What is A in the following reaction?

Step 1: Identify the reaction

This is the Gabriel phthalimide synthesis for preparing primary amines.

Step 2: Reaction mechanism overview

Gabriel synthesis steps:

1. Phthalimide deprotonation:

   • Phthalimide + KOH → Potassium phthalimide (nucleophile)

2. N-alkylation: $\ce{Phthalimide-N-K+ + R-Br -> Phthalimide-N-R + KBr}$

   • Benzyl bromide: $\ce{C6H5CH2Br}$
   • Product: N-benzylphthalimide

3. Hydrolysis: $\ce{Phthalimide-N-R + OH-/H2O -> R-NH2 + Phthalic acid}$

   • Cleaves the phthalimide
   • Releases primary amine

Step 3: Identify product A

Starting with benzyl bromide ($\ce{C6H5CH2Br}$):

Final product: Benzylamine ($\ce{C6H5CH2NH2}$)

Answer: (a) Benzylamine

Key Points:

Gabriel synthesis:

• Used to prepare primary amines only
• Cannot make 2° or 3° amines (phthalimide N is not nucleophilic enough after first alkylation)
• Works best with primary alkyl halides
• Benzylic halides are excellent substrates

Why this method?

• Direct alkylation of $\ce{NH3}$ gives mixture (1°, 2°, 3°, quaternary)
• Gabriel synthesis gives only primary amine
• Clean, selective method

Alternative methods for primary amines:

• Reduction of nitriles: $\ce{R-CN ->[LiAlH4] R-CH2-NH2}$
• Reduction of amides: $\ce{R-CONH2 ->[LiAlH4] R-CH2-NH2}$
• Hofmann bromamide: $\ce{R-CONH2 ->[Br2/OH-] R-NH2}$