Mechanism of Substitution Reactions

Nucleophilic substitution reactions are of two types

(a) S_N1 type

Unimolecular nucleophilic reactions proceed in two steps:

Rate, r = k [RX). It is a first order reaction.

Reactivity order of alkyl halide towards S_N1 mechanism

3° > 2° > 1°

Polar solvents, low concentration of nucleophiles and weak nucleophiles favour S_N1 mechanism.

In S_N1 reactions, partial racemisation occurs due to the possibility of frontal as well as backside attack on planar carbocation.

(b) S_N2 type (Bimolecular nucleophilic substitution)

These reactions proceed in one step and is a second order reaction with r = k[RX] [Nu].

During S_N2 reaction, inversion of configuration occurs (Walden inversion) i.e., starting with dextrorotatory halide a laevo product is obtained and vice-versa, e.g.,

Reactivity of halides towards S_N2 mechanism is

1° > 2° > 3°

Rate of reaction in S_N2 mechanism depends on the strength of the attacking nucleophile. Strength of some common nucleophiles is

:CN- > : I- > : OR- > : OH- > CH₃COO: > H₂O > F-

Non-polar solvents, strong nucleophiles and high concentration of nucleophiles favour S_N2 mechanism.

Relative rates of some alkyl halides in S_N1 and S_N2 reactions are in the order

Resonating structure of benzyl carbocations are

Relative reactivity of alkyl halides for same alkyl group is

RI >RBr> RCI > RF