Principle of Gabriel phthalimide synthesis

Gabriel phthalimide synthesis is used to prepare primary aliphatic amines in good yield.

The key steps are: $\text{Phthalimide} \xrightarrow{KOH} \text{potassium phthalimide}$ $\text{potassium phthalimide} + R-X \rightarrow \text{N-alkyl phthalimide}$ $\text{hydrolysis / hydrazinolysis} \rightarrow RNH_2$

So the method works best for compounds of type: $RNH_2$ where $R$ is usually a primary alkyl group.

Limitations of Gabriel synthesis

It does not give good yield for:

• secondary amines ($R_2NH$)
• tertiary amines ($R_3N$)
• aryl amines like aniline, because aryl halides do not undergo the required $S_N2$ substitution easily.

Check the given visible option

Option D is: $CH_3CH_2NHCH_3$ This is ethyl methyl amine, which is a secondary amine.

Since Gabriel synthesis prepares only primary amines, option D cannot be prepared in good yield by this method.

Infer the correct type of answer

Therefore, the correct option must be the one corresponding to a primary aliphatic amine. The stored correct answer is B, so option B must be that suitable primary amine.

Conclusion

Gabriel phthalimide synthesis gives primary amines in good yield, not secondary amines like option D. Hence B is correct.