Question

Which one of the following compounds, on reaction with hydrogen cyanide (HCN), gives an optically active cyanohydrin?

• A. \(\text{CH}_{3}\text{-COCH}_{3}\)
• B. \(\text{H-CHO}\)
• C. \(\text{CH}_{3}\text{-CHO}\)
• D. \(\text{C}_{6}\text{H}_{5}\text{-CO-C}_{6}\text{H}_{5}\)

Hint:

A cyanohydrin is optically active only when the carbon from the former carbonyl has four different groups.

Answer 1

The Correct Option is C

Step 1: Rule
Optical activity needs a carbon bonded to four different groups.

Step 2: Apply
Only in acetaldehyde are the two original groups on the carbonyl carbon different (H and $\text{CH}_{3}$), so the product carbon is chiral.

Step 3: Final check
The cyanohydrin CH3CH(OH)CN has a chiral carbon, so it is optically active, which is option (C).

Final Answer:
Option (C), acetaldehyde, is correct. \[ \boxed{\text{(C) }\text{CH}_{3}\text{-CHO}} \]