Question

1. Define and describe what is meant by Sn1 and Sn2.

2. Describe the differences between Sn1 and SN2 reactions, including mechanistic details.

3. Discuss the difference between the intermediates for each mechanisms- which you should draw.

Answer 1

Types of nucleophilic substitution reactions:

The nucleophillic substitution reactions can be classified into two types.

S_N¹ reaction mechanism[ studied as substitution nucleophilic, first order reaction]:

This type of reaction proceeds in two steps as:

The first step is slow and is the rate – determining step. As the nucleophile (Z^-) is not involved in the rate - determining step, the reaction depends only upon the concentration of alkyl halide (RX) and is therefore, a first order reaction.

                                                  i.e.    Rate = K [RX]

The order of reactivity depends upon the stability of carbonium ion formed in the first step. Since the 3⁰ carbonium ion is most stable, the ionization of tertiary alkyl halide is favoured. The order of reactivity for S_N¹ reaction is

Tertiary > secondary > primary

S_N² reaction mechnism[studied as substitution nucleophilic, second order reaction]:

This type of reaction occurs in one step through the formation of transition state as:

Here, the rate of reaction depends upon the concentration of both the alkyl halide and the nucleophile.

i.e.   Rate = K [RX] [Z^-]

The transition state form of tertiary alkyl halide is less stable due to steric hindrance. i.e. crowding of alkyl groups. The order of reactivity is

Primary > Secondary > Tertiary