Question

Gabriel phthalimide synthesis is used for the preparation of which of the following?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Gabriel phthalimide synthesis is mainly used for preparing primary aliphatic amines It cannot prepare aromatic, secondary, or tertiary amines efficiently.

Answer 1

The Correct Option is D

Step 1: Describe the Gabriel phthalimide synthesis steps.
Step 1: Phthalimide reacts with KOH to give potassium phthalimide (K⁺N^- salt). Step 2: Potassium phthalimide reacts with a primary alkyl halide (RX) via SN2 to give N-alkylphthalimide. Step 3: Acid hydrolysis (or hydrazinolysis) of N-alkylphthalimide liberates the primary amine (RNH₂) and phthalic acid (or phthalyl hydrazide).
Step 2: Explain why only PRIMARY ALIPHATIC amines can be prepared.
The key step is SN2 attack of the phthalimide nitrogen on the alkyl halide. SN2 reactions require primary (or sometimes secondary) alkyl halides. Tertiary alkyl halides cannot undergo SN2 (too much steric hindrance). Therefore, only primary aliphatic amines result from this synthesis.
Step 3: Explain why aromatic amines cannot be prepared.
Aryl halides (like C₆H₅Cl) are not reactive toward SN2 reactions under normal conditions because the sp² carbon of the arene ring does not undergo backside attack. Therefore, aromatic primary amines (anilines) cannot be made by Gabriel synthesis.
Step 4: Explain why secondary and tertiary amines cannot be prepared.
In Gabriel synthesis, only ONE alkyl group is introduced onto the phthalimide nitrogen (only one N-H in phthalimide). After alkylation and hydrolysis, the N has only one R group: RNH₂. Secondary (R₂NH) and tertiary (R₃N) amines cannot be formed because the phthalimide nitrogen can only be mono-alkylated.
Step 5: Confirm option 4 as the correct answer.
Option 4 states that Gabriel synthesis prepares primary aliphatic amines only, excluding aromatic, secondary, and tertiary amines. This is consistent with all the mechanistic reasoning above.
Step 6: Final answer.
Gabriel phthalimide synthesis is used exclusively for preparing primary aliphatic amines via SN2 alkylation of potassium phthalimide followed by hydrolysis.
\[ \boxed{\text{Primary aliphatic amines only (option 4)}} \]