Q.Gabriel Pthalimide Synthesis is used for preparation of –

Gabriel phthalimide synthesis gives only primary amines.

In the Gabriel phthalimide synthesis, potassium phthalimide is reacted with an alkyl halide (R-X) to form N-alkylphthalimide, which is then hydrolysed (usually with aqueous NaOH or hydrazine) to give the primary amine (R-NH2) and phthalic acid/hydrazide. Since only one alkylation step is possible (the nitrogen is locked in the imide ring until hydrolysis), this method exclusively yiel …

The correct option is (i) 1$^\circ$ (primary) amine.

Concept. In the Gabriel synthesis, phthalimide is treated with KOH to form potassium phthalimide, which is then alkylated by an alkyl halide. Hydrolysis (or hydrazinolysis) then releases a primary amine and phthalic acid:

$\text{Phthalimide} \xrightarrow{KOH} \text{K-phthalimide} \xrightarrow{R{-}X} N\text{-alkylphthalimide} \xrightarrow{H_2O/H^+} R{-}NH_2 + \text{phthalic acid}$ …

Gabriel synthesis converts an alkyl halide into a pure primary aliphatic amine via N-alkylation of potassium phthalimide followed by hydrolysis; it cannot be used for aromatic amines because aryl halides do not undergo the required nucleophilic substitution.

The Gabriel phthalimide synthesis proceeds in two stages:

1. Phthalimide is treated with ethanolic KOH to form potassium phthalimide (the N–H is acidic enough to be deprotonated). This is then reacted with an alkyl halide, $R-X$, in an $S_N2$ nucleophilic substitution, giving N-alkylphthalimide.
2. The N-alkylphthalimide is hydrolysed (acidic or alkaline hydrolysis, or hydrazinolysis) to release the free primary amine, $R-NH_2$, along with phthalic acid/hydrazide by-product. …

Phthalimide's N-H is converted to a nucleophilic salt, alkylated by an alkyl halide, then hydrolysed to cleanly release a primary amine without any secondary/tertiary amine contamination.

The Gabriel phthalimide synthesis is used to prepare pure primary amines (free of secondary/tertiary amine contamination, unlike direct ammonolysis of alkyl halides):

Step 1: Phthalimide is treated with ethanolic KOH to form its potassium salt (potassium phthalimide), since the N–H hydrogen in phthalimide is acidic (flanked by two carbonyl groups).

Step 2: Potassium phthalimide, now a good nucleophile at nitrogen, is reacted with an alkyl halide (R–X) in an SN2 reaction, giving N-alkylphthalimide.

Step 3: N-alkylphthalimide is hydrolysed (with aqueous acid/alkali, or preferably with hydrazine — the Ing–Manske modification) to release the primary amine R–NH2 and phthalic acid (or phthalhydrazide).

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