Question

When C$_6$H$_5$COCOC$_6$H$_5$ is reduced with LiAlH$_4$, the product formed has __________ stereoisomers.

• A. 2
• B. 3
• C. 4
• D. 6
• E. 8

Hint:

When a molecule has $n$ identical chiral centers, the number of stereoisomers is reduced because of the meso form. For $n=2$, total isomers = $2^{(n-1)} + 2^{(n/2 - 1)} = 2 + 1 = 3$.

Answer 1

The Correct Option is B

Concept: Lithium aluminum hydride (LiAlH$_4$) is a strong reducing agent that reduces ketones to secondary alcohols. The number of stereoisomers depends on the number of chiral centers formed and the symmetry of the molecule.

Step 1: {Identify the reduction product.} The starting material is benzil (C$_6$H$_5$COCOC$_6$H$_5$). Reducing both carbonyl groups yields hydrobenzoin: C$_6$H$_5$CH(OH)CH(OH)C$_6$H$_5$.

Step 2: {Identify chiral centers.} The product, 1,2-diphenyl-1,2-ethanediol, has two identical chiral carbons. Each carbon is attached to -H, -OH, -C$_6$H$_5$, and the other chiral center.

Step 3: {Count the stereoisomers.} For a molecule with two identical chiral centers, there are three possible stereoisomers: a pair of enantiomers (d- and l-forms) and one optically inactive meso form due to internal symmetry.