Q.58 - Organic Chemistry

Question

The following reactions do not afford the major product that is given. Explain why this is so, and draw the structure of the major product actually formed.

a.

b.

c.

d.

Step-by-Step Solution

Answer

The major E2 products from the alkyl halides are shown below:

Bases and nucleophiles

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

Halides

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

Observation of the major product formed

Formation of major product in a

Bulky bases always promote E2 reactions, and they cannot act as a nucleophile since the oxygen center cannot attack the carbon center due to the presence of the butyl group around it.

Formation of major product in b

When the substrate used is a primary halide and the base is strong, it promotes both  ${\text{S}}_{\text{N}}\text{2}$  and E2 reaction, but the major product will be a substitution product.

Formation of major product in c

When the substrate used is a tertiary halide, it undergoes a  ${\text{S}}_{\text{N}}\text{1}$ reaction since the intermediate tertiary carbocation is stable. Elimination reactions can also be formed, but the major product will be a substitution product.

Formation of major product in d

Here, the leaving group and base are both powerful, so in these situations, ${\text{S}}_{\text{N}}\text{2}$ reactions occur.