Write about Gabriel phthalimide synthesis ?

Hint : It is a simple reaction involving phthalimide and a basic hydroxide. This reaction is used to get primary amine from primary alkyl halide. This three stepped mechanism normally involves use of potassium hydroxide as basic hydroxide.

Complete answer :
This a chemical reaction used to transform primary alkyl halides into primary amines. It is named after scientist Siegmund Gabriel.
In this reaction, the potassium salt of phthalimide is N-alkylated with a primary alkyl halide giving N- alkylphthalimide which later on give amine.
It is a three step mechanism.
Step 1:- The potassium hydroxide is added to phthalimide in the beaker. The hydroxide ion from potassium hydroxide deprotonates the imide ion. The proton formed here is more acidic than in any simple amine because two adjacent carbonyl-like groups are present which show resonance. This will stabilize it and as a result a strong nucleophile the imide ion is generated.

Step 2:- The imide ion attacks the electrophilic carbon of the alkyl halide. The halogen is replaced by the N atom in alkyl halide. Thus, an NC bond is formed. As a result of this, there is formation of an N-Alkyl Phthalimide.

Step 3:- This mechanism is similar to base-catalyzed hydrolysis of esters. It is just that here Nitrogen is attached to the R group instead of oxygen. The hydroxide ion from sodium or potassium hydroxide attacks the carbon atom bonded to the nitrogen atom. As a result, the N-alkyl phthalimide is cleaved. The cation in the base gets bonded with oxygen atoms.

Note :
The Nitrogen atom attached to the carbon of R group forms a bond with the hydrogens ejected from the hydroxide ion when it is replaced by the oxygen atom in the phthalimide.
Further, instead of using basic hydroxide, the reaction also occurs by acidic hydrolysis and even with hydrazine. But both have disadvantages. In case of acidic hydrolysis, we get low yield and in hydrazine, the conditions get harsh. So, these two are avoided.