Gabriel synthesis is a process of producing amines from alkyl halides.
1. Phthalimide is converted to potassium salt by treating it with potassium hydroxide (KOH) solution.
2. The potassium salt reacts with the 2-bromobutane forming the intermediate, 2-butyl phthalimide (box 1) and potassium bromide (KBr).
The reaction is governed via nucleophillic substitution. The negative charge in the potassium salt reacts with the carbon where the bromide is attached. The bromide is released and is attached with the potassium ion.
3. 2-butyl phthalimide is hydrolyzed forming the 2 aminobutane.
The hydrogen in the 2 aminobutane came from the water molecules allowing the cleavage to from. Hydrolysis forms the dicarboxylic acid (box 2).
Please see the answer on the attached image.