Question

Which of the following compounds will give racemic mixture on nucleophilic substitution by \( \text{OH}^- \) ion? \includegraphics[width=5cm]{71.png}

• A. \( (i) \)
• B. \( (i), (ii), and (iii) \)
• C. \( (ii) and (iii) \)
• D. \( (i) and (iii) \)

Hint:

Racemic mixtures form in \( \text{S}_\text{N}1 \) reactions due to the formation of a planar carbocation intermediate, which allows the nucleophile to attack from either side.

Answer 1

The Correct Option is B

Compounds that undergo the \( \text{S}_\text{N}1 \) mechanism in nucleophilic substitution reactions produce a racemic mixture. This is because the \( \text{S}_\text{N}1 \) mechanism involves the formation of a planar carbocation intermediate, which can be attacked by the nucleophile from either side, leading to the production of a 1:1 mixture of enantiomers. The reactivity order for \( \text{S}_\text{N}1 \) reactions is: \[ 3^\circ>2^\circ>1^\circ \, \text{(alkyl halides)}. \]
Compound (i): Has a secondary carbon, and follows the \( \text{S}_\text{N}1 \) mechanism.
Compound (ii): Contains a tertiary carbon and strongly favours the \( \text{S}_\text{N}1 \) mechanism.
Compound (iii): Has a secondary carbon, and follows the \( \text{S}_\text{N}1 \) mechanism. Hence, all three compounds will produce a racemic mixture.
Final Answer: \[ \boxed{(i), (ii), \text{and} (iii)} \]