Question

Given below are 4 reactions. Two of these reactions will give product which is an equimolar mixture of the d and l forms. Identify these 2 reactions. Reactions $ \text{[A]} \quad \text{2-Methylpropene} + \text{HBr} \rightarrow \text{--------} $ $ \text{[B]} \quad \text{But-1-ene} + \text{HBr} \rightarrow \text{--------} $ $ \text{[C]} \quad \text{3-Methylbut-1-ene} + \text{HI} \rightarrow \text{--------} $ $ \text{[D]} \quad \text{3-Phenylpropene} + \text{HBr (Peroxide)} \rightarrow \text{--------} $

• A. C & A
• B. D & B
• C. B & C
• D. A & D

Hint:

In addition reactions involving peroxides (as seen in [D]), the addition is anti, which often results in stereochemistry that leads to a racemic mixture, especially in the case of asymmetric substitution.

Answer 1

The Correct Option is C

In the given reactions, the formation of a racemic mixture (equimolar mixture of d and l forms) occurs in reactions where the product is formed by a mechanism that leads to the creation of a chiral center with both possible configurations being equally likely.
1. Reaction [B] - But-1-ene + HBr The addition of HBr to but-1-ene occurs via a free-radical mechanism (due to the absence of peroxides). This mechanism will produce a mixture of two enantiomers, forming a racemic mixture of the product.
2. Reaction [C] - 3-Methylbut-1-ene + HI The addition of HI to 3-methylbut-1-ene will form a product where a chiral center is generated, leading to a racemic mixture of the d and l forms.
Thus, reactions B and C lead to the formation of racemic mixtures.