Hexanediamide, $\mathrm{H}{}_2\mathrm{N}-\mathrm{CO}-(\mathrm{CH}{}_2){}_4-\mathrm{CO}-\mathrm{NH}{}_2$, is treated with excess $\mathrm{Br}{}_2$ and aqueous $\mathrm{NaOH}$. The organic product is

Each amide group undergoes Hofmann degradation, in which the carbonyl carbon is lost as carbonate and the alkyl group ends up bonded to $\mathrm{NH}{}_2$. Two carbons (both carbonyl carbons) are removed from the six-carbon chain, leaving the four $\mathrm{CH}{}_2$ groups with an $\mathrm{NH}{}_2$ at each end, i.e. $\mathrm{H}{}_2\mathrm{N}-(\mathrm{CH}{}_2){}_4-\mathrm{NH}{}_2$.