23 Haloalkanes

Compounds formed by replacement hydrogen atom(s)

of in an
haloalkanes.
c-F C-CI C-Br
atomare called
aliphatic hydrocarbon by halogen Increasing bond length
Nomenclature
Increasing bond strength
to commonn or trivial system,
According
monohaloderivatives are named as alkyl halides.

For example.
Preparation
CH CH-I From alcohols: Alkyl halides can be prepared from
CH-Cl
(methyl chloride) (ethyl iodide) alcohols in a number of ways:
According to IUPAC system, the monohaloderivatives - Action of hydrogen halides:
of alkanes are named as haloalkanes. HCl/anhyd. ZnCl
The numbering of the parent straight chain
alkane >R-Cl+H,0
is always done in such a way that the carbon HBr/H,SO4 refux R- Br +H,OO
atom gets the lowest
R-OH or KBr +conc. H,SO4 reflux
atom carrying the halogen
possible number.
HI reflux R-I+ H,0
orKI+H3PO, reflux
CH-CH-CH3 CH-CH-CH2-Br - Order of reactivity of halogen acids towards the

Br CH3 above reactions are: HI> HBr>HCI

(2-bromopropane) (1-bromo-2-methylpropane) - Order of reactivity of different alcohols towards the
If different halogen atoms are present in a chain
abovereactions are:
then we wite them alphabetically and also counting
will be done from that side by which halogen benzyl> tertiary> secondary> primary
followed - Action of phosphorus halides:
(bromine) gets the least possible position
by chlorine, fluorine and iodine alphabetically.
PCl R- Cl+ POCl+ HC
CH-H-H-CH PClhR -Cl+H,PO3
R-OH PBr
CI Br
(P+ Br,) R-Br H,PO,
HPOs
R-Br ++

(1-bromo-3-chlorobutane)
Pl
(P+12)
R-I+ H,PO,
Nature ofC-X bond
covalent bond due to large - Action ofthionyl chloride (Darzen's process)
C-Xbond is highly polarised carbon and halogen
difference in the electronegativities of
the clectrons away from carbon
R-OH+ SoCI, rdne R-Cl+ so,1 +HCl
atoms. Halogentends to pull
due to high electronegativity. Physical Properties
susceptible for
Lower members (CH,CI, CH,Br and CH,C) are ga
carbon makes it
This positive charge on
and the others are sweet smelling liquids.
nucleophilic attack.
In haloalkanes, bond strength of C-X bond
decreases with Alkyl halides are polar in nature but are gene
moves from fluorine to insoluble in water due to their inability to fom
an increase in bond length
as one

with water or break the existing H-bonds.

iodine.
Halbatkanes
considerably
higher boiling 291
Ikanes because of greater points
loalkanes

than corresponding alkar ********----------1----------

Rate of reaction: ******
higher molecular weights.
polarity Rate of reaction:
3>2°1°> CH -CH> 1°>2°>3° halides
Gradatio in oiling points and densities of (fastest) (slowest) (fastest) (slowest)
nalides for a given alk group follows the order: ureater the
stability of| Less the steric hindrance in
RI> RBr> RCI>RF Carbocation, faster will be | transition state, faster wil
the reaction.
For a
given halogcn, the boiling points of alkyl
given be the reaction.
halides increases with the increase in size of the Tends to proceed with weak
-

Tends proceed with

to |
alkyl g r o u p . nucleophiles e.g., CH,OH, | strong nucleophiles, e.g.,|
<
CH,CH,CH,Xx H,O, CH,CHOH, etc. CH,0, CN", OH, etc.
CH X< CH,CH2X Configuration is retained Inversion of configuration
Chemical P r o p e r t i e s
but in front attack inversion | takes place (Walden
following reactions: takes place (racemisation
alkanes undergo | inversion).
Vincleophilic substitution reactions and inversion).
Elimination r e a c t i o n s
Because of the presence of a polarC-Xbond, alkyl halides
with metals are highly reactive compounds and undergo following
1 Reactions
Nucleophilic substitutions reactions substitution reactions:
aq. NaOH or KOHR-OH
Reactions : Substitut nucleophilic unimolecular Hydrolysis (alcohol)
abbreviated as Syl. Consider a general reaction: MoistAg,O
tions are OH
R-LG+Nu R-Nu+LG" (AgOH) (alcohol)
for nucleophile and LG stands for leaving alc. KCN R-CN (alkyl cyanide
here Nu stands
or alkyl nitrile)
AgCN(alc.) >R-NC(Alkyl isocyanide)
are-[R-LG]
first order kinetics. The rate of the alc. NH
The reaction follows >R- NH, (amine)
concentration ofthe nucleophile.
zarion is independent of the RX- sod. alkoxide (RONa
Tie Sl mechanism is a two step process, first one being R-0-R(¢ther)
Silver salt of fatty acid
ie slow and the rate determining step.
(RCOOAg) >R-COOR (ester)
slow + LG
Ssep-1c-LG A
Na in ether R - R orR- R'
Step-2 (Wurtz reaction) (higher alkanes)
fast C - N u + Nu-C* ether
+Nu Mg in dry RMgX
(Grignard's reagent)
stands for substitution
2 reactions: S2 mechanisma example
AgNO2 RNO,(Nitroalkane)
clcophilic bimolecular. Consider general KNO2 RONO (Alkyl nitrite)
R- Nu + LG
Nu +R-LG- no
E
N2 mechanism is a single step process w1 of optical isomerism
breaking take place Some basic concepts
ediate. Bond making and which rotate the plane of
Samultaneously with the reaction centre. Optical
rotation : Substances known as optically
either left or right are
+LG polarised light to optical rotation.
rotation is known as
> Nu-C active and the
ToNuL If the
substance rotates
the plane of polarised
it is
light
called
direction to the right)
(i.e.,
Differenc Between S1 and Sn2 Reactions: in clockwise and if the substance rotates
the
o r d- form
dextrorotatory direction (i.e. to
Bimolecular (Sy2) light in
anticlockwise
Unimolecular (Sy1) plane of polarized laevorotatory o r /-form.
'is first reaction. called
order reaction. second order the left), it is is
It is
rotation of optically active compound
an
The optical
enerally
ar
carried
carried out in Carried out in polar aprotic
solvents like acetone, expressed in
terms of specific
rotation.

observed rotation in degrees

protic
water, alcoholsolvents
like
and
acetonitrile, or Specific rotation =

length (dm)x conc.(g/mL)

acid. etic | DMSO,
DMF
isomerism:
The compounds
which differ only
Optical are called optical
akes place in two Takes place in
one step
behaviour
towards polarised light
step>
eps transition
state. in the 1s called optical isomerism.
phenomenon
irbocation as the throught and the
Aht-----ermediate--*****
.******---~--*******
isomers
 MtG Chapterwise WB JEE
EXPLOREDR
292
Alkyl halides show optical isomerism. Basic requirement Formation of Grignard reagent
for the presence of optical isomerism is; compounds These prepared in the laboratory by the action of
must have a chiral or asymmetric carbon atom i.e., a
are
metal in the presence of dry eth
alky
halides on magnesium er.
carbon atom should be linked to four different groups. RX+Mg RMgX
Ether
Mirror images must be non-superimposab!e (non-
superimposable miror images are said to be enantiomers). Applications of Grignard reagent
Formation of alkanes : Grignard reagent reacts wih
Lowest molecular mass, optically active haloalkane must
water, alcohols and amines (having active hydropem
contain four carbon atoms.
In the given structure, there is presence of
CH to form alkanes.

one chiral carbon atom which is linked to CHMgl + HOH CH4 +Mg(OH)I
four different groups. These are CH Mgl+ C2H,OH CH4+ Mg(OC;H:)I
CHs C2H6 t+ Mg(NHCH.
- H,-X, - C H and-CH2-CH. CHMgBr +C2H,NH2
Higher alkanes are formed by reacting with alu
CH CH halides.
H-C-C Cl-C-H CHMgl+CH,CH,Br CHCH;CH, +Mgl(Br
- Formation of alcohols:
C,H CHs CH
CH
(4) (B)
H-C=0+CH3Mgl->HC-OMgl 20 HCOH
A and B mirror images of each other or enantiomers
are

(enantiomers have same physical

and chemical H H
Ethyl alcohol (1)
Also they rotate the plane of polarized light
properties). Formaldehyde
directions.
H
upto the same extent, but in opposite H
CH C=0+CH,Mgl->CH,-C-OMgl 4,0
H
II. Elimination reactions
reaction in the presence
Acetaldehyde
CH
Alkyl halides undergo B-elimination CH-CH-OH
ofpotassium hydroxide in ethanol (high temperature) to yield
alkene by El or E2 mechanism according
to the structure CH,
Isopropyl alcohol (2)
of alkyl halides.
E1 mechanism: CH
R CH-C=O+CH,MgBr CH,-C-OMgBr
Rr aleKOH>R--CR"+X CH CH
R- Slow
H Acetone CH,
H CH¢OH
R CH
R--R" R-ç= -R"+H0 Tert- butyl alcohol (1)

H H -

Formation of aldehyde
Rate k [Alkyl halide]
H-CEN+CH3Mgl>CHj-= NMg|H,H0,
E2 mechanism:
H H H
-H aleKOH,R-c2c- CH-= 0+ NH+ Mg()OH
- Formation of ketones
Acetaldehyde

Transition state
OMgl
R-=c-H CH-C-Cl +CHMg >CH1-¢-C
H+HO+ KXJ
Rate k [Alkyl halide] [Base] CH3
CH-C=O +Mg()CI

1. Reaction with
metals CH3
bromides and iodides react Formation of acids:
Most of the organic chlorides,
with certain metals to compounds form containing carbon- OMgl
metal bond. Such compounds
are called organometallic CHMgl + =0->CH,-C=0H#9
compounds. CHC=0+MgI(OH)
halides are called Grignard reagents.
Organo magnesium These are highly reactive OH
Their general formula is R Mg X. Acetic acid
useful synthetic reagents
on a large scale.
and are extremely
Halbalkanes 293
nmental Effects of Some Important
n dE n v i r o n m e n t a l

Uses
and
Polyhalogen Compounds
Name Uses
Environmental effects
Dichloromcthane As a solvent inpaint remover. Have harmful effects on the human central
C H , C l , )
An cffectivc solvent for extraction in nervous system.
phamaceutical in the manufacturing of drugs Cause dizziness, nausea, tingling and
and food industries.
numbness in fingers and toes.
As a propellant in aerosols.
Direct contact with the eyes can even burn
As refrigerant and dewaxing agent. the cornea.
Trichloromethane or As an
important solvent particularly for fats, Breathing of chloroform for a short time
Chloroform (CHCI) alkaloids, iodine, waxes, rubber, etc. causes dizziness, fatigue and headache.
As an anaesthetic.
May damage liver and kidneys because
As a laboratory reagent. chloroform is metabolised to poisonous
Used in medicines. phosgene. The use of chloroform as an
anaesthetic has been replaced by less toxic
and safer anaesthetic such as ether.

In large quantities in the manufacture of

Tetrachloromethane
Exposure to carbon tetrachloride causes

refrigerants and propellants for aerosol cans.| liver cancer in humans.

(CC1)
As a feedstock in the synthesis of The most common effects are dizzriness.
chlorofluorocarbons. light headache, nausea and vomiting.
As a solvent for the manufacture of| which can cause permanent damage to
pharmaceuticals and oils, fats, waxes, etc. nerve cells.
When carbon tetrachloride is released into
air, it rises to the atmosphere and depletes
the ozone layer. Depletion of the ozone
layer increases the human exposure to
ultraviolet radiations which may lead to
increase skin cancer, eye diseases and
disorders and possible disruption of the
immune system.

4.Tri-iodomethane or As an antiseptic and this nature is due to free Causes nausea, vomiting.
iodine that it liberates and not due to iodoformn Irritating to mucous membranes and
lodoform (CHI,)
itself. respiratory system.
In the manufacture of pharmaceuticals. May cause eye irritation.
In stratosphere, îreons undergo
5. Freons (Chloroflouro As refrigerants in refrigerators
and air |
photochemical decomposition and initiate
carbons) conditioners.
radical chain reactions and deplete the
As propellants in aerosols
and foams (i.e, hair
ozone layer surrounding our
deodorants, cleansers, shavingcreams,| protective
earth. Therefore, the use of freons as
sprays,
and insecticides etc.). propellants and refrigerants has been
drastically discouraged. It has also been
banned in many countries.

ighly toxic towards fish.

D.D.T. is high
6.
DDT insecticide for killing |
(Dichloro diphen
enyl| It is widely used as an
D.D.T. is not biodegradable. Its residues
trichloroethane) mosquitoes and
other insects.
accumulate in environment and its long
term effects could be highly dangerous
CLCHC D.D.T. is not metabolised very rapidly by
animals rather it gets deposited and stored
in fatty tissues.
294 WutG Chapterwise WB JEE
EXPLORE

WB JEE WORKOUT
11. 1,3-Dichloropropane reacts with Zn and Nal and o
CATEGORY 1:Single Option CorrectType (1 Mark) (a) propane (b) propene gives
1 The IUPAC of the ()cyclopropane (d) n-propyl iodide.
name compound,
CH3 12. Haloforms are trihalogen derivatives of
(a) methane (b) ethane (c) propane (d) benzena
HC-C-Br is
13. Haloform reaction is used for preparing
CH-CH, Br (a) CH,Ch (b) CC (c) CHC (d) CHCL,
(a) 1,3-dibromo-3-methylbutane 14. The Grignard reagent, CH,CH,MgBr, can be used
(b) 3-methyl-1,2-dibromobutane prepare
(c) 3-methyl-1,3-dibromopropane (a) ethane (b) 3-methyl-3-pentano
(d) none of these. (d) all of these.
(c)propanoicacid
2. Amongst the C- X bond ( where X =
CI, Br, 1), the 15. Which is best reagent to accomplish the following
correct bond energy order is conversion?
(a) C-CI>C-Br>C-I CHCHBr CH,CH
(b) C-I>C-Cl>C-Br (a) conc. H,SO4 (b) Na
(c) C-Br>C-Cl> C-I
(c) conc. HCI (d) Mg, then H,0
(d) C-I >C-Br>C-Cl.
16. What is the major product of the following reaction?
3. The fire extinguisher 'pyrene' contains
(a) carbon dioxide b) carbon disulphide
(c) carbon tetrachloride (d) chloroform. CH3CH- H +CHMgBr
4. Which one of the following compounds when heated (a) 1-Butanol (b) Butanal
with KOH and a primary amine gives carbylamine test? (c) 2-Butanol (d) Butanone
(a) CHCl3 (b) CH3Cl 17. Ethylmagnesium iodide reacts with formaldehyde to
(c) CH,OH (d) CHCN
give a product which on acid hydrolysis forms
5. R-OH + HX>R -X+ H,O (a) an aldehyde (6) a primary alcohol
In the above reaction, the reactivity of different (d) a secondary alcohol.
(c)aketone
alcohols is 18. Which of the following compounds will react with
(a) tertiary> secondary > primary
methylmagnesium iodide followed by acid-hydrolysis
b) tertiary> secondary < primary
to give ethyl alcohol?
(c) tertiary < secondary> primary
(d) secondary < primary < tertiary.
(a) Ethylene (b) Acetaldehyde
(c) Formaldehyde (d) Acetone
6. Butane nitrile is formed by reaction of KCN with 19. Which of the following gives a tertiary alcohol when
(a) propyl alcohol (b) butyl chloride
treated with Grignard reagent?
(c) butyl alcohol d) propyl chloride.
7. The order of reactivities of the following alkyl halides
for a Sy2 reaction is
(a) H-Ë-H (b) CH-H
RF> RBr> RCI>RI
(a) RF>RCI> RBr> RI (b) None of these
(c) RCI>RBr> RF > RI (d) RI> RBr> RCI> RF (c) CH3-C-CH3 (d)
Which of the following are arranged in the decreasing
20. The progressive increase in the boiling pon
8. from CH3Cl (249 K), CH,Br (277 K) and CH
order of dipole moment?
(316 K) is due to
(a) CH,CI, CH,Br, CHF (6) CH,CI, CH,F, CH;Br (a) higher polarity of C-Ibond and higher molar
mass

(c) CH3Br, CH,CI, CH,F (d) CH,Br, CH,F, CH,CI (b) higher polarity of C-Ibond and lower molar mass
Haloform reaction cannot be used to prepare (C) lower polarity but higher molar mass in CH3l
9.
(a) CHF3 (b) CHCl3 (c) CHBr (d) CHl3 (d) lower polarity and lower molar mass in CH3
10. The order of polarity of CH3l, CH3Br and CH3CI 21. Which one is correct?
molecules follows the order (a) Freon-14 is CF4, Freon-13 is CFCl, Freon-1
(a) CH3Br> CH,Cl> CH3I CFClh and Freon-11 is CFCl3.
(b) CH3I >CH,Br> CH,CI (6) Freons are chlorofluorocarbons.
(c) CHCI>CH,Br>CH3I (c) Freons are used as refrigerants.
(d) CHCI> CH3l> CH,Br (d) All the above.
 295
HaloaManes
fire extinguisher, because
CATEGORY2: Single Option Correct Type (2 Marks)
fire
as aa
usedas
sed
1s
poimt
is low
.CCly boiling

a) its point is high 31. A compound is formed by substitution of two chlorine

nieltng

b) its i n c o m b u s t i b l e vapour
IOr two hydrogens in propane. The number of possible
t gives covalent bond. isomeric compounds is
forms
it (d) 2
d) the following reaction is (a) 4 b) 3 (c)
T h emajorproduc product P in
chloride 1s
H -CH=CH; PeroxideP 32. The best method to prepare neopentyl
a) CH,CH,CH,I
(b) CH3-CH-CH (a) (CH3), CCH,OH PCls, A
(6) (CH3), CCH,OH
e) CH-CH=CH, (d) CH2-ÇH2 C(CH3), CCH,OH0Ch. pyridine

Reaction
of ROH with R'MgX produces (d)(CH) cCH, A
4 (b) R'H (c) R- R (d) R'- R' treatment with
RH 33. n-Propylmagnesium bromide on
(a)
an lkane with chlorine and irradiating carbon dioxide and further hydrolysis gives
On mixing it forms only one (b) propanoic acid
with ultra-violet light, mono-
(a) acetic acid
chloroalkane. The
alkane is (c) butanoic acid (d) formic acid.

(a)propane (b) pentane 34. Identify (C) in the following sequence of reactions
(c) iso-pentane (d) neo-pentane.
The ease of dehydrohalogenation alkyl
alcoholic KOH is
of halide with
H-C-H (1)
2)CH Mgl (A)
H,O/H" NH3(C)
(B)HOHO
(b) 3°>2°> 1° (b) CHCHNH2
(a) 3 2 0 1 °
(a) CH,NH2
() 30<2°>1° (d) 3°>20°<1° (d) CH CN
(c) CH,CH,CI
reaction will (1) CO;
.Under identical conditions, the Syl occur
HBr(A)-* (B)
35. CHCH=CH2 peroxide );(C) is
most efficiently with (2) H,O/H
a) tert-butyl chloride (b) 1-chlorobutan (a) CHNH,CH,OH (b) CH3CHCH,COOH
c) 2-methyl-1-chloropropane (c) CHCH,CH,CHO (d) CH,CH3
) 2-chlorobutane. 36. Compound (4) reacts with ethylmagnesium bromide
3. In Syl (substitution nucleophilic unimolecular) to give a product which on hydrolysis yields 3-methyl-
Teaction, the racemisation takes place. It is due to 3-pentanol. What is the structural formula of (A)?
(a) inversion of configuration
b) retention of configuration
(c) both (a) and (b)
(a) CH,CH,CH,CHO (b) CH--CH,-CH,
d) neither (a) nor (b). (C) CH,--CH, (d) CHCH,CH,COOH
S. O
The C-Mg bond in CH,CH,MgBris
a) ionic (b) non-polar covalent 37. Compound (4), which is an alkyl cyanide reacts
c) polar covalent with ethylmagnesium iodide to give compound
M. (d) hydrogen.
Arrange the iven compounds in decreasing order of
(B). Compound (B) reacts with dilute HCI to yield
boiling points. 3-pentanone. What is the structural formula of (4)?
(a) CH,CN (b) CHCH,NH,
CH3
CH,CHCH,CH,Br I
CH3-C-Br
() CH,CH,CN (d) CH3NH
38. A ketone CgH1,0 (A) reacts with ethylmagnesium
CH3
II
bromide to give a product which on
hydrolysis yields
an alcohol (B). Compoun (B) undergoes dehydration
CH-CH,-CHBr
with concentrated sulphuric acid to form (C) which on
CH
a) 1> Ill> III ozonolysis gives 3-pentanone as the only product. (4) is
c) 1>I>II (b) II>I>III (a) 4-ethyl-3-hexanone (b) 3,4-diethyl-3-hexanol
(d) II> I>II (c) 4-ethyl-3-hexene (d) 3,4-diethyl-3-hexene.
 wtG Chapterwise WB JEE EXPLORE
296
39. Identify the final product (Y) in the following sequence
44. CHyl- DMSO M. The major product |X] is
of reactions :
H-C=CH )CH,CH,Mgbr (a) IMe,S=0 (b) [Me,SOMe]
(2)M,O
both (a) and (b) (d) none of these.
(a) CH,CH,CH,0H (b) CH,CH,CH,OH (c)
(c) CH-CH CHCH (d) None ofthese 45. What is the product X in the following reaction?
H
OH
Cl NaOH> ()
40. Identify the set of reagents Xand Yin the following set Br (1 eq)
oftransformations: D
CHCHCH,Br Product> CH;CHCH3 OH D
CI
Br
(a) X=dilute aqueous NaOH, 20°C;
(a) Xis
H D IH
Y=HBr/acetic acid, 20°C
X=conc. alcoholic NaOH, 80°C; OH
(b)
Y=HBr/acetic acid, 20°C (b) Xis H
(c) X=dilute aqueous NaOH, 20°C; D
Y=Bry/CHCl3, 0°C D

(d) X=conc. alcoholic NaOH, 80°C;

Y=Bry/CHCl3, 0°C (c) Xis H H
D 0 D
CH3
41. KNO2 OH
Br HO.CHOH
Product OH
() Xis H
The main product is

CH CH
enl ONO
l NO CATEGORY 3:One or More than One Option
(a) (b) H Correct Type (2 Marks)
CH3 46. Select the compound from the following which can
i l ONO minor
undergo Sn2, but cannot undergo (or gives
( H Enantiomer product), elimination on heating with CH,0
CH3 (a) 2-bromo-3-methylpentane
nl NO (b) 1-bromo-2,3-dimethylbutane
(d) (c) 1-bromo-2,2-dimethylbutane
Br +Enantiomer
(d) 3-bromo-2-methylpentane.
like
42. An unknown alkyl halide (A)
reacts with alcoholie 47. Sy2 reactions are much faster in the solvents
as the major
KOH to produce a hydrocarbon (CaHg) (a) dimethyl formamide
of the hydrocarbon affords (b) dimethyl sulphoxide
product. Ozonolysis
mole of
mole of propanaldehyde and one
one
(c)hexamethyl phosphoramide
formaldehyde. Suggest which organic compound (d) alcohol-water mixture.
is the correct structure of the
among the following 48. Which of the following statements are correct
above alkyl halide (A)?
(a) In general, for a nucleophilic reaction to
(a) CH,CHBrCH,CH3 must be significanu
Successful, the nucleophile
(b) CH,CHBr)CH(Br)CH
(c) CH3CH,CH,CH2Br
more stronger than the leaving group.
ky
atoms for
(d) Br(CH)4Br (6) The replacement of hydrogen
in
fluoroethane is groups hinders the approach ofnucleophile S
43. The best method to prepare (c) Atertiary halide is mostreactive in an Sy2 rec
HF/SbF's
(a) CH,OH HFH,50 b) CHOH observed in Sv
(d) Rearrangements are generally
HggF/a Fn
(d) CH mechanism.
(c) CaH,Cl-
Haloalkanes
291
53. In the reaction given below:
B r +

NaCECCH,
N a c = C C H

CH,CH2 CHCH3 H, +CI

m a j o r ,products formed are
reaction
the CH -CI +ÖH HO-C
Inthis (b) cyclolhexene
H
(a) prop
lohexylpropyne (d) 2-cyelohexylpropane.
used as refrigerant is Which of the following statements is/are incorrect?
mechanism hence
(a) The reaction proceeds via S2
compounds

M . The.

(b) Cl () CF4 (d) CFCl2

CH inversion of configuration takes place.
(a) following is not correct?
mechanism hence
Which o fthe
reaction involves transition state and (6) The reaction proceeds via Syl
inversion of configuration takes place.
(a) Sy aprotic solvents.
Completed polar
in
mechanism hence
is stereoselective as well as (c) The reaction proceeds via S2
b) Sy2 reaction
their is no change in the configuration.
mechanism hence
(d) The reaction proceeds via Sl
stereospecific.

Walden
inversion occurs in Syl reaction.
there is no change in the configuration.
(c) benzyi halides show low reactivityy alkene by
and alkane
) Allylic 54. Alkyl halide can be obtained from or
towards Syl reaction.
reaction with
will be correct about the major (a) HBr (b) HCI (c) PCls (d) Cl2
Which observation(s) reaction?
Yof the following form stable
nroducts Xand
a
55. Which of the following compounds
RD Grignard reagent?
SOLl Y alc. KOH/A y
peroxide (a) CH3-CH2-0-CH2-Br Et20
DD CHBr
DC D Mg/CO
(b) Xis Et,o
(a)Xis
D D
N(CH3)
DD
Br
DD Br

(c)
Mg/Et, d) MgEtO
c)Yis (d) Yis
COOH Me

HINTS &SOLUTIONS
cannot be used to
reaction. Hence, haloform reaction,
CH prepare CHF3.
L (a): CH,-c- H,-CH,-Br follows the order
10. (c):Since the electronegativity
Br
CI> Br>1, the order polarity CH,CI> CH,Br> CHI.
ofis

-dibromo-3-methylbutane 11. ():

u CH,C+ Znal,
CHCH.CI a A
3. () CH,
a:CHClh gives carbylamine test.
CH +ZnCl,
. CH
a:Tertiary> Secondary> Primary. Cyclopropane
d): 12. (a)
CHCH,CH, CI -KCINCH,CH,CH2CN Butane nitrile 13. (d): Haloform reaction is used to prepare CHCl
1.
(d): Weaker the
,RI> RBr > C-Xbond, greater is the reactivity, 14. (d)
RCl> RF 15. (d): CH,CH, Br+Mg> CH3CH,MgBr
.6):CHCI>
a:F2 Feacts CHF> CH3Br
OF) and not hypofluorite ion (OF) needed for haloform
with NaOH to give oxygen difluoride CH,CHt+ Mg(OH)Br
 298 wutG Chapterwise WB JEE
EXPLORER
16. (c) 17. (b) 18. (c) CHCH,CH,CH,Br> CH,CH-ÇH-Br>
CH
19. (c): CH-C-CH, + CHMgBr CH3
QH OMgBr CH-C-Br
H,O
CH-C-CH; MOBr CH-C-CH CH3
CH CH 31. (c): The compound is C3H,Ch and the number of
3° alcohol
20. (c): The boiling point of CH,I is higher than CH,CI possible isomeric compounds is 5.
asymmetric
and CH,Brbecause of higher molar mass yet it has low
H HH Carbonatom
polarity ofC - I bond. H H H

21. (d) -f--H H-C

22. (c): CC; is used as fire extinguishers under the name
form protective layer on the CI H C
pyrene. The dense vapours (d,Ipair)
come in
burning objects and prevent the oxygen or air to
and gives incombustible H CI H
contact with the burning objects H H H
vapours.
23. (b): The product formation will be accordingly
effect is
Cl-C-C-C-H
H---H
to Markownikoff's rule, because peroxide CI H H H CI H
HBr and not effective in
effective only in the case of
case of H. CH3 PCl5, A
OR
R 32. (d): CH-C-CH,OH or HCl, A or
+RH
24. b): Mg HOR Mg SOClh/pyridine
25. (d): In chlorination of alkanes, hydrogen is replaced
CH3
by chlorine. In neo-pentane, only one type of hydrogen is CH3 CH3
present, hence replacement of any hydrogen atom will give
CH-C-CH2>CH,-C-CH,CH
the same product.
CH3 CHC CH
HC-C-CH, H,C--CH
CH3
CH CH3
CH3-C-CH,CH
26. (b): The ease
of dehydrohalogenation of alkyl halide
with alcoholic KOH is 3°>2°> 1°. Cl
This order of alkyl halides can be explained on the basis of
the stability of the alkene formed after dehydrohalogenation On the other hand, free radicals do not undergo
of haloalkanes. 3° alkyl halides on dehydrohalogenation rearrangement and hence the best method of preparation is
form more being more stable are
substituted alkenes, which
through free radical chlorination.
least
formed at faster rate, while primary alkyl halides yield
substituted alkenes, which being less stable are formed
as H3 CH
slower rate. CH-C-CH C,/0CH,-C-CH,C1
chloride will form 3° carbocation
27. (a): As tert-butyl CH CH3
which is most stable thus Syl reaction will occur most

efficiently with tert-butyl chloride. 33. (c)

28. (c):n Syl mechanism, the intermediate formed is 34. (b): (A) = CH,CH2OH; Ethyl alcohol
carbocation. As carbocation is planar thus the incoming
side. Thus, both retention (B)= CH,CH,CI; Ethyl chloride
nucleophile can attack from either
racemisation.
and inversion takes place resulting in (C) CHCHNH,; Ethylamine
29. (c): C Mg bond is covalent but polar. 35. (b): (4) = CH,CH,CH,Br; n-Propyl bromide
of van der Waals' forces goes on
30. (a):Magnitude (8)-CH,CH,CH,MgBr, »-Propylmagnesitmbron
mide

decreasing with branching. Hence, the order of boiling

points is (C)=CH,CH,CH,COOH; Butanoie acid
Haloarkanes 299
Methylation of DMSO with Mel gives two isomers
%. (b):C H ; -
-CH-CH +CH,MgBr (4) and (B) from S-methylation and from O-methylation.
(A) respectivcly.
Since 'S' is better nucleophilic sitc (because O ' is more
CHs clectronegative than "S'), (A) is the major product.
- -CH-CH, H0
>CH-Ç-CH2CH3
ÓMgBr OH
MeS IMeS=O]r
CH,CH;-CEN
N+CHsMgl- (4) (major product)
o ) :C H

(4) >[Me^S- OMe] I

CHCH:CCH;CH3 Ho CHCH-C= NMgl

SCH T () (minor
produc
45. (b): Br is better leaving group than Cl hence (X)
C2Hs
(B)
OH H
formed by S2 mechanism involving
(a): (4) =
4-Ethyl-3-hexanone
ECI is
(B)=3, 4-Diethy1-3-hexanol H
D D
3,4-Diethyl-3-hexene

replacement of Br by OH.
9 )
H,SO,/HgsO0
H,O CH,-C-H CH,CH,MgBr
46. (b, c) :
CH-CHCH-CH,Br
(2) H,O/H CH, H,
Acetaldehyde
Acerylene OH primary alkyl halide has more tendency to undergo substitution)
CH
CH-CH-CH,CH,
2-Butanol CH-CH--CHBr
.CH.CHCH,Br NaOH(Alc)60C) CH,CH = CH CH
() (primary alkyl halide without o-hydrogen)
HBr 47. (a, b, c) : SN2 is always favourable in polar aprotic
CH-CH-CH3 CH, COOH (20°C) solvents.
Br ()
48. (a, b)

CH3
CH
H KNO H 49. (a, b): Br+ Na-C=C-CH
4.(9: Br H0,C,H,OH +CH-C=C -H
SNl condition

50. (d)
CH3
ONO 51. (a, c, d): Syl reaction involves carbocation as the
+Enantiomer N=O intermediate. Walden inversion occurs in Sn2 reaction.
H
D D C
42. (C) sO,Cl/peroxide
52. (b, d):
free radical substirution
LHCH,CHCH,Br alc. KOH CH,CH,CH= CH2 D D DD
(A) CH
racemic mixture
CH,CHC=O0 +0=CH2 Os
Zn/H,O D D
Formaldehyde alc. KOH
Ftopanaldehyde
3. (e
: Fluoro derivatives of alkanes can be obtained by D D
alogen exchange method using inorganic fluorides such as
(h
F, AgF,
HgF2, etc. 53. (b, c, d) : Inversion of configuration takes place in Sy2

4. (a): DM
HCI+ Hg,F2 2C,HsF + HgC2 mechanism.

MSO (Me,a=ö) is an ambident nucleophile. 54. (a, b, d): PCl, is used to prepare alkyl halides from
alcohols.
cleophiles
wo) suitableble atoms
which have more than one (generally 55. (a, b, d): The presence of acidic hydrogen in
strate are through which they can attack the COOH group destabilises Grignard reagent.
known asas ambident nucleophiles.
 wutG Chapterwise WB JEE EXPLORER
300

8 WB JEE Previous Years Questions

2013-2020

CATEGORY 1: Single Option Correct Type (1 Mark)

(d) CHCH-CH-CH,CH,CH,
(2016)
NH, NH2
CH3
HBr (1 equiv.), 5. The number of possible organobromine compounds
1. CH which can be obtained in the allylic bromination of
with N-bromosuccinimide is
HC 1-butene

The major product of the above reaction is (a) 1 b)

(c) 3 (d) 4 (2018)
CH3 H3C
a. OH
(a) CH3 b) 6. CH-0-CH,-Cl
H,C Br
CH-0-CH-OH
H3C Which information below regarding
this reaction is

(c) CH2 d) applicable??

H3C Br Sy2 pathway, because it is a primary
HC Br
(2015)
(a) It follows
alkyl chloride.
reaction: It follows Syl pathway, because the intemediate
2. Inthe following (b)
carbocation is resonance stabilized.
RMgBr+HC(OEt)Enher p followed, because the
(c) Syl pathway is not
The product P' is intermediate carbocation is destabilised by-effect
(a) RCHO b) RCHOEt of oxygen.
(c) RCH (d) RCH(OEt)h (2015) (d) Amixed Syl and S2 pathway is followed.
The total number of alkyl bromides (including (2020)
3.
stereoisomers) formed in the reaction
MeC CH=CH2 +HBr will be CATEGORY 3:One or More than One Option
(b) 2 Correct Type (2 Marks)
(a)
(d) no bromide forms.
c) 3 7. The major product(s) obtained in the following
(2020)
reaction is/are

CATEGORY 2:Single Option Correct Type (2 Marks) HCccHs

of reactions:
Br2
4. Identify Xin the following sequence
1. NaNH,
X CH CH3
CH-CH-CH-CHCH 2. Na in liquid NH H- Br
Br Br H Br
-Br
(6) Br-
(a) CH-CH-CH-CH,CH,CH,
CHs C,H
Br NH2
CH H CH CH3
(b) C=C Br. -H Br- -H
H CHCH,CH -Br (d) Br -H

CH,CH,CH CGHs
C=CH
(c)
C,Hs
H
(2018)
Haoakanes 301

L HINTS& SOLUTIONS J
CH3
CH,/ HBr
(1 eq) HC- NBr
H,C NB

b )HCC
:

CH3 CH-CH-CH=CH
CH3 /B HC C CH,Br

H CH- H-CH=CH, > CH-CH=CH-CH2

GOBt
( a ) : RMgBr + H - C - O E t .
OEt
CH-CH-CH=CH2
OEt OEt
Br
AHO +2EtOH R-CH +Mg Chiral centre so, dextro
OEt Br and laevo isomers are possible.

CH3 CH CH3 CHBr

) : CH-C-CH==CH,+H-Br
H
c=c CHBr H
CH3 trans- cis
CH3 CH3 Total 4 isomers are possible.
--CHCH + CH, - -CH--CH2
Br CH3 H CH3 Br H H
Major product) (R+S) Minor 6. (d):CH,- -CI+OH
b): H
CH-CH-CH-CH2-CHa-CH3
Br r OCH3 OCH3
NaNH2, E path (NaNH, will perform HO - -C CI HO- H +CI
double dehydrohalogenation)

CH-CC-CH,-CH,-CH3 Substitution nucleophilic bimolecular (Sy2)

Na/liq. NH, (Birch reduction;
anti-addition of H2)

H C = cCHH , C H , C H 3
Also,CH--CH,-
Stabilised by resonance (Sy1)
d): NBS allylic
(N-Bromosuccinimide) does addition is anti-addition.
on
through free radical substitution. 7. (a, d): Br,