Question

Why gabrielpthalmide is preffred method for synthesis of primary amines?

Answer 1

This is because, in all the other methods of synthesizing primary amines, sufficient amt. of byproducts are obtained.

Answer 2

The Gabriel synthesis is a chemical reaction that transforms primary alkyl halides into primary amines. 
Potassium phthalimide is a -NH2-synthon which allows the preparation of primary amines by reaction with alkyl halides. After alkylation, the phthalimid is not nucleophile and does not react anymore. Product is cleaved by reaction with base or hydrazine, which leads to a stable cyclic product.