Question

Which of the following reactions proceeds with retention of configuration?

• A. A secondary alcohol reacts with sodium (Na) to form an alkoxide, followed by reaction with methyl bromide (CH$_3$Br).
• B. A secondary alcohol reacts with p-toluenesulfonyl chloride (TsCl) to form a tosylate, followed by reaction with sodium hydrosulfide (NaSH).
• C. A secondary alcohol reacts with phosphorus pentachloride (PCl$_5$) to form an alkyl chloride, followed by reaction with sodium ethoxide (EtONa) and heat ($\Delta$).
• D. Propan-2-ol reacts with SOCl$_2$ to form an alkyl chloride, followed by reaction with KCN in DMSO.

Answer 1

Answer: (A), (C)

Option 1 and Option 3

Answer 2

Option 1: The reaction involves a secondary alcohol reacting with sodium (Na) to form an alkoxide, followed by reaction with methyl bromide (CH$_3$Br). This is a Williamson ether synthesis. The alkoxide attacks the methyl carbon of CH$_3$Br in an SN2 reaction. Since the chiral center of the alcohol is not involved in bond breaking or formation, the configuration is retained.

Option 2: The reaction involves a secondary alcohol reacting with p-toluenesulfonyl chloride (TsCl) to form a tosylate, followed by reaction with sodium hydrosulfide (NaSH). The tosylate group is an excellent leaving group, and the reaction with SH$^-$ is an SN2 reaction, which proceeds with inversion of configuration.

Option 3: The reaction involves a secondary alcohol reacting with phosphorus pentachloride (PCl$_5$) to form an alkyl chloride, followed by reaction with sodium ethoxide (EtONa) and heat ($\Delta$). The reaction of secondary alcohols with PCl$_5$ generally proceeds with inversion of configuration. The subsequent reaction of the alkyl chloride with sodium ethoxide is an SN2 reaction, which also proceeds with inversion of configuration. A double inversion results in the retention of configuration.

Option 4: The starting material shown is propan-2-ol, which is achiral because the central carbon is bonded to two identical methyl groups. The concept of retention of configuration is not applicable to achiral molecules.