Which of the following amines cannot be prepared by Gabriel synthesis?

N-Methylbenzylamine 

The correct answer is option 4. N-Methylbenzylamine.

To understand why Gabriel synthesis cannot be used to prepare N-Methylbenzylamine, let’s break down the process and examine the characteristics of each amine listed:

Gabriel synthesis is a method specifically designed to prepare primary amines. It involves two main steps:

1. Formation of N-alkylphthalimide: Phthalimide reacts with an alkyl halide (like an alkyl bromide or chloride) to form N-alkylphthalimide. This reaction does not produce secondary or tertiary amines.

2. Hydrolysis: The N-alkylphthalimide is then hydrolyzed (usually with a strong base such as potassium hydroxide) to release the primary amine and phthalic acid. The primary amine is the product of interest.

Analysis of the Options

1. Butylamine:

Butylamine can be prepared using Gabriel synthesis. The process involves reacting phthalimide with butyl bromide to form N-butylphthalimide, which is then hydrolyzed to yield butylamine.

2. Isopropylamine:

Isopropylamine can be prepared using Gabriel synthesis. In this case, phthalimide reacts with isopropyl bromide to form N-isopropylphthalimide, which is then hydrolyzed to produce isopropylamine.

3. 2-Phenylethylamine:

2-Phenylethylamine can be prepared using Gabriel synthesis. Phthalimide would react with phenylethyl bromide to form N-phenylethylphthalimide, which upon hydrolysis yields 2-phenylethylamine.

4. N-Methylbenzylamine:

This amine cannot be prepared using Gabriel synthesis. Gabriel synthesis is used to produce primary amines, not secondary or tertiary amines. In Gabriel synthesis, the final product is always a primary amine, and since N-Methylbenzylamine is a secondary amine, it does not fit this method.

Why Gabriel Synthesis Doesn’t Work for Secondary Amines

Primary Amine Focus: Gabriel synthesis specifically targets the creation of primary amines through the intermediate formation of phthalimide derivatives. Secondary and tertiary amines involve different bonding and structures that are not achievable through this method.

Reaction Specificity: The hydrolysis step in Gabriel synthesis is designed to release the primary amine from the N-alkylphthalimide. This process does not accommodate secondary or tertiary amines due to their different bonding and structure requirements.

Conclusion

Gabriel synthesis is designed to produce primary amines and cannot be used to synthesize secondary amines like N-Methylbenzylamine. This method involves steps that specifically generate primary amines, making N-Methylbenzylamine unsuitable for preparation by this method.