(limited)
pyrolysis occurs in alkanes, the process is term.ed crac~ . j
Bum •' The alkanes, on heating under high temperature m absen~e ~ I
CH4 + Oz -- C +2HzO ,,-
(limited) Carbon black air, are broken down into a mixture of lower alkanes, alk~ I
Carbon black is used for making printing ink, black ·
and hydrogen. During cracking of alkanes, either J rup!Ult;
carbon-hydrogen bonds or rupture of carbon-carbon bo
paints, polishes and as a filler in rubber vulcanization.
(li) Reaction with steam : · Methane reacts with steam
(Occurs by a free radical mechanism) takes place. The rup:
over nickel suspended on alumina at J073 K, hydrogen gas is of C-H bonds is catalysed by the oxides of chron11dl~
formed with carbon monoxide. · vanadium, molybdenum, etc. and the rupture of c-C ~n art
catalysed by silica, alumina, zinc oxide, etc. The followl.Ilg
Ni/AJ2o3 some of the examples of pyrolysis:
1073K CO+3~2 ..
· CH 1300-1500K C+ZHz
4
~c) Ca~~c ~i:idatioo: Lower alkanes . undergo .· · Methane
restncted ox1dat1on m presence of metallic catalysts such as
700-SOOK CH + J-12
copper at high temperature and pressure to fonn alcohols : C2H 6 ----➔ H 2C= 2
aldehydes, ketones and acids. . . • Ethane Cr2O3 + Al 2O3 Ethene
(i) When methane and oxygen mixed in the ratio of 9 : 1
are compressed to I 00 abnospheres and passed through copper ·
tube at 473K, methyl alcohol is formed,
A ·
.CJ~~:7£.--,
•
~H3C~=C~2 + H2 (C- H
Pr0panc A · ·
fi~on)

C-C fi~ on)

. H20-CH 2 +CH4 (
 l l~t~ of 1111 lllll 1
(i) Melhanc is used in the manufacture of compounds like
Cll 30 II, I ICHO, Cll 3CJ,Cf l 2Cl 2, CllCl3 andCCl4 et~.
(i i) Methane is used in the preparation of carbon bla~k which nd
is used for making printing ink, black pamts a
automobile tyres. . th
(iii) Methane in the form of natural gas is used for runnmg e
cars, buses, etc. LPG (mixture of butane and isobutane)
is used as a fuel.
(iv) Ethane is used for making hexachloroethane which is an
artificial camphor. .
(v) Higher alkanes in the form of gasoline, kerosene oil,
diesel, lubricating oils and paraffin wax are widely used.
The pyrolysis or cracking of alkanes in 1 b .
c-C d C H b vo ves reaking of ILLUSTRATIONS OF OBJECTIVE QUESTIONS
~ I :-- f on~s and occurs by a free radical
mechamsm. t 1s o great importance to the petro1eum mdustry.
T " _ • • ""
. 1. A compound with no tertiary hydrogen is:
. • ·. c,
LU . : The process of co nvers1on · of one (a) (CH 3)3C-CH(CH3)i
ISomer mto another
. isomer is called 1·somer·1sat·10n. Alkanes (b) (CH 3)3C-CH2CH3
when beated 10 presence of anhydrous aluminium chloride and (c) (CH 3)iCH-CH 2CH2CH3
concentrated
. hydrochloric acid or anhydro us aIummmm · · (d) none of the above
bromide and hydrobromic acid at about 523K under a pressure [Ans. (b)]
of 35 atmospheres are converted into one or more other 2. Which of the following gives methane on hydrolysis?
isomers. (a) A1 20 3 (b) CaC2
CH 3 (c) Fe 30 4 (d) Al4C3
[Ans. (d)]
A1Cl 3 + HCI (cone.) I
CH3-CH-CH 3. Which of the following has highest octane number?
3
523K, 35 atm lsobutane (a) n-Hexane ' (b) n-Heptane
(c) n-Pentane (d) 2,2,4-Trimetbylpentane
A1Cl3+ HCI (cone.) [Ans. (d)] .
2-Methylpentane /':,, 4. Relative reactivity of halogens on alkanes follow the
2,3-Dimethylbutane
order:
(a) F2 >Cl 2 ?' Br2 > 12 (b) Cl2 >Br2 >1 2 >F2
This isomerisation involves 1,2 shift of hydride or methyl
(c) F2 > 12 > Br2 >Cl 2 (d) l2>Br2 >Cl 2 >F2
group with the formation of intennediate carbocation (by
AlCI 3 and HCI) which initiates the chain reaction. [Ans. (a)]
The branched chain alkanes have lower b.pts. than straight 5. The reaction,
chain, hence isomerisation is important in petroleum industry. ( i) Li CH B
C2H 5 Br -'-'----4
(ii) Cul [X] 3 r [Y]; is called:
~'· / '"OD ,t· t'on : The conversion of aliphatic
compounds into aromatic compounds is known as (a) Wurtz synthesis
aromatisation. Alkanes having 6 to 10 carbon atoms are (b) Wolff-Kishner reduction
converted into benzene or its homologous at high temperatures
(c) Corey-House synthesis
and in presence of a catalyst. (d) Kolbe's synthesis
/CH3 [Ans. (c)]
H2C
H I
2C'-c-CH2
CJ-13
I
Cr2OJ Al2O3
873K/15 atm. 0
Benzene
6. What are [X] and [Y] in the fioll owmg

(CH )iCHBr
3
(i) Li
(ii)Cul
·
.
. reaction?

CH 3CH2Br
[X) - - ~ [Y)
H2
n-Hexane

6
. (a) [(CH3)iCH]iliCu and (CH ) CHCH

V
)

n-Heptane
, Cr2OJAl2O3
873K 6
Methylcyclo-
hexane
Toluene
(b) [(CH3 )iCHJi LiCu and (CH3) 2 CHC 3
(c) [(CH3)iCH]iliCu and CH
(d) [(CH3 )iCH]LiBr and (CH 3) C2
[Ans. (b)J
3
3 2 HCH2CH 3
bJ
CH H2CH3
 GRB Organic chemistry for Medical Entrance ~a
158
- - ~
jlllll-i•)ti!l-1•)8"#•1Ai1•1=1•81~U~
7. Propane is obtained from rrorcne by whkh of the »11') are a/kanes relatively unr~ .
1
following method? rrobIUll 1• ? !Ve?
(a) \\'wtz reaction t,;oluflon : Alkancs are inert s~bstance~ at room tern
arc not affected by acids, alkalies and O .• Per.
(b) Dehydration olure os th eY h h C-C X1d1s·
This is due to the fact t at t e and C- Ji b ing
(c) FranllanJ rca'-'ti'-'° agents. h h . on~-
(d) Catal)1ic hyJn,gcnath.m in alkancs are non-polar, 1. e., I cy_ ave no reaction sites Wh;
the polar reagents can attack. c
[ \ "' (d)] d · order of
"• \\lien these coml')unJs arc arrange in Problem 2. Why do the C- C bonds rather th
increasing lx)iling point, C-ll bonds break during pyrolysis of alkanes? an tJie
CHlCH,CH CH 2CJt 3, CH3CH2CH(CH3 )CH3, Solution : Bond energy of C--C bond is lower than the
• P~t~ (I) 2-Mcthylbutane (II) bond energy of C-H bond. The C-C . bond energy .
83.0 kcal/mol while C-H bond_energy 1s gg_g kcal/11\ois
Ot 3C{CH3) 2CH3
Thus, c-C bonds break more easily than C-H bonds. l
~,;-Dimcthylpropanc (TU)
what is the correct order? Problem 3. Methane does not react with chlorine .
(a) J<H<III (b) II<I<III dark. Explain by giving reason. . ~
(c) 11<111<1 (d) III<Il<I Solution : Chlorination of methane ~s ·a free radicai
{ .\ n~ (d)] substitution. To conver_t Cl2 ~ole_cules mto_ chlorine-free
\ ,n nl .ind When a higher
dt·,ct·nt of ~eries : radicals energy is required which 1s not available in da.~
homologue is prepared from a lower homologue, it is called Thus, ~ethane does not react with Cl2 in dark due to absen~
ascent or series. Similarly the preparation of lower
of free radicals.
homologue is called descent or series.
.\~ent of all..ane series: Use of Wurtz reaction is Problem 4. The fire ofburning liquid paraffin cannot he
extinguished by throwing water over this. Explain by giving
made.
1i) \1cthane to ethane : reason.
Cl Na in dry ether Solution : The liquid paraffin is lighter than water. h
CH
4
--4 CH3CI---;_..-➔ CH3-CH3 flows over water surface and continues burning.
Methane UV (Wurtz-reaction) Ethane
Problem 5. Why is the Wurtz synthesis not a good merhal
Butane from ethane :
f ii)
for preparing propane?
Cl, Na in dry ether
C2H6 ---=-+ C2HsCI ----➔ C2Hs-C2Hs Solution : Two different alkyl halides (methyl chloride
Ethane UV Ethyl chloride (Wurtz reaction) Butane and ethyl chloride) are to be used to prepare propane. Thus.
(e~)
three reactions may occur giving a mixture containing ethane,
~~t:nt of alkane st·ries : Use of decarboxylation
butane and propane.
reaction is made. II is a multislep conversion.
f.th11nt' to methane : Problem 6. Write the stroctural formulae and !UPAC
names of the different alkanes formed when a mirt11re of
C2H6 ..£!4 C2HsCI KOH(aq.) C H 50H [OJ>
2 1-bromopropane and 2-bromopropane is reacted with sodium
Ethane UV Ethyl chloride A Ethyl alcohol
(QCCII) in presence of ether. What is the name of the reaction?
Solution : (a)
CH3CHO [OJ• CH3COOH~ CH3COONa
~ i> Acetic acid Sodium acetate CH3CH2CH/~r +2Na + Br[CH 2CH 2CH 3
NaOH/CaO CH 1-Broroopropan~ 1-Bromopropane
4
· A Methane
Higher Clz. Alkyl KOH (aq.) roJ
alk:ine W halide A Alcohol~ Aldehyde [OJ•
. NaOH
Acid --➔ Sodiwn alt of NIIOH/CaO
the acid A Lower alkanc
 - - - - - - - - !_ _ __ ~ 1~59

- - -- ---~-- -
~ 3 arran ~eme nt of atoms
or group s (co nfig ura
tio n). He nc e,
t
< r str uc tur e but dif fer en
(c) ~_~!{~-~-1?fjCH
Cff3CH2CHiC!!f_
CH
CH 3
stere ois om crs ha ve the same mo lec ula
1-Brom>propane configu ra tion. typ es :
2-Bromopropane r classified into thr ee
Stereoiso me rs are furthe
merism
CH3 (i) Conformational iso
(ii) Optical isomeris m
I rism
CH3CH2CH2- CH-C
H3 (iii) Geometrical isome alk an es. •
us no w dis cu ss co nfo rmation al isomeris m in
2-Methylpentane Le t
on is W ur tz rea cti on . AL IS OM ER IS M
The name of the rea cti 2.8 CONFORMATION
mass 72fiormed as in eth an e ~s
rro bJ em 7.. An alkande with molecular joi nin g the two ca rbon ato ms
.
o uct. Suggest a str ucture fo r the The sig ma bo nd ax is. Th is
l ab ou t the nu cle ar
only one subistltullon pr cylind ricall y sy mm etrica
pe rm .its the fre e rot ati on of the rn: 0

a/kane. mula of the alkane be symmetry of the bo nd bo n_d ax is

Le t the mo lec ula r for wi th res pe ct to ea ch oth er along the
Solu I ion : ca rbon ato ms sta tio na ry
bre ak ing the bo nd . If on e CH 3 gro up is ke pt
C11 H211+2· alk an e = nC + (2n + 2)
H witho ut e thr oug h C ~
Molecular ma ss of the up is allow ed to ro tat
and oth er methy l gro are po ssi b~ e
= n X 12 + (2n +2 ) xl inf ini te nu mb er of atomic arran ge me nts
axis, an the CH 3 gr ou p is
=l 4n +2 gle (0) through wh ich
depending upon the an ms in sp ac e th at
Thus, I4n +2 = 72
rotated. The different
arrangements of ato
ps ab ou t C -- C bo nd
l 4n 72 ._ 2 = 70
= tation of grou
or result from the free ro nal iso me rs
70 ations or conformatio
n= - =5 axis are called conform the ph en om en on as
So, 14 rs an d
or rot ati on al iso me uc tur e of the
alk an e is C 5 H12 . lt can ris m. Th e ba sic str
Thus, the mo lec ula r fon nu
la of the co nfo rm ati on al iso me s, ho we ve r,
an d va rio us bo nd lengths an d bond gle
an
have three iso me rs. molecule co nfo rm ati on s.
these arrangements or
CH 3 CH3 remain the same in all
I oj ec tio n)
I hane (Sawhorse Pr
Co nfo rmati on s of Et nts of eth an e,
CH2CH3; CH 3- C- CH 3 ofpo ssi ble arr an ge me
H3CH
CH3CH2CH2CH2CH3;C 2-Methylbutane Of the infinite number es e ar e ca Jie d
/
nn ati on s rep res en t the ex tre me s. Th
n-Pentane CH3 two co nfo the sta gg er ed
(Tsopenlane) pse d co nf or ma tio n (I) an d
2,2 -D imethylpropane
the ecH co nf on na tio n, th e
ati on (II). In the case of ec lip sed
(N eop entane) co nfo rm hin d the oth er,
of on e car bo n ato m are dir ec tly be
hy dro ge ns ms is ma xi mu m
su bstituted the rep uls ion in the se ato
on ly on e mo no con seq ue ntl y, on , the hyd.r9g en s
Since, the alkane forms gen ato ms . sta gg ere d co nfo rm ati
ly on e type of hydro (0 =0°). Wh ile in the th respect to on e
product, it must have on of the two carbo n ato
ms are sta gg ere d wi
ane.
2,2 -di me thy lprop ximum dis tan ce
Therefore, the alkane is a result, the y are at ma
using the another (0 =60° ). As them. An y other
tan e fro m ch /o, vetha ne um rep uls ion between
Problem 8. Prepare bu ap art an d have mi nim ese tw o ex tre me
will be between th
Corey-House synthesis. - arr~n_g ement which
I) is kn ow n as Gauche
or Sk ew fior m .
conv erted into lithium pos1t10ns as shoHwn in (II
· Solution : Ch lor oe tha ne is first
thane to form
n rea cts with chloroe ,,·H
diethylcuprate wh ich the
butane. --H
Cl H 60°
Ether CH 3CH2 Li + Li H
(i) CH 3CH iC I + 2Li H
+ Lil
~ Li(CH3CH2 )i Cu
(ii) 2CH CH Li +C ul
3 2 Lit hium dielhylcuprate H
H
(""")
+C H 3CH 2CI ~ (i) Eclipsed fon n (ii) Staggered form Gauche or Sk ew fon n
(iii ) Li(CH 3CH 2 )iC u (8 =60") Ill
3CH2Cu + LiCI (9 = O")
(B"" anything else)
CH3CH2CH 2,CH3 + CH (Sawhorse Projection)
Butane
Newman Pr oje cti on
n, the two
IS 4
2.7 STEREOISOMER e to the difference in. relative . In Newman pro jec tio c ~ n ato ms fo nn in
th e
_ nd are rep res en ted b be h. :
Y two circles, on e
. du sigma bo
S...-.w. ...ne..sm occurs groups in· space (Greek . ste. reo -
a , J Dt i,f atoms or oth er, so that on ly the
&
Iro n~ ~ ca rb on is see
n. The thehy:.O
--.
~ . --I•p ar t.
) S mers are compounds ha
•so
tereot .. fon nu lae but
vmg theI
.
dif fierent spalta atoms attached to the
arbon are rep res en ted lty C
g';';
- ular __ ...a 5trUc......... 1
~ • · ~ ... .. u
 for Medical Entrance E)(
GR B Or ga nic Ch emistr y
~
--
16 0

bQ nJs fro m Ute cen tre of cir

-
de. ThC' C'- 11 t-..lOds llf the:
b:,ck Th e oth er
It is not pos sib
~
con
.
tom
. ns lte
tatw
.
bet we en the se two i
le to iso late tl1e I diff
.
'erent con.fomn1atto
nve rting. In
s~ability.
n
his or
llft he cin :le. st~m t Y mte rco
car lxm are <lr.1wn fro m the drc
umf'i:ren1.•e eth ane bt."Cnuse tl1ey are con gber
en ato ms of eth ane are re
Ht t alk ane s, one or mo re hyd rog arb ons and sub ~lat eq
H her hyd r~c
ll H by alky l gro ups. Oth er hig sti
tuteq

~
atio ns. Th e Stu dy
con fo rm of th e1r.
eth ane s all exi st in these &' t·1on a I ana lysis.
H~ H~' ).:! ~H pro per ties is kno wn as
e
co n,o rm a

II Ill II/ N~ H H
. Conformatfons of Pr op an
H pan e are sim ilar to th ose 0f
(ii) ~'m .-d form (iii) Gao
oie or ske w furm Th e con fom 1ation s of pro db
(i) Fd i~ li.'1ffl1 the hyd rog en ato ~s is replace
(:S ~m aa Pn,J«doa) eth ane ex cep t tha t on e of ma t1o ns of pro Ya
reme con for
me thy l gro up. Th e tw o ext : Pane are
·ii iati on int o jec tio ns)
A mf3tkln of t,0° t.-onn!rts
a sta gge red con fom sho wn bel ow (N el'·ma n Pro
or ,·ice-l'ersa. Ro tati on bet we
en 0° p o tential ene rgy between th ese
an ecl ips ed oonti..lm1:uion. ent s in Th e dif fer enc e in the -1
ma ny oth er arr ang em is abo ut 3.4 kca l 11101
to 60" gen ern t~ one of the s ent ext rem e con for ma tio ns (or
ed for ms . Th ese arr ang em
~tw «n smggered and ecl ips
1
14.2 kJ mo l- ).
form.
are called Gauche or Sk e"·

i~~
e
Co ,,f. :r 'l'l t · ~ is of Eth an
Relati>,e Sta bil itie s of the H~ C H1
eth ane mo lec ule cha nge s
The polential ene rgy of the of
the C- C bon d on acc oun t
somewhat \\it h rotation aro und bon ato ms .
en ato ms of the two car H~ H
the distance bet w« n hyd rog mi nim um and of H H
gge red for m is
The potential ene rgy of sta
eclipsed form is ma
content between the two ext
1
xim um . Th e dif fer enc e in the ene
rem e con for ma tio ns
bar rie r of rot atio n is
is 3
als
kca
o cal
rgy
l/m o)
led
Eclipse d
'---- - - --......r ____
Conformations of pro pane
Staggered
/

(or 1.'.?.5 kJ roo l- ). Th is sm all ven t c--c

rgy is not large eno ugh to pre bar rie r o f rot ati on about
Torsional barrier. Th is ene ene rgy bar rie r is E_ven the n, this _ene rgy on. As a result, the
perature, the no t pre ven t rot ati
rotation . E\'en at ord ina ry tem for ma tio ns kee p bon d 1s so smal~ tha t tt can rtible by rotation
and thu s, the con dil y int erc o nve
O\'ercome_ through collisions e var iati on of two con for ma tio ns are rea separa1e
on cha ngm g from one fon
n to the other. Th d thu s, it is no t po ssi ble to
C- C bon d in eth ane has bee
n thr oug h an ang le of 60° an
ut
ene rgy wit h rotatio n abo the se tw o con for ma tio ns.
shown in figure. e
Co nfo rm ati on s of Butan
Ec:lip.,ed i
be con sid ere d as a dimeth1
{Jess stable) . n-~ uta ne mo lec ule can carbon~
ich on e H- ato m of eac h
den vat ive of eth ane in wh a n ole (8) is the
up an d the dih ed ral
rep lac ed by a me thy l gro

I l gro up s. t,
ang le bet we en tw o me thy
H H

I
j
I 2 I 31
H 3C -C -C- CH 3
I I
4

l ----------------- ~ - -· H H

tn
(llll ble)

180"
fix ~~h
~;e eron e of _these cen tra l car
bo n ato ms (C 2 or CJ l is
0th 18 rot ate d rou nd the cen tra l (C 2 - C3) t,oo~
0
throug
t ma ny con for ma tion s. 04!
120" ~
AllllcofROlllion
Fig . 2.3
an ~g le 360~,
the seo nJ
Ne~t;:~ are(60°
the ir
of
0
we
l'D1;8t1on s
ge
eac h tim e) are
important an
OJ e~t ion s giv en as fol low s:
The eclipsed conformad ause the · . ·;. ..
hyd rog ens and bon din g pairs
ato ~ are as nea r one ano
o~~ Jb least stabl~ bec
ectrons ~n adJacent carbon
the r _
i as Possible. This
maxunum repulsion. Thus the ecl Pse d form is less abundant
Th e sta ggt -nd confo;ma
causes rkH H
HCytyH
H yi
,
rs :;: ,fs lllost stable becaus
e the CH~~
hrd rog ens and bon din g pai ma -ri, _._
distance. This cau ses minimu
m ectr_ons are at a fl ,:~·"'r
repuJsion. Thus, th is ~
mo st abundant. Sl.c:w u1v
( t) - 3(J(f)
(fll}
 Gll3
- , uche strucUm"5
I fowevcr. in some ,cax:"., ga
161

. are m<1rc ~:;;J,!e

1:~:I
than a , ' . 0luufar Jf-1-.M;d:r,g arid
d:pr,!e-
r~m
di 1 n:,1 be<:~use u~ mt germ1etric.aJ1y p(m;l-.,Je <,r.ly ;.1
raction wh i~h IJ
po_e att .am/.
ne 1,2~io1 and 2-fluwJ ctr
tha
gauche structure. e.g., me

,.~ (f• O' )

(IV)
%t or-; "• n 1.,
(8 • 12(/')
(VI)
,.
CH3
Eclipsed (Partial)
Co er tor,~ of r:, 1 0

cycloalk ane molecule

planar molecules havin
respccti ve1y.
s.
..,
Confonnationa1 isomcri.s
Cy
, r.,

m ha~ alw be1..-1J sf.r

clopropane and cycfobu
g bond angJcs vf M l,
,wn in
tarie are
ari d 'J(J~

Fig. 2.4 Different conrorm

stable
nfonnation (I) is most
The maximum staggered co art as far as possible due to
in which methyl groups are
minimum repulsion betwee
far ap
n methyl groups and is
al ~ called
60° gives
6 El
Cyclopropane Cyclobut..ne
Cycl<Jpent..ne Cyc:fol;.e
xane
co■f or ma ti on . Th is on rotation through
Aad- on one
rm ation (II ), in which methyl group molecules are quite dif:c-
rcr.t from
eclipsed co nfo
the hydrogen atom on
the other The bond angle in these 9S" ) v.hich
ca,bon ii overlapped by the normal tetrahedral bo 3
nd angle of(109'-18' orH1
ms. Th is
carbon. one of for sp -hybridized carbon ato
be obtained by rotating is a no nn al an gle
Other confonnations can °. In al tetrahedral angle
int roduces
ca rbo n ato ms through an angle of 60 de via tion from no nn
s of the:.e
the C2 or C3 Gauche), the ain in the molec ule
(II I) and (V) (Skew or considerable angle str
staggered confonnations h oth er (Ill and ater the deviation fro m the normal
sed that they repel eac cycloalkanes. further, gre
methyl groups are so clo caus es Gauche ain and greater the instab
ility of the
This repulsion angle, the greater is the str
V are mirror images). 1 y than , cyclopropane with a bo
nd angle
tions to ha ve ab ou t 3.8 U mol - more energ ring co mp ou nd . fo r ex am ple
ck.,butane
confon na
rmations (JI) and (V
I) are d hence very reactive. Cy
anticon fon na tio n. Th e co nfo eir of 60° is much strained an and nce lt: n
he
ec lip sed an d un sta ble because of repulsion. Th with a bond angle of
90° is less strained
partially 1 ne.
potential energy is 14 .6
U mo1- • reactive than cyclopropa nt of the
ully eclipsed), the steric
strain is
Ba ey er '1 strain theory, the amou
In confonnation (IV) (F Ac co rding to wbich a
ble and ha ve nal to the angle through
mu m, e, this co nformation is most unsta strain is directly proportio
mw he nc of repulsion from its normal posit ion, i.e.,
mol- 1• This is because valency bond has deviated
potential energy 18.4 U
between methyl-methyl groof up which are very se clo
these conformation s
together.
is: Amount of deviation (d
=; ) (J 09°28' - Valency
angle)
The order of stability Eclipsed I 24"4 4'
Partially Eclipsed> Fully e.g., In cyclopropane,
d = 2 (I 09°28' - 60°) =
Ami>Skew or Gauche > (IV )
(V J)
(I) om and (V) ence be(ll)twand een various confo ation
rm s is l
d = 2 (109"28' -90° ) =9
"4 4'
The energy differ In cyclobutane,
I
shown below: Io cyclopentane, d = 2 (109°28' - 108°
) =0 °4 4'
I = - 5"16'
E.cliped and in cyclohexane, d = i (109028 ' -1 200J
Fullyf.cli,-1 (VI) . . ..
(JV) ts that angle strain 15
(Negative sign represen i.ru1de the
ring).
Thus, strain is minimum in cyclo tan e and hence it is
reacti :
more stable and less an e . ic lo pr op ane an
d
, cy clo :x
cyc!obutane. As a result stable and JS rec_from angJe of
stram and hence is quite . Therefore,
n-planar st ru c~ ve
cy cl o ~ adopts a no
120' IIO' 7M1' JOO" Moll; the ·
According to Sac•1e and(or folded c y c l~ e exti1s in
d
Al lp ., ... ... - two non-planar or puckere ha t ) confonnauo-r ns ctJJcd
fg . 2.5 form) and die ,- ·
the c• au (Z (C fonn) co nf -· ·- ions as
r. __.., _ is more stable tha n the
l,etw
mown in the fioa,-
•- ··
. 1s·
onn-coo101u-~- cen
l,ccaUSC of Jess rcpu aon
-• •"'..,•ci..1111':.5:11111ib·oo
 ganic Chem istry for Medi cal Entrance I:
162 GRB 0 r ~
follow ing diagr am s_how s the _ conforrnatio
The d their relati ve energ ies) as ns or
H cyclo hcxa~e ~a~rc onve rts to the other chair confoO
~e _chair
confo rmati on in with methyl grou p in the equat
orial at1~1\.
Cyclohelxa~ean in the axial p ositio n.
is more stab e 1 Position
CH
H

. . Boat confo
~ C H3 ~. °: ~~:;"
Ch•i r ~\'mt1.\mt:1t~)n
•T. f,.,m, 1 ~t·,,•~~n.>ti ,,.,m,) ·c- fonn (eclipnnatio n
sed fonn) M ethyl grou(? _in (Less stable 5%)
·. ..: : ,·"· ·c•,
=.. , a ,d Boat conforn1al1.ons o f cyclo hexan e
equato rial Posm on
(More stable 95% )
-
The nan~ s of these fom1s are d ue to .the resem blance of
Each carbo n
thdr share s with chair and boat respectively. The
bond is
2 _9 PETROL EUM
atom of cyclo hexan e is bond ed to two hydro
gens. . This The term pet ro leum (Lati n: Petra .
= rock ; O/eum == oil) .
. .d . h
one of these hydrogens lies in the plane of th: . th dark- colou red 01ly h qm w it offensive oct is
hvJrol!cn is called equatorial hydrog_en. !he
other hvdro.ren atom is parallel to the axis; this by
;:n
n~g~o the

ogcn
atom
f
apph ed to _e depth s in m any reg io ns below
found at vario us
. also calle d rock
. . .
the surfac our
011, mme ra l o il or crude oil It.
e of
is called· - Ea h f · carbo n atom 0 the earth . It IS
axial hydrogen. c o six . s nd unde r the rock y strata of the earth 's crust. andIS
cyclohexane has one cqu~torial and one axial hydro en in all gener a IIY fiou ·
g ' often floats over salte d wate r. It is db
six axial and six equat onal hydrogen. cove re y an atmosphere
R~,a~ \e St'.'lb 1lities of Chai r and Boat o f agas c Ous mixtu re know .
n a s natu ral gas.
.
Conf orma tions The chief oil prod ucmg centres m . .
0 1 c,, c!ohe :xane India are. Rudrasagar
The chair conformation of cyclohexane is more stabl and Lakw a in Assa m; Ankl eshw ar, Cam bay
e and Kaloi in
than the boat conformation due to the following reaso Guja rat and Mum bai high (offs hor~ _area) .
ns:
(i) In chair conformation, the adjacent hydrogens Com posit ion : The comp osit10n of crude
on petroleum
c 1 -C., , C 2 -C 3 ,c3 -C4 ,C4 -C 5 ,Cs -C6 andC 6 -~1 ~e varie s with place s of occu rrenc e but essen tia lly
it
quite s;agg ered (more stable) and the force of repuls is a mixture
10~ m of hydro carbo ns. It conta ins main ly the fo llowi
them is minimum. On the other hand, in boat confo ng:
rmation, (i) AJl-:10e~ : The perce ntage of a lkane s can
the adjacent hydrogen atom on C -C and Cs -C6 vary from
2 3 are in the 30 to 70. Alka nes conta ining upto 4 0 carbo
less stable eclips ed orientations. n atoms arc
prese nt. Alka nes are main ly straig ht c hain
The two fonns have not been isolated.so far, becau but few are
se the branc hed chain isom ers.
energy difference between the two fonns is too small
(29.9 kJ (ii) C)clo all,. :rn •~S : The p ercen tage of cyclo
0101- 1 )whil e 37.7 to 46.0 kJ mol-1 is the energy of barrie alkanes
r and varie s from 16 to 64. The cycloa lkan es main
one form readily changes into the other. ly present in
petro leum are: Meth yl cyclo penta ne, cyclo hexa
(ii) The two hydrogen atoms (marked as Hf) called ne and methyl
the cyclo hexan e. The oil rich in cyclo alka nes
flag pole hydrogens and in chair forms, C-H flag poles is know n as
at C 1 aspb altic oil.
and C 4 are in the trans side (distance 2.29A or 229 pm)
havin g (iii) Arom atic h yJrc,cMb011..,.
minimum strain and thus more stable. While in boat T he perce ntage of
form the arom atic hydr ocarb ons varie
C-H flag poles at C 1 and C are on the same side s from 8 to 15. The chief aromatic
4 (dista nce comp ound s prese nt in petro leum are: Benz
1.83 A or 183 pm) and so steric hindrance is incre ene, toluene,
ased and xylen es and naph thale ne.
thus stability is decreased.
Chai r> Twist boat> Boat > Half-chain (iv) Sulp hur, 11itro gen .md O' J ,,. n ,
1 , 1 • Besid~s
hydro carbo ns, there are also prese nt
certa in organic
half chair half chair comp ound s conta ining oxyg en, nitro ge n a nd
sulph ur comp ound s are prese nt to the ex ten t
sulphur.
of 6% and th e)
T~:
f ~elu de merc aptan s (R-S H) and sulph ides
· disag reeab le smel l of petro leum is due
(R- S--R). Th~
to these sulphur
l comp ound s Th ·
·
neces sary othe .
e1r remo val from petro le um prod ucb •~
h . .
· ver)
. • .
rwise t ese will get ox1d1 sed to sulp h urous and .
_sul~h unc acids durin g comb ustio n in inte
rnal combu~uvn
en~n e and ~ill c~e corro sion of meta l.
· . ·~
Rdation quino1:esch1e! rutro gen comp ound s arc alkyl pynJ1nc~
petrol _an PYm>les. The oxyg en comp ounds pre~ent J:,
. . ~m inclu de alcoh ols, phen ols and re,in s. Coin
r''un
 Hydrocarbons
----
ll.ke
chlorophyll, hacmin · I
· I
(gree n and
ants and ammaJ s, respective y) arc also prcsc
red colou
·
ring
·
iit 111
,
m.i 1lcr of
petrol eum
_ _ _ _ _ _ _ _ 163
fractions condense lirst whcreas the low boiling fractio ns -rise
upw d, d
p
' (v)• Natu ra gas : The gas found above ti1c petroleum· . ar an arc collect ed as soon as they condcn~e. The ga~es
· .~ d low whic11 do not condense at aI1 (low boiling points such as
deposits 1s rc1_crrhc tko as natural gas. It is a mixture of
molecular we1g t a1 ancs, namely methane (8001 10
) met I1anc' ethane, propane and butane) are collected at the top
( 1 o/c) , ethane . .
3%) b
(13%), propane ( o , utane o , the vapours oflo b0 .,.
pcntanes and hexancs (0.5%) and nitrogen (1.3%)
mainl y eonsi
rt
:t: e
' rng
natura l
of the. colum fh
. n · So,ne port·ion o t e crude oil does not vapon se
and is obta,~1ed as a residu e or pitch.
gas in liquid form
under
d
press ure
mg propane - The mam fractions obtained are:
·
and butanes )1s u.s \. ~~ ~ooking gas (LPG = liquef
ieci Boiling point
I
petroleum gasd. t_1s 1g y mflammable . The gas bums with
Fraction
Approximate range upto room
blue flame pro ucmg lot of heat. The gas conta i'ni·n th composition tempe rature
h · g me ane
d sold as bottle d gas in com ressed '
nitrogen an_ et ane 1s state 1. Vncondensed gases C1 - C4
in steel cylmders. P
2. Crude naphtha (16%) 303-4 23K
Theories of origin .of petroleum •• Many th cones . have C5 - C10
the 423-5 2JK
3. Kerosene (25-30%) • C11 - C16
been proposed to explam the origin of petroleum below
crust of the e~rth bu~ none is sati~factory in itself. Any theory 4. Heavy oil (25-30%) c,6-c,s 523-o 7JK
Above673K
proposed must explam the followmg characteristics associated 5. Residual oil (about 30%) C17-C40
with petroleum: 6. Non-volatile residue
~!) its association w~th brine (sodium chloride solutiinon),it
(11) the presence of mtrogen and sulphur compounds Different fractions are further refined and subjected to
(iii) the presence of chlorophyll and haemin in it ' refractionation to yield various useful products.
(iv) its optically active nature. ' Purification : The fractions obtained above are called
ve ·
sour fractions. These fractions are purified in order to impro
Mining of petroleum : Petroleum deposits occur at ess and other
odour, stability to air oxidation, corrosiven
varying depths at different places ranging from 500 to 15000 properties. The actual process of purification differs from one·
oil
feet. This is brought to the surface by artificial drilling. The fraction to the other depending upon its commercial utility and
well is drilled till the oil bearin g regio n is reach ed and pipes
nature of unwanted substances present. .
are fitted. Sometimes, the oil rushes out through these pipes (i) Treatment with concentrated sulphuric acid :
The
the press ure exert ed by the natura l gas. As the gas uric acid to
due to
d to raise the oil from gasoline or kerosene fraction is shaken with sulph
pressure subsides, air press ure is applie ur
remove aromatic compounds like thiophene and other sulph
and
the well. compounds which impart offensive odour to gasoline
The crude oil thus, obtained is either stored in big steel kerosene and also make them corrosive. It is then allowed to
tanks or sent to refineries by pipelines for processing. settle and the upper layer is withdrawn. It is treated with
eum obtain ed by ed
Petroleum refining : The petrol sodium hydroxide to remove excess of acid and finally wash
, with water several times. It is then redistilled.
mining contains impurities such as sand, brine or sea water
as cr~de (ii) Doctor sweetening process : Sometimes sodiu
m
sulphur compounds and resins. It is technically called
calculated quan tity
oil. The crude oil is mechanically freed from sand and brme plumbite is used in presence of alkali and
ry of sulphur to remove mercaptans. Merc aptan s are oxidised to ·
and then subjected to fractional distillation in a refi~e
because crude oil as such is not suitable for most techm
cal disulphides.
pwposes. · • 2RSH + Na2Pb02 +S ~ RSSR + PbS +2N a0H
The process of dividing crude oiJ into uscf ul fra~tJ Ons Disulphide
nd esirab le
with different boiling ranges and free from u (Iii) Treatment with adsorbents • Vi .
Impurities is termed refining. . . . are passed over adsorbents like alumina •or s1·1~no us fractions
is came d out . 1ca or clay. etc
The fractional distillation of crude oil w hen t he undes irable comp ounds get ad sor be d. • ·•
• . spec1.a11y des1gn · ed tall fractionating tower or .
contmuously ma . A comp Iete I1st of petroleum d .
The crude oil is heated m ah furnac e to n, boilin g range and th . pro .uct~, approxnnate
b T comp ositio
coIumn, made of steeI. H' eir uses ts given ahead :
about 673K and introduced in this tower. ig er o1 mg
 GRB Organic Chemistry for Medical Entrance E

---------------- - -----
Table : Petroleum Products
~
Uses
lmatn compo,if111n
S • K) Appro1 ~ _ --- - - - - - - - -
__ n_.ge: __(~ ---1---c:-c-4
=lll=n:g_:r•__:
_.!:N~o~.~---_:F~r:•c~ti:o:_n_____j~.:_Bo ~f--------
Fuel gases; refrigerants; production O
black; hydrogen; synthesis of carbon
1 U Upto room temperature Cr -
. ncondensed gases chemicals. organic

c5 - Cro
(303--423 K)
2. Crude naphtha on refractionation Solvent.
yields: C5 -C6
(303- 343K) Motor fuel; drycleaning; petrol gas.
(i) Petroleum ether c6 -Cs
(343-393K) Solvent; drycleaning.
(ii) Petrol or gasoline c 8 - Cio
(393--423K) Fuel; illuminant; oil gas.
(iii) Benzene derivatives C11 -C16
(423-523K) As fuel for diesel engines; converted to
3. Kerosene
c15 - C,s gasoline by cracking.
(523-673K)
4. Heavyoil

Refraclionation gives:
;
(i) Gas oil
(ii) Fuel oil
(iii) Diesel oil
Above673K
5_ Residual oil on fractionation by
vacuum distillation gives: Lubrication.
C17-C20
(i) Lubricating oil Candles; boot polish; wax paper; etc.
C20 - C30
(ii) Paraffin wax Toilets; ointments; lubrication.
C70 - C30
(ill) Vaseline Paints, road surfac ing.
C30 - C40
(iv) Pitch As fuel.
6. Petroleum coke (or redistillint tar).

various gases such as ··ethane, propane and butane including

LPG and CNG (Petroleum Gases) small amounts of other gases nitrogen, helium, carbon dioxide,
I. Liquefied petroleum gas (LPG) : The petroleum hydrogen sulphide and water vapours, etc. Because of the
gas liquefied under pressure is called liquefied petroleum gas gaseous nature of this fuel, it must be stored in either a
(LPG). It is chiefly a mixture of butane and isobutane with a compressed natural gas (CNG) state or in a liquefied natural
small amount of propane and is easily compressed under gas (LNG) state. The CNG is now being used as a better fuel
pressure as liquid and stored in iron cylinders. It is supplied in than gasoline for running buses~ cars and three-wheelers in
liquid fonn, so that a cylinder of even small volume may metropolitan cities like Delhi, Mumbai, etc., bl.!cause of its
contain an appreciable amount of the gas. A domestic gas
complete combustion and no unburnt carbon is being released
cylinder contains about 14 kg of LPG A strong foul smelling
substance called ethyl mercaptan (C2 H5SH) is added to LPG into the atmosph~re to cause air pollution. Recently a plant for
cylinders to help in the detection of gas leakage. The gas used LNG has been established in Gujarat. CNG or LNG are very
for domestic cooking is called Liquefied Petroleum Gas c~ean fuel~ which cause very little pollution and also havevery
(LPG). It is mainly used as a pollution free household neat and high calonfic value. Natural gas has octane rating of I30.
~lean goo~ fuel because the combustion of butane and
iso~utane ts comple~. When we tum the knob of the gas 2.1 OARTIFICIAL METHODS FOR MANUFACTURE
~Yl!nder, the p_ressure is released resulting decrease of ressure OF PETROL QR GASOIJNi
inside the cyhnder, so that highly volatile LPG chanp . 01
gas. When ignited, the gas bums with a blue fl ges_ mto P~trol or gasoline obtained from crude petroleum is 0.
lot _of heat. It has a high calorific value (abo:n;ci~u)c~a sufficient and can meet only half of world's requirements. It is,
maJor sources of LPG are natural as and fr ~· e ~erefore, necessary to find alternative methods of obtain!ng
. · cracking of petroleum. g om refimng and ~ trol. The deficiency is partly met these days by converung
2. Compressed natural gas (CNG) • ke~ 0ther les~ valuable fractions such as heavy oil a11d
compressed at very high pressure is called • The natural gas osene fractions into petrol with the help of the cracking
gas (CNG). It consists mainly of me compressed natural
relatively unreactive hydrocarbo thane (95%) which is a
:;:e:s, Synthetic methods have also been discovered for~~
u.1acture of petrol Th I ·d,or
•. J n and makes . the artifi ·a1 · e methods generally emp oyc ~
·, comp etc combustion possible. The other 50 . It nearly main hea':ls: production of petrol can be studied undl'r fll
. ¼ 18 made up of
 ::::.:"s= --------- - - - - - - - - - - - - - - - -~1=65
HY(Jrocarb<>
~king, (ii) Synthesis. It is observed that petrol obtained by cracking is far
(~) cracking : It is a process in which high boiling superior to petrol obtained by direct distillation of crude
('.) consisting of higher hydrocarbons are heated petroleum on account of its high octane number (due to
5
rractiO: to decompose them into lower hydrocarbons with presence of large quantities of unsaturated hydrocarbons).
str00;oiUng points. The process of cracking involves the Petrol is also obtained by the polymerisation and
alkylation of simple hydrocarbons obtained during cracking.
towaki g of carbon-carbon and carbon-hydrogen bond
breul _ng in the formation of smaller molecules of various Olefins undergo polymerisation reactions in the presence of
res tilldepending on the cond". catalysts like H2so 4 or H3 PO 4 . Simple olefins fo~
1t1ons used. molecules having 6 to 8 carbon atoms and can be converte
type}or example, whe~ long chain alkanes. are hea~ed well
b e their boiling pomts (673- 1073K) with or without a into petrol by hydrogenation.
:a~~yst, they are broken (or cracked) to yield smaller alkanes, HC>
3 C=CH 2 + H-CH2--C= CH2
70% H2S0~
aJkenes and hydrogen. H3C J
673- 1073K CH3
Long chain alkanes - - - - Smaller alkanes
+ Alkenes + H2 2-Molecules of 2-methylpropene

Thus, by cracking of a single alkane (e.g., decane), a large H 3C> C-CH -C=CH2 ~ (CH3)JC-CH2CH(CH3)2
number of hydrocarbons like heptane, hexane, pentane, H3C J 2 J 2,2,4-Trimethylpentane
butane, propene, butene, pentene, hexene, etc., are produced. CH3 CH3 (Iso-oetanC)
CH 3 (CH2)6CH3 + C2H4
Octane Ethene 2,4,4-Trimethylpentene

CH 3 (CH 2 ) 5CH3 + C3H6

Heptane Propene
The octane number of polymer gasoline i~ ~enen;lly~!!e
Alkylation occurs under similar cond1ttons. s~ bout
reacts with isobutane in presence of cone. H2S04 a a
CH3(CH2)sCH3_c_ra_c_ki~ng:...+-~ CH3 (CH2)4CH3 +C4Hg \ 3 I 3K to form iso-octane.
Oecane 773K Hexane Butene/
CH3
CH 3 (CH2)JCH3 + C5H10 I
Pentane Pen!f;oe CH --C=CH2 + CH3-C-H
3
I . I
CH 3 (CH2>zCH3 + C6HI2 CH 3 CH3
Butane Hexene 2-Methylpropane
2-Methylpropene
(Isobutene) (Isobutane)
Similarly,
CH3
Cracking
C12H26 - 973K =+
C 7H16 + C5H10
Heptane Pentene
I
CH3-C-CH2-CH-CH3
Dodecane
(b.pt. 489 K)
(b.pt. 371 K) (b.pt. 309 K)
I I
CH3 CH3
Cracking is carried out in two different ways:
lso-octanc
(a) Liquid-phase cracki~g and
. (b) Vapour-phase crack1~g. . this rocess, the heavy1 CH3 CH3
(a) Liquid-phase cracl<lngh" h~empe~ature (750-800K) I I
oil or residual oil is cracked at 3 ~! heric pressure). The high: CH 3--C-
I
H + H2C=CH2 ---+ CH3-C-CH2-CH 3
I
under high pressure (7 to '.O a~ociuct in liquid state. The1 Ethcne
pressure keeps the _reaction ~ d the resulting petrol has CH3 CH 3
conversion is approximately 7o an 70
O
lsobutane Neobexane
the octane number in the range _65 to se~ce of some catalysts
. be done Ill.dpreferric oxide and aIumma
. (ii) Synthcsh : Petrol can be synthesised &
The cracking can . . coal
(Synthetic ~troleum) by the following two meth~~
like silica, zinc oxide, t1tan1~~dox~f ;etrol are generally higher ,
(a) Berg1us process and (b) Fischer-Tro h ·
(catalytic cracking). The y1e :emperature. ' (a) Bergius process (H cir . psc process.
when catalyst is used at Iowe~ • In this process, kerosene method was invented by Berit ~eaation of coal) : Thia
(b) Vapour-phase crackiDgh.se The temperature is kept
d · vapour-p a · h world war. Finely powdered co':
m Germany during 1he fint
or gas oil is cracke Ill
5
is about 3.5 to 10.5 atmosp eres.
870-1070K and the pres ur~ se of a suitable catalyst. The
The cracking is facilitated y u
yields are about 70"/4.
oxide, Fe 2o 3 or tin) is made . and a catalyst (usually &aic
paste is then preheated and · mto a paste with heavy oil
Pumped to the convener. a.. t
 166 GRB Organic Chemistry for Medical Ent ra nce
0
-- ----- - - - - - ~ rtiss
p ·
- - - •·
J . Its the loss o f energy. A fuel which h
I
. -----......:.

2 astc Is heated _to 7~0-770K a nJ hyJmgcn is passed at _al'Kl~J~ an ~~u . ro erty. is always pre ferred. as rn1niniuni
so_atmosphenc r ~~'l1rc. The product of hyJro~cnarmn is kno~ h g
35
~! observed that knocki ng property is
1
subjected to frac tional d1sti llation. h t (compos ition) of fue l. The straight cha~e at~d \\ 1th
t e nature . 1n ahph
Fc:~O_l
Coal+ H., ---_..;;...-.. d ·1 hydrocarbons have a higher tendency to knock while branc~t1c
Mix. of hydrocarbons or cru e 01
7~0--7701\: chain or unsaturated hydroca rbons have less tcndenc ed
Y_to
foll ow,n
~50atm. (synthetic petroleum] kn Ock • ; . e. , the tendency to knock fa lls off in the
~~ g
The follo" i ng fractions arc obtained: Straight chain alkanes > branched chain alkanes ::,
(i) G asoline upto 423K
olefins > cyclo alkanes > aromatic hydroca bo
(ii) ~fiddle oil between 423-473K . . r ns
(iii ) Heavy oil 473- 573K To indicate th~ qualtty of_gasolme ~etr?I), a methoct of
( iv) Residue gradation has been mtroduced m 1929 which 1s termed octane
Heavy oil and residue are again mixed with coal and the rating or octane numbe~. Two compound~ have been taken as
above process is repeated. Middle oil fraction can be standards. Heptane which causes max~um knocking is
separately hydrogenated in the vapour phase in contact with a assigned the octane num?er O (zero) ~ h_1le 2,2,4-trimethyl-
solid catalyst to form more of gasoline. pentane (iso-octane) which causes mrn1mum knocking is
The }icld of gasoline by this method may be as high as assigned the octane number 100.
6~'0- CH3CH 2CH2CH2CH2CH2CH3 ;
1) ! ... , IH.r-Trop ... ch process: In this method, steam is Heptane
first passed over bot coke to form water gas. (octane number = 0)
ISOOK
C + H 2 0 - - - - , CO+H2 CH3 CH 3
Coke Steam Water gas I I
CH3-C- CHz- CH-CH 3
·water gas is mixed with half its volume of hydrogen and I
then passed over a catalyst at 473K under 5-IO atmospheric CH3
pressure. The product is synthetic petroleum. 2,2,4-Trimethylpentane
(octane number = 100)
xCO+ yH2 Co or Ni Mixture of hydrocarbons+ H20
A 'The octane number of any sample of gasoline is
The best catalyst for this process is a mixture of cobalt determined by matching its knocking property with tht'
(100 parts), thoria (5 parts), magnesia (8 parts) and kieselguhr mixture containing iso-octane and heptane in an experimental
(200 parts). engine. The octane number of a given sample may be
The artificial petroleum obtained is then fractionally defined as the percentage by volume of iso-octane present
distilled. The various fractions separated are: petrol, kerosene, in a mixture of iso-octane and heptane which has the same
lubricating oil, diesel oil and paraffin wax. The high boiling ; knocking performance as the fuel itself. Fo r exampk. a
fractions are cracked to get more gasoline. The overall yield in ' given sample has the knocking performance equivalent to a
1
this process is slightly higher than Bergius process. mixture containing 60% iso-octane and 40% heptane. Th~
octane number of the gasoline is, therefore, 60.
2.11 KNOCKING AND OCTANE NUMBER
A ntiknock compounds : To reduce the knocking pro~-
Petrol is used as a fuel in the internal combustion engines of . · hern1•
erty or to improve the octane number of a fue l certain c
c~, scooters, aeroplanes, etc. A mixture of petrol vapour and nd5
air 1s COll1preued by I piston .th.10 th. . ,;als are added to it. These are called antiknock compou j
to its one-sixth or WI e cylinder of the engine
is ignited by I one-tenth volume. The compressed mixture Oqe such compound, which is extensively used, is tetraelbY
delivers a
movement of
srn:: .!:
the
~The fuel b~s and energy produced
Pillon the PlltOo: This result in the
lead {TEL). TEL is used in the form of following mixrure:_ d
= =
TEL 63%, Ethylene bromide 26%, Ethylene chlon e
~!.~
wheel of the vehicle. The effl.... 18 ~mitted to the = 9% and a dye = 2%. 11on
the compression ratio. 'Ji;":"- 1 0
•the engine depends on \ Generally l to 3 ml of this mixture is added to on~ ga 10
fuel-air mixture beyond I C:::- ?' compression of the ,.of
gasoline. It is believed that tetraethyl lead dissocia;es,h~
burning of the fuel which causes·C:m t ~ta in irregular
. gsv~ free ethyl radicals which convert some
O
b ns
gives rise to violent metallic ~ l&linat the piston and I Stra.ight-chain hydrocarbons to branched-chain hydrocar -~ a
produced due to Irregular blhillna f : • llletallle ■ound
1 ~ thus, improves the octane number. However, ~r: ';.0
th
knocking. The knocking Iowera the·~ . '-11 ta termed as
. . ~&UCt".°cy .c,f the ~
·) re:~e
)disadvantage that the lead is deposited in the enginci,ich
th~ free lead, the ethylene halides are added w
· j, co me With lead to form volatile lead halides.
. I j
 \
~
~~s
.-- :~~;,:: :=~:
pb+ Br- CH 2-C H2
~:: ::---~------____
Br ~
___ - PbB r2 +H 2C CH
No-lead gasoline sold t0 daY is • h
16~7
_.!.

Ethylene bromide Volatile Ethene 2

isomerisa (o d . t e gasoline obtained by
·.
1
n an a 1kyla tion blen ded with BTX Its octane
ber 1s 90. ·
s for Improving Octane Nu,nber of Gasolm . . num
Other Method e Th oved:
·
toxi c in natu h .e octa_ne number of petrol can, thus, be impr
nds are -chain or
Since, lead com pou
· re, t e lead . (i) by increasing the proportion of bran ched
'de escaping from th poll utes the atmosphere cychc alkanes
broDU d. h fie engm ~ (") b '
rove the octa b · bons (BT X),
The modem tren 1s, t Th ere ore, to imp
ne num er .~~ Y addition of aromatic hydrocarol
· TEL (111) by addi tion of meth anol or ethan
without us;:g. · . e thoctane number of a fuel can be H
(iv) by addition of tetraethyl lead (C2 5 ) 4
Pb.
jmproved Y mcreasmg e_ perc enta ge of branched-chain
hydrocarbons The 10II owm ·
aJkanes, alkenes and arom atic
&'
. g 2 12 CE
. with better • TANE NUMBER
processes are used fior gett mg no-lead gasoline l oils. Two
octane number. Cetane number is used for grading the diese
. Hex adecane
(i) Isomerisation (Reforming) : By pass
ing gasoline compounds have been selected as standards ethyl
while a-m
vapours over aluminium chlo ride (AICl 3 ) at 473K
. (cetane) has been assigned cetane number 100
on the basis of
naphthalene is assigned zero cetane number
CH 3 ignition property.
AJCl3 I The cetane number of a diesel oil is the
perc enta ge of
CH 3CH2CH2CH 2CH 3 CH 3-C H-C H 2CH 3 of cetane and
473K cetane (hexadecane) by volume in a mixture
e igni tion prop erty
Pentane Isopentane a-methylnaphthalene which has the sam
(octane nwnber = 62) (octane nwnber = 90) expe rime ntal
as the fuel oil in question und er similar
conditions. Thus, the diesel oil having
cetane number 75
(ii) Alky1ation : Isobutylene formed as a by-product
ure of75 % cetane
during cracking of petroleum, alkylates with isob
utane to fonn · would have same ignition property as a mixt
H SO . and 25% a-methylnaphthalene.
iso-octane in pres ence of concentrated 2 4
CH3 CH 3 CH3 CH 3 2.13 FLASH POINT
I I H2S04 I I Volatility of a liquid hydrocarbon determin
es its expl osiv e
CH3 -CH + H 2 C= C-C H3 - - CH3CCH2CHCH3 nature. Kerosene used for illuminating purposes
should not be
I Isobutylene I sufficiently volatile at ordinary temperatures
so as to form
CH3 ture at which
CH3
Isooctane explosive mixture with air. The lowest tem pera
osive mix ture
hobutane (octane number = I 00) an oil gives sufficient vapours to form an expl
the oil.
with air is referred to as flash poin t of
d h of fixed for a
. AtQlnat1zau. •on ( . A mixt
Pl ~ t&-,orm•·ng) ·•. ure The flash point or ignition temperature
(Iii} depends on its
bcmene toluene and xylenes known as BTX nes) is obtame w en particular oil varies from country to country,
vapours are enta ge of high ly vola tile
crude nap'htha (C 6 -Cg straight chai n alka climate and controls the perc
t 773K So the octane flash poin t is high for a hot
+ Al 0 3) a ~ ·ng 'low-octane hydrocarbons in the oil. The
t in India is
over a cata lyst (Pt 2 try. The flash poin
J)ISSed 1
country and low for a cold coun
Dumber is improved significantly by conve at 308K , and in Eng land at
fixed at 317K, in France it is fixed
alkanes into high-octane co?1ponents. 295.SK. The flash point of an oil is usua lly dete rmin ed by
CH3 means of "Abel's apparatus".
HiCI/ <;;H3 Pt/A li03
2.14 PET ROC HEM ICA LS
I 773 K, (- H2)
H2 C, ....,........CH2 source of
Cyclohexane Petroleum is the biggest and cheapest
CH2 bons can be conv erted into
hydrocarbons. These hydrocar
n.Hexane ds. AU such chem icals which
CH , various useful organic compoun
HC~ · .._C H natu ral gas are called
are derived from petroleum or
pt/AJi03 - j II petrochemicals. The modem petro chem ical indu stry
CH ul orga nic com po ds
7 73 K, (- 3H2) HC ~fi produces about half'da million usef un f.
tur
. I
Petrochem1ca s prov1 e raw· materials for the man &-.
Benzene . r-.b u1ac eo
dyes, dru_gs, PI~~hes, ,a ncs, insecticides, dete ents' f,--...1

6
CH3 rg uvu
preservatives, d1smfectants, etc.
+4H ,
CH3(CH2)sCH3 Toluene
. Heptane ( octan e no. = I 04)
(octane no. =0)
 GP; Orga nic Qiem istry for Med ~!_51"0-ar,.e...f:;.,-·t
168 ......

are obtained from petroleum:

The folJowing list includes some of the chemicals l'fach
_
. ,.__ _.acf t dff'iY d • of -
fj ~
JI) drocaroon1 I
'-"'""" r--
formic acid. frwn. hydrogen ar S)T,~lC 1a ~il'rt1x•.a.
--~ ---- .

~I~:• deby'1e, ethylene ethyl a,ccta:te, nitroctl-.ane, aw.x

ar.J::,d.-4
I Methyl chloride, chJorofonn, methanol. fonnaJ
l y ~ , styrene, butadi enc, ~ ic ac,-id.
2. E&.ane iErh;1 chloride, ethyl brooude, acetic acid, - oethane, nitrr.,propane.
e. gJy _ 1, po
3. uhjle ne ; Ethanol, ethylene oxide, glycol, vinyl chJond acefOO C nirrom edJane , DI1T
l ether, ' _ . dod,ecyfbcnzene, cumene, bahl;:e.
4 . Propane , Propaoo1, propionic acid, isopropyl alcoho l, acrole in. rurrog Jycenn e,
, __ ,
_/Glycerol, alfyl akobo l, iauy...,,,.,.
1
5. Pro;,ylc:ne
6. Hexan e Benzene, DDT, gammexane.
Toluene.
7. Hepta ne
8. Cycloalkanes
I.Benzene roluene, xyleoes. adipic acid. . _ _..,,, nylon, cycJo heune, ABS deterg errtJ.
, BHC (insecticide), ad1p1c - -
.
/ ' _.._,. _ T. t,enzyl chloride, benz;sl chlondc.
.
Ethylbaazale. styre ne,,-. -..
9. Benze ne I
I Benzoic acid, TNT, benzaldehyde, aacchaml. chJoramme-
'
JO. Toluene
(lsobutane - ane + yleneJ lso-bu!jt..c
CH3 -C= CH2
PART-II ALKENES
The open chain hydrocarl>ons which contain lesser n ~
ng aJkanes C O D ~
'
CH3 .
a
- - ,ydrogen atoms than the correspondi are caJJed aasat ur• CH= CH2
- ,.,ic;,n,.1e number of carl>on atoms CH3 CH2 CH2
gen
h,dro carbo ns. The membeB which contain two hydro tJ
a(oms Jess than the corresponding saturated bydrocarl>o
ns are CH3CH2 CH= CH- CH3
Ja
known as olefim • and are represented by the general formu doubl e fit) , c1 ,, : Olefi ns are na~ as
These are charac terised by the presen ce of a
d by
Cn H2n · substituted derivatives of ethylene. The names are deri\e
bond between two adjacent carbo n atoms >, C=C < , in the
adding the name of alkyl groups repla cing the hydrog
~
en
name ethyle ne.
est• atom/atoms of the ethylene to the parent
mulccuJe. In JUPAC system, the olefins are termed as aJken l or
Alkenes seldom occur free in nature. Lower aJJcenes occur diaJkyJ derivatives are of two types, symmetrica
alkyl groups are
in coal gas in minute quantities. Ethylene is present in nalur
al unsymmetrical, depending upon whether the
attached to the same carbon atom or to different carbon
nts by atornS.
gas. Jlowever, alkenes are produced in large amou
crack ing of petroleum. CH3 -CH =CH2 Methyl ethylene
CH3 -CH =CH -CH3 Sym-dimethyl ethylene
2 1 S NOMENCLATURE OF ALKENES
CH3 -C= CH2 Unsym-dimethyl ethylene
There are three ways of naming the aJkenes or oJefins.
(11 'J ht commoft \) ,tern: The common names
of first I
three members are derived from the corresponding aJkan
es by CH3
g -ane to -yJene . Greek letters are used to ing
changing the endin
the positi on of doubl e bond. (iiiJ Jn•A C w~te, 11 : The name is obtained by dropp
di~tinguish isome rs as to indica te and
the suffix -ane from the name of the corresponding alkane
u., /3, y, etc., are written to indicate the carbon atoms having adding the suffix -ene.
double bond.
H2 C=C H2 (Ethan e - ane + yJene) Ethylene Allcane- ane + ene = AJkene
the
CH3 - CH=C H2 (Propane - ane + yJene) PropyJene The position of the double bond is indicated by
ered) involv ed in
a number of the fJISt carbon atom (lowest numb nd
a-ButyJene the double bond. Alkyl groups are numbered, name d a
CH3 -CH 2 -CH =CH z (Butane - ane + yJene)
two or
placed as prefixes in alphabetic order. When there are
'
CH3 -CH =CH -CH 3 (Butane - ane + ylene) IJ-Bu
tylene :;:-ee
rresp
doub!e bonds i~ the molecule, the ending -ane of the
ondmg aJ.kane 1s replaced by -adiene or -atriene.
of
•The term olefin h11 itt oriain from III oJd Dutch name H 2C=C H2 CH3CH= CH2
crhylenc 'Oleliant gu' (oil fonni np gu) which mers to the ·1 &hene Propene
y
nature of the product (echylene chloride> formed by the com bi~•on CH3CH2CH=CH2
of crhylc ne and chlorine. CH3CH= CHC H3
But-J-ene Bul -2-cne
t Alke11es containing two c:arbon-ca,t,oa double bond, are caJled
dic:nt•s or afkadJenn.