Haloalkanes 393

The IUPAC nomenclature of haloalkanes is similar to

substituent. A few examples (for alkanes and in doing so halogen
as a details see Chapter 2) of is treated
follows: haloalkane nomenclature are as
CH,
CH,CH,CH,CH,Br
CH, CH,
CH,CHCHCH,CH, CH,-CCH,C-CH,
CHAPTER Br
Br C
1-Bromnobutane

Haloalkanes 2-Bromo-3-methylpentane 2-Bromo-4-chloro-2,4-dimethylpentane

11.1.1 Physical Properties

(a) Bond strength: In haloalkanes bond strength of
carbon-halogen bond
increase in bond length, as one moves from fluorine to iodine. This isdecreases with' an
attributed to the
size of p orbital, which increases from fluorine to iodine and thus makes the
less effective.
sp-p overlap

11.1 INTRODUCTION
Increasing size of halogen p orbital and C-X bond length

Haloalkanes are the halogen derivatives of hydrocarbons with general formula

C,H)X and are
In haloalkanes the carbon B
represented as R-X where R is an alkyl group and X represents halogen.
to carbon, halogens
halogen bond is formed by sp-p overlap. Being more electronegative compared and carbon acquires
impart a polar character to C-X bond where halogens acquire a negative charge Increasing C-X bond strength
a positive charge.
(b) Solubility and state of matter: Haloalkanes are insoluble in water but soluble in organic
solvents. Lower molecular mass haloalkanes are gases at room temperature while higher
sp°p overlap X (halogen) being molecular mass haloalkanes are liquid at room temperature.
more electronegative
withdraws the bond
(C) Boiling point: Boiling point of haloalkanes is higher compared to coresponding alkanes
pair towards itself due to dipole-dipole interaction. With an increase in molecular mass, there is an increase
C in boiling point. For the same alkyl group the boiling point increases from fluoroalkane
(R-F) to iodoalkane (R-).lodine has a larger surface area and outer electrons are loosely
bound. This makes iodine a highly polarizable atom. A polarizable atom has increased
Dipole (polarity) in C-X bond
so hybrid orbitals Dorbital of halogen London forces of attraction (section 1.7), which causes an increase in boiling point. The
of carbon branched chain haloalkanes follow a similar increasing order of boiling points.
(d) Density: The densities of haloalkanes increase with atomic mass of the halogen and
(CHI)
Depending upon the number of carbons attached to the carbon to which halogen is attacned decrease with increasing size of the alkyl group. In monohaloalkanes, iodomethane
haloalkanes may be classified as primary (1), secondary (2), or tertiary (3). has the maximum density.

Increasing trend of boiling points°C and densities g/mL (in bracket) of haloalkanes
with
R
respect to methane
R-CH,-X R-CH--X R-C-X CH,F CH,CI CH,Br CH,I
CH,
-78 -23.7 4.6 42
R R -161.4
1° Haloalkane (0.920) (1.732) (2.279)
2° Haloalkane 3° Haloalkane
C,H,CI C,H,Br C,H,I
12 38 73
(1.46) (1.94)
392
394 Organic Chemistry
Haloalkanes 395
CH,CH=ECH, + HCI
chain
CH,CHCH,
Increasing trend of boiling points°C and densities g/nL (in bracket) of branched CI
Propene
haloalkanes (2° haloalkanes) 2-Chloropropane
(CH;),CHF (CH),CHCI (CH),CHBr (CH,),CHI CH,CH,CHCH,
HBr Why no peroxide
-11 35 CH,CH,CH =CH, Br effect is observed
(0.863) (1.425) (1.703) 2-Bromobutane in case of HE, HCI,
and HI?
Peroxide (see p. 237)
The general characteristics of monohaloalkanes can be enumerated as follows:
CH,CH,CH,CH,--Br
-Bromobutane
(a) Fluoro and chloroalkanes are less dense than water whereas bromo and iodoalkanes are
denser than water. From alcohols

(b) Monofluroalkanes are unstable and on heating. H-F is eliminated to produce alkene.
(c) Bromo and iodoalkanes are generally photosensitive and are stored in brown opaque () By reaction with HCI or HBr
bottles. Otherwise, they liberate free bromine and iodine respectively. Reaction of alcohol with HCI in presence of anhydrous ZnCl, results in the formation of a
chloroalkane. The reagent, HCI combined with anhydrous ZnCL, is known as Lucas Reagent and
PREPARATION OF HALOALKANES is used for distinguishing l°, 2° & 3°alcohols. Similarly, reaction of alcohols with HBr in presence
11.2 of H,SO. results in the formation of bromoalkanes. For example,
The various methods used for the preparation of haloalkanes are discussed in this section. Some of
the methods for the preparation of haloalkanes are already discussed in details in other chapters as
the chemical properties of other functional groups from which these are derived. Such reactions are CH,CH,OH HBr/H,S04, CH,CH,Br + H,0
indicated at appropriate places in the present text. Ethanol Bromoethane The mechanism
is discussed in
Lucas reagent
By halogenation of alkanes CH,-CH-OH CH,-CH-CI + H,0 details in
Section 13.3.3
Direct Halogenation of alkanes in presence of light (photohalogenation), heat (thermal
CH, CH,
halogenation), or catalyst (catalytichalogenation) results in formation of haloalkanes. The reaction
proceeds through a free radical substitution mechanism. For example, Propan-2-ol 2-Chloropropane
Order of reactivity of alcohols is 3°> 2° > 1°
CH, + CI, CH,CI HCI Various aspects of
CH,CHCH, + HBr halogenation are () By reaction with phosphorous halides and thionyl chloride
CH,CH,CH, + Br, discussed thoroughly
in Section 5A.3.1)
Phosphorous pentachloride (PCIs), phosphorous trichloride (PCl,), and thionyl chloride (sOCl,)
B react with alcohols to produce corresponding chloroalkanes. The reaction of alcohols with thionyl
chloride to produce chloroalkanes is one of the best methods for its preparation since both the
be
Note: byproducts SO, and HCI being in the gaseous state, can be removed easily. The reactions can
(i) Order of reactivity of halogens is F> Cl> Br >I represented as follows:
(ii) Ease of abstraction of hydrogen from alkanes by halogens is 3° > 2> 1°
With SOCl,, reaction
By addition of hydrogen halides to alkenes
3CH,CH,OH + PCI, 3CH,CH,CI + HPO, follows the Si
CH,CH,0H + PCl, CH,CH,CI + POCI, + HCI mechanism (see details
The electrophilic addition of hydrogen halides to alkenes (Section 6.3.2; p. 225) results in the
formation of haloalkanes. The reaction is regioselective and follows Markovnikov's rule. CH,CH,CH,CI + So,f + HCIT
in Section 13.3.3:
p. 466)
CH,CH,CH,OH + SC1,
The addition of HBr in presence of a peroxide results in the formation of 2° haloalkane
(Kharasch effect). A free radical mechanism is followed in presence of peroxide. For example,
 Haloalkanes 397
396 Organic Chemistry
of alcohols with (b) Formation of flouroalkanes
iodoalkanes are prepared
conveniently by reaction phosphorous
The bromo and phosphorous triiodide (Pl) respectively. The flouroalkanes are prepared by reaction of chloroalkanes with inorganic fluorides. For example.
tribromide (PBr;) and
PBr, and Pl, are produced
3CH,CH,Br + H,PO, in-situ by reaction of 2CH,CI + Hg,F, ’ 2CH,F + Hg,CI,
3CH,CH,OH + PBr; phosphorous and halogen Chloromethane Fluoromethane
3CH,CH,I + H,PO, 2P +3X, ’ 2PX,
3CH,CH,OH + PI, (X= Br or I)
41.3 CHEMICAL PROPERTIES OF HALOALKANES
acids (Hunsdiecker reaction)
From silver salts of carboxylic carbon tetrachloride recsle:
The reactions undergone by haloalkanes can be broadly classified into two categories:
undergoes reaction with bromine in
The silver salt of carboxylic acid of carbon dioxide. The reaction is known (1) Substitution reactions, which involve reaction occuring at sp carbon directly attached to
evolution
in thà formation of bromoalkane and halogen.
Hunsdiecker reaction. For example, (2) Elimination reactions, which involve the Bcarbons of haloalkanes.
’ CH,Br + CO, + AgBr Bromoalkane obtained Besides this, the haloalkanes are key reactants for synthesis of various organometallic compounds
CH,-Ç-04a + Br, has one carbon less and are also used in the well-known Friedel-Crafts alkylation reaction. Some of the chemical
than the reactant acid
reactions of haloalkanes are discussed in details in other chapters at appropriate places and their
Silver ethanoate reference has been given in the present chapter. The various chemical reactions of haloalkanes are
discussed as follows:
mechanism as follows
Mechanism. The reaction follows a free radical

CH,COOBr + AgBr 11.3.1 General Discussion

Nucleophilic Substitution Reactions: A
CH,COOAg + Br,
CH,COOBr CH,COO + Br (Chain initiation) In haloalkanes, the C-X bond is polarized due to high electronegativity of halogen, and thus carbon
is electrophilic nature (electron deficient). The substitution reactions in haloalkanes involve the
CH,COO CH, + Co, replacement of halogen (leaving group) by a nucleophile (entering group) and are called
CH, + CH,COOBr CH,Br + CH,COO (Chain propagation) nucleophilic substitution reactions. Halide ions being weak bases are easily displaced by
nucleophiles and thus behave as good leaving groups. The general reaction is expressed as
From other haloalkanes
R-X :Nu R-Nu
(a) Formation of iodoalkanes
Haloalkane Nucleophile Nucleophilic Leaving
(Ö) Finkelstein reaction. The chloroalkanes or bromoalkanes on treatment with sodium (Substrate) (Entering substituted group
iodide (or potassium iodide) in presence of acetone results in the formation of iodoalkane. group) product (Halide ion)
The replacement of chloride and bromide occurs readily by iodide ions (halide exchange
reaction) because sodium iodide (or KI) is soluble in acetone. NaCl and NaBr are insoluble Depending upon the nature of substrate and to some extent on the nature of reagent and reaction
in acetone and get precipitated during the course of reaction. This reaction is known as conditions, the course of the nucleophilic substitution reactions may follow the following
Finkelstein reaction. mechanisms (or pathways):
(ii) via Grignard reagent. Grignard reagent on reaction with iodine results in the formation (a) Substitution Nucleophilic Unimolecular mechanism (referred as Syl mechanism)
of iodoalkane.
(b) Substitution Nucleophilic Bimolecular mechanism (referred as Sy2 mechanism)
Finkelstein reaction
EXPLORE MORE
(c) Substitution Nucleophilic Internal mechanism (referred as Syi mechanism)
(The Syi mechanism is discussed thoroughly in section 13.3.3; p. 466)
Acetone
CHCH,CI + Nal -NaCI
..through solved
Acetone
CH,CH,I problem 73 Substitution nucleophilic unimolecular (SN1) mechanism
CH,CH,Br + Nal
-NaBr The nucleophilic substitution a unimolecular mechanism, that is, the Syl mechanism is favoured
Grignard reaction In presence of weak bases or poor nucleophiles. Also, it occurs in two steps. The first step involves
CH,CH,Br Mg/Dryether
CH,CH,--MgBr
 Haloalkanes 399
398 Organic Chemistry
formation of In other words, Syl mechanism involves retention as well as inversion of configuration. This
results in removal of halide ion and
heterolytic cleavage of C-X bond that carbocation to form the can be illustrated as follows:
carbocation. The second step involves the
attack of nucleophile (Nu) on
substituted product as R

+ INu
’ R,C* ’ RC-Nu
RC-X
Carbocation R, R,
Rear side attack of Front side attack of EXPLORE MORE
follows.
The reaction mechanism can be explained as nucleophile (Nu) Nu nucleophile (Nu) SET - I

Heterolytic cleavage in C-X bond: formation of carbocation .through solved

Step 1.
heterolytic cleavage results in the formation
problem 72
In haloalkanes the C-X carbon is sp hybridized and can be depicted in the reaction form
of carbocation (sp² hybridized), a reactive intermediate. This
as shown.
R R
sp (Tetrahedral) sp (Planar)
R R

Nu Nu
R R
Slow step X
Rate determining step R,
(Inversion of configuration) (Retention of configuration)
R R R
R

under.
Some important points to be kept in mind as regards the formation of carbocation are as
mechanism!
(a) The formation of carbocation is a slow step and is thus the rate determining step. The Complete racemization is not observed during Sy1
if obtained in equal ratio result
rate of reaction is dependent only on the concentration of haloalkanes. Once a carbocation The prodscts with retention and inversion of configuration
in the formation of racemic mixture. However SNl mechanism, the product with inversion
is formed the attack of Nu in next step is very fast. (~60%) conpared to the product with
(b) Since only one molecule interacts in rate determining step and rate is dependent on the of configuration is obtained. in a higher amount
concentration of haloalkanes only the reaction is unimolecular and follows the first order retention of configuration (~40%).
the vicinity of carbocation in the first
kinetics. (That's why it is called Sy1) The leaving group ie. halide ion, X, does not leave
somewhat shielded fron the front. As no such
(c) The ease of formation of carbocation is directly related to the stability of carbocation. step. As a result, the attack of nucleophile is
the back, the product with inversion
This means that a more stable carbocation is formed more readily. The order of stability shielding is there when attack of nucleophile occurs from
percentage.
of carbocations is 3° > 2° > 1° > CH,. Hence, the order of reactivity of haloalkanes is of configuration is formed more readily and in arelatively higher
3° > 2° >'> CH;X. Front side attack of nucleophile is
(d) Since reaction proceeds through the formation of carbocation, there apossibility of Rear side (Back side)
hinde (shielded) due to the
rearrangement to form a more stable carbocation. presence of leaving group X
attack of nucleophile
is not hindered in the vicinity
R
Step 2. Attack of nucleophile (Nu) on carbocation (fast step) R

The carbocation (sp hybridized) has a planar structure and can undergo attack by nucleophile from S lreaction
front side or rear side resulting in the formation of a substituted product.
An attack from the front results in the formation of product having 'same configuration
(retention of configuration) as that of the initial haloalkane while a nucleophile attack from the Ri R
R
back results in the formation of a product having a configuration opposite (inversion of R
cÑnfiguration) to that of haloalkane.
400 Organic Chemistry
Haloalkanes 401
aqucous NaOH as depicted here summarizes
The hydrolysis of S-3-bronmo-3-methylhexane with (ii) A bulkier group attached
to carbon, makes the transition
interactions. More the number of bulkier state unstable
alkvl groups. more is the stericbecause of steric
the overall reaction mechanism:
lesser will be the stability of transition hinderance and
state. This in turn causes a slow rate
CH, CH, CH, Hence, the order of reactivity of of reaction.
CH, The effect of substituents on the stability haloalkanes is CHX >1> 2>3
of transition state in SN2 reaction is
Slow step OH illustrated as follows:
fast
Br
OH HO
Br C,H, CH C,H, C,H,
C,H, C,H,
C,H, C,H,
Retention of Inversion of Nu X
[Planar)
configuration configuration
Nu--
S-3-methylhexan-3-ol R-3-methylhexan-3-o1 H H
s.3-bromo-3-nethylhexanc R,
1° Haloalkane 3 Haloalkane

Substitution nucleophilic bimolecular (SN2) mechanism Characteristics:

of haloalkane and
In abimolecular mechanism, the attack of nucleophile on sp hybridized carbon
. Least steric hinderance " Maximum steric hinderance due to presence of
nucleophile
step. Thus,
removal of halide ion, (the leaving group) occurs simultaneously in a single bulkier alkyl groups
group).
has no other option but to attack the carbon fromn aside opposite to halogen (leavingnucleophile Low energy in transition state " High energy in transition state
The reaction proceeds through a ransition stage, which has partial attachment of More stable transition state " Less stable transition state

(Nu) and partial detachment of leaving group X

Effect of solvent on SN1 and SN2 reactions
The Sy2 mechanism is stereospecific in nature and always leads to the formation of substituted
words, S2
product with a configuration opposite to that of the initial haloalkane. In other The solvent plays an important role in the substitution reactions and polar solvents have a marked
mechanism involves the inversion of configuration (also known as Walden Inversion). This has been effect on the rate of SNl and S2 reactions.
illustrated by considering the example of the hydrolysis of 2-bromobutane in aqueous alkali as (Recall: A polar solvent where hydrogen is attached to oxygen or nitrogen is termed as polar protic
follows: solvent whereas a polar solvent where hydrogen is not attached to oxygen or nitrogen is termed as
polar aprotic solvent. For example Water, alcohol, and formic acid are polar protic whereas acetone,
DMSO, and DMF are polar aprotic solvents. (Section 4.7)}
CH, CH, CH, Sy1 reactions
In Sy1 reactions, the reactants in rate determining step involve neutral molecule that is haloalkane.
Rear side attack OH
Apolar solvent stabilizes the intermediate carbocation and thus enhances the rate of SNl reactions.
HO -Br
(less hindered) Front side -B
A polar protic solvent stabilizes not only the carbocation but also the leaving group. that is, halide
attack is HO
H
B
hindered
H CH, H ion which further enhances the SNl reactions.
C,H, C,H,
R-2-bronobutane Trans1tion state S-butan-2-ol S+ S+
(Inversion of configuration] H
Protic polar solvents
(like water) stabilize
The rate determining step involves interaction of two species, namely the haloalkane and the the carbocation through
Slow step solvation and thereby
nucleophile. Thus, rate is dependent upon the concentration of these two species and is referred as -X increase the rate
bimolecular reaction, which follows second order kinetics. It is to be noted that the mechanism is of Sl reaction
favoured in presence of strong nucleophiles., Carbocation
The order of reactivity of haloalkanes is dependent on the stability of transition state. The
stability of the transition state is governed by the following factors:
(i) A primary haloalkane preferably undergoes SN2 mechanism because of least steric
Sy 2 reactions
hindrance in the transition state, which makes the transition state more stable. In SE2 mechanism, the reactants in rate determining step involve both haloalkane and nucleophile
(a charged species). A polar protic solvent interacts with nucleophile and solvolyzes it. As solvent
402 Organic Chemistry

electrophilic carbon gets hindered anaas Haloalkanes 403

molecules surround the nucleophile, its approach towards CH,
rate of nucleophilic substitution reaction
decreases NH, ÖH
polar protic solvents and to increosas
Thus, the SN2 reaction rate is decrcascd in presence of Across a period
rate of S,2 reactions. polar aprotic solvents are uscd. Polar aprotic solvents provide a polar mediuns nucleoph1licity
towards clectrophilic carbon, as thev d Increasing nucleophilicity increases with
but do not inhibit the approach of nucleophile increasing basicity
solvolyze the nucleophile'
(ii) The polarizability of
considered when the attacking nucleophilic atom is another important factor,
nucleophilic which is
difference in their size. For example, atoms
R R are different and also have an
in a set of nucleophiles appreciable
of periodic table (as in case of belonging to the same group
halide ions). the larger the size of nucleophilic
The molccules of protic higher will be its nucleophilicity. A atom the
charge by delocalization and is more larger
polar solvent suround
atom can easily accommodate the
N thc nucleophile and inhibit
stable and thus more nucleophilic. However.negative
the approach of nuclcophile nature of solvents greatly affects the the
nucleophilicity.
nucleophilicity is expressed in the following examples. the relative variation in the
towards substrate therehby
R decrcasing the rate of S2
rcaction.
R
SH OH
Moving down the
QEXPLORE MORE
SH CH,SH CH,OH group. nucleophihcity
R Increases wIth increasing
...through solved SIZe
problemns 79,80,87 Br > C1 > F

More about nucieophiles EXPLORE MORE

As discussed earlier in Chapter 4. a nucleophile is defined as an electron rich species that may be .through solved
negatively charged or neutral. The nucieophilicity is the ability of the nucleophile to form
bond problems 78

with an clectrophilic (electron deficient) centre. The nucleophilicity is dependent on following

factors. In presence of polar protic soBvent, the rate of S2 reaction for halide ions as
nucleophiles follows the order I- > Br> Ct>F. However in aprotic solvents, the order
(i) The availabil1ty of electron density on an atom through which nucleophilic attack occurs is just reverse.
Higher the electron density. higher is the nucleophilicity. Thus, the negatively charged In case of halide ions, the fluoride (F) ions are solvolyzed to the maximum extent. The
1Ons are more nucieophilic compared to coresponding neutral species. For example, the fluorine has a small size and has maximum eleetron density. Thus, its interaction with polar
nucleophilhcity of some charged ions is mathematically quantified as follows. protic solv ents is strong and it forns strong bonding with solvent molecules The cage of
solvent nolecules around F inhibits its approacl as a nucleophile to carbon of the suhstrate.
Thus, the reaction is slow wih F in th presence of polar protic solvents.
OH HOH NH, > NH, On the other hand, Ehas alurger size and electron density is spreaded over alarge area (ie
OCH, > CH OH less elecron densily). So it does not form a strong bonding with polar protc solvent molecules
and is surrounded by le'sser solvent molecules compared to F. Thus. the approach of l as
SH HSH Ö-C-CH, HO -CH, a nucleophile is much easiei d nuleopbilic substitution occurs faster in presence ot I

Increasing order of nucleophilicity in proti solvents

In a similar manner, if attacking nucleophilic atoms are different but of almost same
Size (that is they belong lo the same period of the periodic table), a strong base behaves
as a strong nucleophile
In other words, if atlacking nucleophilic atons are either same or have a similar size, Increasing order of mceophiticity n aprotic sotents
the nucleophilcity is directly related to basicity of the species. That is,
notsolvalized as discussed and under such
In case of apIoiC sol,ents, the haile ivas ar.
corelated to the rate of SN2 rcactions.
COnduons, he 0ider of basivity of halide ion is diretly
404 Organic Chemistry
Table 11.1 A Comparison Haloalkanes 405
More about leaving groups of SN1 and Sh2
on it. A species Sy1
mnechanisms (Contd.)
A good leaving.group is one which can accommodate the negative charge present a larger size sDecis
with a larger size can easily accommodatc negative charge. In other words, in Concentration of nucleophile does not SN
the dispersion of negative charge is more. A large sized, negatively charged species thus, behaves affect the rate. concentration of
as a weak base and this is the reason that wcak bascs arr generally good leaving groups. rate of reaction. haloalkane increases the
A Reaction proceeds through the formation
of carbocation. (iv) Reaction proceeds through
Halide ions are considered to be good leaving groups of transition state. the formation
() Energy profile:
The leaving group ability of halide ions follow the order I> Br > Cl>F. Iodide ion has a Jaroer (v) Energy profile:
size than all the others and can accommodate the negative charge easily. thus behaving as the best
leaving group.
Sulfonates (sufonic ester groups) are good leaving groups
AS negative charge is dciocalized among oxygens in the sulfonates, for example, methanesulfonate
CH,SO, (mesylate), trifluoromethanesulfonate CF;SO, (triflate ion). p-methylbenzenesufonate ion
(tosylate ion), these groups act as good leaving groups.

-Gu
CH,-S-0 CF-S--ô CH, H
-Nu
X
Mesvlate ion Triflate ion Tosylate ion

Water is good leaving group Order of reactivity

The hydroxide ion is a strong base and is not a good leaving group. On the other hand water is () Reactivity order stability of carbocation () Reactivity order o stability of transition state
a neutral molecule and behaves as a good leaving group. Thus, removal of -OH occurs through
formation of oxonium ion as neutral water molecule (refer p. 463). (ii) Reactivity of RX follows the order: (i) Reactivity of RX follows the order:.
3°>2°>1°>CHX CH,X >1°>2°>3*
Stereochemistry
ROH H, RLÖH, R + H,O
(i) Products formed with retention as well as () Product formed with inversion of
Oxonjum ion Good leaving inversion of configuration. It is not a
group
configuration only (Walden inversion).
stereospecific reaction. It is a stereospecific reaction.
Table 11.1 compares both reaction mechanisms in the light of various characteristics. Nature of nucleophile
Table 11.1 A Comparison of SN1 and SN2 mechanisms SN1 reaction is favoured in the presence of SN2 reaction is favoured in the presence
weak bases or poor nucleophiles. of strong nucleophiles.
Sy1 Sy2 Effect of solvent
Type of reaction Polar protic solvents (H,0, ROH etc.) Polar protic solvents decrease the rate of
() Unimolecular reaction with first order ()) Bimolecular reaction with second order enhance the rate of reaction. reaction. However polar aprotic solvents
kinetics kinetics. (acetone, DMF, DMSO etc.) enhance the
(ii) Two step mechanism. rate of reaction.
(i) One step mechanism.
(ii)) Rate a [RX]
(i) Rate a [RX][Nu]
Increase in concentration of the haloalkane An increase in the concentration of
causes an increase in the reaction rate.
nucleophile as well as increase in the
(Contd.)
412 Organic Chemistry Haloalkanes 413
Alcoholic
OH
NH,
R-NH,
CH,-CH-CH-CH, KOH, CH,CH=ECH-CH, +
R-OH
Alcohol
Primary amine iH Bri H CH,CH,-CH=CH,
NH,R 2-Bromobutanc But-2-ene
OR R-NHR' But-1-ene
(major product) (minor product)
R-OR't Secondary amine
Ether CH,-CH-CH-CH,
(CH),S=0 NHR', ’R -NR,
CH,-C=CH-CH, + CH-CH-CH=CH,
Carbonyl Tertiary amine CH, Br CH,
compounds 2-Bromo-3-methylbutane
CH,
00CR'
R'C=C R-CECR' 2-Methylbut-2-ene 3-Methylbut-1-ene
R OCOR' (major product) (minor product)
Alkynes
Esters
SH R' The elimination may proceed through a unimolecular (E-1) or bimolecular (E-2) mechanism and the
R ’R-R'
R-SH
Alkane reaction follows Saytzeff elimination (section 6.2.2).
Mercaptans
SR CH(CO,Et)) Elimination unimolecular (E-1) mechanism
R SR' Malonic ester products
Sulfide The unimolecular elimination mechanism is a two step process and proceeds through the formation
so CH(COMe)CO,Et
Acetoacetic of carbocation intermediate. For example, the dehydrohalogenation in case of 1-bromopropane
RSO, results in the formation of propene as follows:
Sulfonate ester products
Y' Step 1. Heterolytic cleavage of C-X bond: formation of carbocation.
’R-X
Haloalkane Slow step
CN ON=0 CH,CH,CH, -Br Rate determining step
CH,CH,H,
R-NO, or R-ONO Carbocation
RNC or R-CN
Isonitrile Nitrile Nitroalkane Alkylnitrite sp? sp²
(Tetrahedral) (Planar)
Fig. 11.1 Synthetic utility of nucleophilic substitution reactions of haloalkanes: Functional group
Thus, it's a unimolecular reaction
transformations from haloalkanes using oxygen, sulfur., nitrogen and carbon nucleophiles. The rate determining step involves only the haloalkane.
and follows first order kinetics.
stability of
the rate of reaction. The order of
The stability of carbocation governs
11.3.3 Elimination Reactions: A General Discussion carbocations and hence, the order of reactivity
of haloalkanes towards elimination reactions
In haloalkanes, the reaction with an electron rich species (Nu) may either be a nucleophilic is 3° > 2° > 1° > CHX. there is a possibility of
carbocation intermediate. Therefore,
substitution reaction or an elimination reaction. In fact. the substitution and elimination reactions The reaction involves a
compete with each other. The reaction of haloalkanes with aqueous sodium or potassium hydroxide carbocation rearrangement.
carbocation by strong base:
formation of an alkene
results in the formation of alcohol (substitution reaction). However, the reaction of haloalkanes with Step 2. Abstraction of Bproton of the
alcoholic potassium hydroxide results in dehydrohalogenation to produce an alkene (elimination
reaction).
Bhydrogen
In general elimination reaction occurs: (i) in the presence of a strong and/or bulkier base; H
(ii) at high temperature: and (iii) with bulkier haloalkanes. OCH,CH, CH,-CH=CH, + CH,CH,--OH
The alcoholic KOH results in the fomation of a strong base, ethoxide ion (0C,H). The much CH,-c H, Propene
stronger ethoxide ion abstracts a proton to form an alkene.
H Bhydrogen
CH,CH, -OH + KOH CH,CH,0 K + H-OH Elimination bimolecular (E-2) mechanism removal of halide ion
Bproton by base and
Ethoxide ion where abstraction of the formation of
(astorng base) O Single step mechanism
involves removal of hydrogen halide and
Simultaneously. The elimination
CH,CH,CH,-CH, Alcoholic KOH, CH,CH,CH=CH, a r-bond.

B
 Haloalkanes 415
Step- 1
Step-2
EXPLORE MORE Base
414 Organic Chemistry
SET- 1 Slow step
CH,-CH=CH, + CH,OH .through solved
Fast
step
Sc=c<
problem 88

CH,-CH CH, Haloalkane Carbanion Alkene

C,H,0 + (acid) (conjugate base)
Strong base
well as
the haloalkane so. it's called a
base as Doted
involves the should be
step (only
step)
order
kinetics. It 41.3.4 Substitution Versus Elimination
determining
follows a second the
reaction.
The rate which
bimolecular
reaction,
alkene is
favoured
during
that is,
removal of B-hydrogen and halide Both substitution and elimination reactions occur simultaneously. The ratio of substituted and
Amnore
substituted
involves anti
elimination,
elimination products formed depends upon two factors, namely the structure of haloalkane and the
(i) mechanism
nature of base used.
(ii) An E-2 opposite
sides. mechanism. as it does not involve
from (E-2)
ion occurs
favour
bimolecular

compares the
E-l and E-2 mechanism.
reactions
(Table 11.2) Structure of haloalkane
following table
elimination
general.
In The
carbocation
rearrangement.
mechanism
In the presence of a strong base, with an increase in the bulk of alkyl groups of haloalkane, the ratio
versus E-1
Table 11.2 E-2 of elimination product increases. In a sterically hindered haloalkane elimination product is preferred
E-2 mechanism over substitution product.
with second order Substitution products Elimination products
E-1 mechanism

(i) Bimolecular reaction

reaction with first order C,H,ONa
(i)
Unimolecular kinetics.
(concerted)
CH,CH,CH,CH,Br CzH,OH CH,CH,CH,CH,0C,H, CH,CH,CH=CH,
mechanism
kinetics (i0) One step no 1° Haloalkane I-Ethoxybutane (90%) But-1-ene (10%)
mechanism. intermediate is formed and
(i) Two step (ii) No reactive
through the rearrangement Occurs. C,H,ONa
(ii) Reaction proceeds and there is a CH,CHCH,CH, CH,CH=CH-CH,
formation of carbocation CH;CHCH,CH, C,H,OH

is directly related to the

rearrangement. But-2-ene (85%)
possibility of carbocation
directly related to the (v) Order of reactivity Br OC,H5
(iv) Order of reactivity is stability of alkene.
2-Ethoxybutane (15%)
stability of carbocation. involved 2° Haloalkane
Stereochemical aspects are
stereochemical aspects are involved. (v) occurs. The elimination
(V) No since anti elimination CH;
and CH, CH,
of HX occurs only when halogen
other in their C,H,ONa
hydrogen are anti to each
conformation. CH,-¢-CH, CGH,OH
CH,-C-CH, CH,--CCH,
2-Methylpropene (93%)
solvent slightly
(vi) Increasing the polarity of solvent increa (Vi) Increasing the polarity of reaction. Br OC,H,
ses the rate of E-1 reaction. Polar protic decreases the rate of E-2 3° Haloalkane 2-Ethoxy-2-methylpropane (7%)
increase
solvent increases the rate of E-1 reaction. However, polar aprotic solvents Substitution is favoured in case Elimination is favoured in case
the rate of E-2 reactions. The competition between
of 1° haloalkanes and follows the of 3° haloalkanes and follows the
subsititution and elimination
(vi) Strong bases increase the rate of depends on the structure of increasing order for substitution to increasing order for elimination
(vii) Strength of base does not affect to occur as 1° < 2° <3° haloalkane
the rate of E-1 reactions. haloalkane Occur as 3° < 2°< 1°
E-2 reactions.

Nature of base
E1cB (Elimination, unimolecular, conjugate base)
mechanism
The presence of electron-withdrawing substituents on B-carbon of haloalkane favours ElcB
4 Srong base abstracts the B-proton more readily and thus favours the EXPLORE MORE
Clmination reaction. For example, when a haloalkane reacts with tert.
mechanism. This is astep wise mechanism where first step is fast and involves loss of Bhydrogen outoxide ((CH),Co1. elimination is favoured over substitution where as ..through solved
of haloalkane, in presence of a problem 86
With ethoxide [C,H-0lthe substitution is favoured over elimination. The
carbanion

base,
intermediate, is a slow, rate determiningto give a carbanion. The loss of halide ion from
step that results Teaction is expressed in the following manner.
in the formation of an a
 Haloalkanes 417
Soemation of Gilman reagent via
416 Organic Chemistry
nrOus jodide in anhydrous ether alkyllithium. The reaction of alkyllithium with
as a solvent results in the
C H , C H , C H , C H , C H , O R

Substitution
product (an ether) dialkylcuprate also known as Gilman reagent. formation of lithium
(R=C,H, or C(CH,),)
C,H,OK/C,H,OH

R-X + 2Li Anhydrous ether

CH,CH,CH,CH,CH,Br
(CH,), COH CH,CH,CH,CH=CH, RLi + LiX
(CH),CO K
Elimination product
(an alkene) Alkylithium
With sterically hindered base, Anhydrous ether
elimination is favoured. 2RL0 + Cu,l,
With unhinderd
R,CuLi
With bulky base base C,H,OK With unhindered base (or less Lithium dialkylcuprate
(sterically hinderd sterically hindered base) sub [Gilman reagent)
stitution is favoured.
base) (CH,),COK

The competition between sub General Characteristics

(89%)
stitution is not only dependent 11.4.1
Substitution (13%)
on the structure of haloalkane
product (as mentioned in earlier reaction) The organometallic compounds exhibit Some general characteristics that can be
(11%) but also on the structure of the base.
enumerated as:
Elimination (87%) (a) The organometallic compounds contain carbon-metal bond.
product ) Due to high electropositive character of metal, the carbon bears a negative charge,
that is,
carbanion character develops at the organic moiety.
Substitution v/s Elimination

elimination Carbon behaves as an Carbanion character develops

Strong bases favour
Weak bases favour substitution and electrophilic centre due due to metal and it becomes

base the 3° haloalkanes undergo elimination whereas 1o to electronegative halogen the nucleophilic centre
REVISITING
With strong
THE KEY substitution.
POINTS haloalkanes undergo preferably

Electronegative Electropositive
ORGANOMETALLIC COMPOUNDS -AN OVERVIEW halogen metal
11.4
[Reaction of Haloalkanes with Metals] (c) A more electropositive metal forms a more ionic carbon-metal bond and the
Haloalkanes (as well as haloarenes) react with metals to form corresponding organometallic electropositive character of metals follows the order: Li > Mg > Cu
compounds. This section discusses some such formations. (d) The preparation and reactions of organometallic compounds are carried out in suitable
(a) Formation of Grignard reagent. The reaction of haloalkanes with magnesium metal in inert solvents like ethers, under anhydrous conditions. In general, the ethers used are
anhydrous ether as asolvent results in the formation of alkylmagnesium halides which are diethy ether, dimethoxyethane (DME), and cyclic ethers such as Tetrahydrofuran (THF)
and dioxane.
referred more commonly as Grignard reagents.

R-X + Mg Anhydrous ether EXPLORE MORE

RMgX SIS
H,CCH,0CH,CH, CH,OCH,CH,0CH,
Alkylmagnesium halide .through solved Diethyl ether DME
[Grignard reagent] problems 52,75 THF Dioxane

b) Formation of alkylithiums.
ether as a solvent results in theThe reaction of haloalkane with lithium metal in
anhydrous
(e) The ethers solvate the organometallic compounds easily, that is, they remain in dissolved
formation alkyllithiums. form and generally exist as etherates.
R-X + 2Li Anhydrous ether R
RLi + LiX C,Hs
H,C,
Alkyllithium o:----- Mg -
H,C, C,H,
X