Question

Assertion (A): Aromatic primary amines cannot be prepared by Gabriel phthalimide synthesis. Reason (R): Gabriel phthalimide synthesis is used for the preparation of primary aliphatic amines.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Gabriel phthalimide synthesis prepares primary aliphatic amines, not aromatic amines.

Answer 1

The Correct Option is B

Step 1: Understand Gabriel phthalimide synthesis mechanism.
Steps: (i) Phthalimide is treated with KOH to form potassium phthalimide. (ii) Potassium phthalimide reacts with an alkyl halide via SN2 (nucleophilic substitution). (iii) The N-alkyl phthalimide product is hydrolysed (acid or base) to release the primary aliphatic amine.
Step 2: Evaluate Assertion (A).
Assertion: Aromatic primary amines cannot be prepared by Gabriel phthalimide synthesis. This is TRUE. Aryl halides (e.g., chlorobenzene) do not undergo SN2 reactions (the halogen is on an sp2 aromatic carbon; backside attack is blocked by the flat rigid ring; also resonance strengthens the C-X bond). Since the key step is SN2, aryl halides cannot participate.
Step 3: Evaluate Reason (R).
Reason: Gabriel synthesis is used for the preparation of primary aliphatic amines. This is TRUE. The method works specifically for alkyl halides (SN2 reactive) and always yields primary amines only (because phthalimide nitrogen bonds to only one alkyl group).
Step 4: Does R correctly explain A?
R says the method is for aliphatic amines. This implies, but does not mechanistically explain, why aromatic amines cannot be prepared. The true mechanistic reason is that aryl halides cannot undergo the SN2 step. So R is a correct statement but not the deepest mechanistic explanation of A.
Step 5: Apply the official interpretation.
According to the official CBSE answer key, both A and R are true, but R is not the correct (mechanistic) explanation of A. The correct explanation would require stating that aryl halides do not undergo SN2 reactions.
Step 6: State the final answer.
Both A and R are true, but R is not the correct explanation of A. \[ \boxed{\text{Both A and R true, but R is not the correct explanation of A}} \]