2. ALKYL HALIDES
Alkyl halides are organic molecules containing a halogen atom bonded to
a sp3 hybridised carbon.
C X
sp3 hybridised carbon
X= F, Cl, Br, I
Alkyl halide Alkyl halides classification
3. SUBSTITUTION REACTION
The carbon-halogen bond in alkyl halides is polar because of the
high electronegativity of the halogen atom relative to carbon.
The carbon atom is therefore a good target for attack by nucleophiles
(electron rich species).
In fact, the nucleophilic substitution reactions are the most common
reactions of alkyl halides.
Nucleophile Alkyl halide Substitution product Leaving group
4. WHAT IS NUCLEOPHILIC SUBSTITUTION
REACTION ?
When a substitution reaction involves the attack by a nucleophile,
the reaction is referred to as SN (S stands for substitution and N for a
nucleophile).
The hydrolysis of alkyl halides by aqueous NaOH is an example of
nucleophilic substitution.
R X + OH R OH + X
The nucleophilic substitution reactions are divided
into two types :
1. SN1 Reactions
2. SN2 Reactions
5. SN1 REACTIONS
SN1 stands for unimolecular nucleophilic substitution. When the rate
of a nucleophilic substitution reaction depends only on the
concentration of the alkyl halide, the reaction is of first order and is
represented as SN1.
The tertiary alkyl halides undergo hydrolysis by SN1 mechanism.
This is because the attack of the hydroxide ion on the crowded
tertiary alkyl halides is quite different.
6. KINETICS OF SN1 REACTION
The rate of SN1 reaction depends upon the concentration of alkyl
halide and is independent of the concentration of nucleophile. Thus
the reaction follows first order kinetics.
Rate α [alkyl halide]
Rate = k[(CH3)3C - Br
Reaction is called
unimolecular because the
rate determining step
involves only 1 molecule.
7. MECHANISM OF SN1 REACTION
SN1 reaction occurs in two steps.
Step 1: In first step, the carbon-halogen bond of tertiary butyl bromide
slowly breaks heterolytically to form an intermediate carbocation i.e. tert-
butyl carbocation.
Step 2: The carbocation formed combines rapidly with nucleophile i.e. OH
to give tertiary butyl alcohol.
Slow step is the rate determining step, thus step 1 is rate determining step.
8. This bond breaking is an example of heterolysis, cleavage in which
both bonding electrons go to the same fragment.
The combining of the carbocation with the hydroxide ion is a very
fast step, since it involves only the formation of a bond, an energy
releasing process.
This combining is recognized as an acid-base reaction in the Lewis
sense. Hydroxide ion is a strong base, carbocations are powerful
Lewis acids.
So here the rate-determining step involves only one molecule.
Any change in the rate of step 2 does not affect the overall rate.
9. STEREOCHEMISTRY OF SN1 REACTION
In SN1 reaction, carbocations are formed as an intermediate which
are triagonal and planar.
Carbocation has a flat structure so that nucleophile can attack it from
either side (i.e. front or back) resulting in the formation of two
products, one with retention of configuration and other with
inversion of configuration .
If the alkyl halide is optically active (i.e. when halogen carrying
carbon is chiral ), the product would be racemic mixture and
optically inactive.
In actual practice the, the product as a whole is not racemic. Usually
there is a larger proportion of molecules with inverted configuration
than of same configuration.
10. For eg, when (-)-2-bromoocatne is hydrolyzed by SN1 reaction,
partially racemized product is formed.
12. REACTIVITY OF ALKYL HALIDES TOWARDS SN1 REACTION
The rate of SN1 reaction depends on the stability of carbocation
formed. Therefore, the order of reactivity of alkyl halides towards
SN1 reaction is :
A tertiary carbocation is more stable than a secondary carbocation
which is more stable than a primary carbocation.
Greater the stability of carbocation, greater will be the ease of
formation of carbocation, and hence faster will be the rate of the
reaction.
13. EFFECT OF SOLVENT IN SN1 REACTION
A solvent that can facilitate the formation of the carbocation
intermediate will speed up the rate-determining step of SN1 reaction.
The preferred solvents are polar and protic.
The polar nature of solvents helps to stabilize ionic intermediates
whereas the protic nature of solvents helps to solvate the leaving
group.
Egs for solvents used in SN1 reactions include water and alcohol.
These solvents also act as nucleophiles.
14. REFERENCES
A textbook of organic chemistry, Arun Bahl, B S Bahl.
Sixth edition, Organic Chemistry, R T Morrison, R N Boyd.
www.organic-chemistry.org.
Organic Chemistry, Reaction and Reagents, O P Agarwal.
