The two compounds shown are mixed (the diamine in aqueous base, the other in an organic solvent) and react at the interface. Which small molecule is eliminated for each new amide bond formed?

A $Cl X_{2}$
B $HCl$
C $H X_{2} O$
D $NH X_{3}$

Answer: B

Hexanedioyl dichloride (adipoyl chloride) reacts with hexane-1,6-diamine to give nylon 6,6 (the nylon rope trick). Each N-H of the amine and a Cl of the acyl chloride combine, so one molecule of HCl is eliminated per amide bond, not water as with the free acid. Base in the aqueous layer neutralises the HCl.

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