L5 Nucleophilic substitution SN1 SN2 Class 12 NCERT Chemistry Chapter 10 HALOALKANES AND HALOARENES

In this video you will learn about one type of chemical reaction of haloalkanes i.e. Nucleophilic substitution reactions including mechanism of SN1 and SN2 reaction. For discussion we use Class 12 NCERT Chemistry Chapter 10 - HALOALKANES AND HALOARENES.
THEORY
10.6 Chemical Reactions
10.6.1 Reactions of Haloalkanes
The reactions of haloalkanes may be divided into the following categories:
(i) Nucleophilic substitution
(ii) Elimination reactions
(iii) Reaction with metals.
(i) Nucleophilic substitution reactions
In this type of reaction, a nucleophile reacts with haloalkane (the
substrate) having a partial positive charge on the carbon atom bonded
to halogen. A substitution reaction takes place and halogen atom,
called leaving group departs as halide ion. Since the substitution
reaction is initiated by a nucleophile, it is called nucleophilic
substitution reaction.
It is one of the most useful classes of organic reactions of alkyl
halides in which halogen is bonded to sp3 hybridised C.
Groups like cyanides and nitrites possess two nucleophilic centres
and are called ambident nucleophiles. Actually cyanide group is a
hybrid of two contributing structures and therefore can act as a
nucleophile in two different ways [C≡N ↔ :C=N], i.e., linking through
C atom resulting in alkyl cyanides and through N atom
leading to isocyanides. Similarly nitrite ion also represents an ambident
nucleophile with two different points of linkage [-O-N=O]. The linkage
through O results in alkyl nitrites while through N atom, it leads to nitroalkanes.
Example 10.5 Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN forms isocyanides as the chief product. Explain.
Mechanism: This reaction has been found to proceed by two different
mechanims which are described below:
(a) Substitution nucleophilic bimolecular (SN2)
The reaction between CH3Cl and OH- ion to yield methanol and
chloride ion follows a second order kinetics, the rate depends
upon the concentration of both the reactants.
It depicts a bimolecular nucleophilic displacement (SN2) reaction;
the incoming nucleophile interacts with alkyl halide causing the carbonhalide
bond to break while forming a new C-OH bond. These two
processes take place simultaneously in a single step and no intermediate
is formed. As the reaction progresses and the bond between the
nucleophile and the C atom starts forming, the bond between
C atom and leaving group weakens. As this happens, the
configuration of C atom under attack inverts in much the same
way as an umbrella is turned inside out when caught in a strong wind,
while the leaving group is pushed away. This process is called as
inversion of configuration. In the transition state, the C atom is
simultaneously bonded to incoming nucleophile and the outgoing leaving group and such structures are unstable and cannot be isolated. This
is because the carbon atom in the transition state is simultaneously
bonded to five atoms and therefore is unstable.
Since this reaction requires the approach of the nucleophile to the
carbon bearing the leaving group, the presence of bulky substituents
on or near the carbon atom have a dramatic inhibiting effect. Of the
simple alkyl halides, methyl halides react most rapidly in SN2 reactions
because there are only three small H atoms. Tertiary halides are
the least reactive because bulky groups hinder the approaching
nucleophiles. Thus the order of reactivity followed is:
Primary halide Secondary halide Tertiary halide.
(b) Substitution nucleophilic unimolecular (SN1)
SN1 reactions are generally carried out in polar protic solvents (like
water, alcohol, acetic acid, etc.). The reaction between tert-butyl
bromide and hydroxide ion yields tert-butyl alcohol and follows
the first order kinetics, the rate of reaction depends upon the
concentration of only one reactant, which is tert- butyl bromide.
It occurs in two steps. In step I, the polarised C-Br bond undergoes
slow cleavage to produce a carbocation and a bromide ion. The
carbocation thus formed is then attacked by nucleophile in step II
to complete the substitution reaction.
(b) Substitution nucleophilic unimolecular (SN1)
SN1 reactions are generally carried out in polar protic solvents (like
water, alcohol, acetic acid, etc.). The reaction between tert-butyl
bromide and hydroxide ion yields tert-butyl alcohol and follows
the first order kinetics, the rate of reaction depends upon the
concentration of only one reactant, which is tert- butyl bromide.
It occurs in two steps. In step I, the polarised C-Br bond undergoes
slow cleavage to produce a carbocation and a bromide ion. The
carbocation thus formed is then attacked by nucleophile in step II
to complete the substitution reaction.
Step I is the slowest and reversible. It involves the C-Br bond breaking for which the energy is obtained through solvation of halide ion with the proton of protic solvent.

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0:25 reactions of haloalkanes 0:54 nucleophilic substitution 7:32 nucleophilic substitution of alkyl halides (R-x) table 10.4 17:20 example 10.5 20:08 mechanisms 21:09 sn2 26:33 mechanisms of sn2 reactions notes 35:17 sn1 mechanisms

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[@basepluscareerinstitute]

Due to presence of bulky groups on the carbon, these bulky groups inhibit the attack of nucleophile on carbon, this is written dramatic inhibition.

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