1,4-Dibromobutane ($1$ mol) is heated with $2$ mol of potassium phthalimide in DMF and the product is then boiled with hydrazine hydrate. The final organic product is

The phthalimide anion is a good nucleophile that is not strongly basic, and DMF increases its reactivity. With 2 mol of reagent, both primary C-Br bonds are displaced by SN2 to give the bis(phthalimide). Hydrazine then removes both phthaloyl groups and gives the diamine $\mathrm{H}{}_2\mathrm{N}(\mathrm{CH}{}_2){}_4\mathrm{NH}{}_2$. The phthalimide nitrogen cannot be alkylated twice, so no secondary amine is formed.