Q.59 - Organic Chemistry

Question

Draw a stepwise, detailed mechanism for each reaction.

a.

b.

Step-by-Step Solution

Answer

The mechanism for the formation of the substitution and elimination products are shown below:

Types of base/nucleophile use

Weak bases and weak nucleophiles such as methanol promote a mixture of both  ${\mathbf{\text{S}}}_{\mathbf{\text{N}}}\mathbf{\text{1}}$ and E1 reactions as observed in reaction a. 

Strong bases and strong nucleophiles such as hydroxides and promote a mixture of both ${\mathbf{\text{S}}}_{\mathbf{\text{N}}}\mathbf{\text{2}}$ and E2 reactions as observed in reaction b.

Types of substrates used

Tertiary halides undergo a  ${\text{S}}_{\text{N}}\text{1}$ and E1 reaction respectively when a weak nucleophile and a weak base are used, such as methanol. 

Secondary halides undergo a  ${\text{S}}_{\text{N}}\text{1}$ and E1 reaction respectively when a weak nucleophile and a weak base are used. They undergo a  ${\text{S}}_{\text{N}}\text{2}$ and E2 reaction, respectively, when a strong nucleophile and a strong base are used.

Explanation

In both (a) and (b), the substrate used is a tertiary halide. The mode of the reactions changes due to the behavior of the base/nucleophiles.

The mechanism for the formation of the substitution and elimination products are shown below:

Representation of detailed reaction scheme of reaction a

Representation of detailed reaction scheme of reaction b