Q.62 - Organic Chemistry

Question

Explain why the reaction of 2-bromopropane with ${\mathbf{\text{NaOCOCH}}}_{\mathbf{3}}$ gives ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{2}}}{\mathbf{\text{CHOCOCH}}}_{\mathbf{\text{3}}}$ exclusively as product, but the reaction of 2-bromopropane with  ${\mathbf{\text{NaOCH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{3}}}$ gives a mixture of  ${\mathbf{\text{(CH}}}_{\mathbf{\text{3}}}{\mathbf{\text{)}}}_{\mathbf{\text{2}}}{\mathbf{\text{CHOCH}}}_{\mathbf{\text{2}}}{\mathbf{\text{CH}}}_{\mathbf{\text{3}}}$ (20%) and  ${\mathbf{\text{CH}}}_{\mathbf{3}}{\mathbf{\text{CH=CH}}}_{\mathbf{2}}$ (80%).

Step-by-Step Solution

Answer

The reason is acetate ion is a powerful nucleophile than a base and only promotes substitution product. But ethoxides act both as a strong base and a strong nucleophile.

Function of ethoxide

Ethoxide is a strong base as well as a strong nucleophile. Hence they give both an elimination and a substitution product.

Function of acetate anion

Acetate anion acts only as a good nucleophile. Hence, they exclusively give a substitution product.

 

Keeping into account all the key points, the products with their mode of reaction are shown below:-

Representation of the substituted and eliminated products

The difference in the concentration is observed here because acetate ion is a powerful nucleophile than a base and only promotes substitution product. In contrast, ethoxides act both as a strong base and a strong nucleophile. Hence, they promote both substitution and elimination products.