Question

The number of stereoisomers formed in a reaction of (±) Ph (C = O) C (OH) (CN) Ph with HCN is_____.
[Where, Ph is – C₆H₅]

Answer 1

Correct Answer: 3

To determine the number of stereoisomers formed in the reaction of (±) Ph(C=O)C(OH)(CN)Ph with HCN, we need to understand the formation of new chiral centers. Initially, the compound has two chiral centers. With the addition of HCN, a cyano group is added, creating a new chiral center at the carbonyl carbon (C=O). Each chiral carbon can exist in two configurations (R and S). 
Initially, there are two chiral centers, so the possible configurations are 2²=4. The process involves adding a third chiral center.
Let's consider the formation of a new temporary diastereomeric pair upon the reaction with HCN:

• The new chiral center forms at the carbonyl carbon.
• The original molecule is racemic, expressed as (±), meaning both enantiomers are present.
• The addition of the new group (CN) leads to the generation of two additional stereoisomers for each original enantiomer.

Therefore, the calculation for the new total number of stereoisomers involves introducing a new chiral center: the formula for the total number of stereoisomers is 2ⁿ, where n is the number of chiral centers. After the reaction, the number of chiral centers becomes 3. The total stereoisomers = 2³ = 8.However, since the initial compound is (±), meaning both enantiomers are equivalent and the reaction does not introduce new symmetry elements, we only form 4 distinct stereoisomers from the new center. Hence, these relate as two pairs of enantiomers.
This explains why the reaction leads to 4 distinct stereoisomers.

Answer 2

Number of stereoisomers = 3

So, the answer is 3.