The number of chiral centres present in [B] is _____.

Solution

Chemistry ma’am · 3 steps

The nitrile becomes a ketone, then a tertiary alcohol, and that carbinol carbon is a fourth chiral centre.

1. Nitrile to ketone. C₂H₅MgBr adds once to C≡N to give an imine salt. H₃O⁺ hydrolyses it to a ketone, so [A] is ring–CH(CH₃)-CO-C₂H₅.
2. Ketone to alcohol. CH₃MgBr adds to the C=O and water gives a tertiary alcohol. [B] is ring–CH(CH₃)-C(OH)(CH₃)(C₂H₅).

   Carbon in [B]	Four groups on it	Chiral?
   Ring C1 (bears side chain)	H, side chain, short ring path to C3, long ring path	yes
   Ring C3 (bears CH₃)	H, CH₃, short ring path to C1, long ring path	yes
   Side-chain CH	H, CH₃, ring, C(OH)(CH₃)(C₂H₅)	yes
   New carbinol C	OH, CH₃, C₂H₅, CH(CH₃)–ring	yes, new

3. So. Three chiral centres carry over from the start, and the Grignard step adds one more. [B] has 4 chiral centres.

The common slip: 3. Counting only the three centres already in [A] gives 3. The carbon carrying OH, CH₃ and C₂H₅ is also chiral.

The idea to keep. When a Grignard reagent attacks a ketone, check the new carbinol carbon. If its two added alkyl groups differ, here CH₃ and C₂H₅, it becomes a new chiral centre.

Answer from the NTA final answer key. Solution by PrepWiser.