2020 H2 Chem P1 Q24 — Nitrogen Compound Solution

The answer is D (equilibrium 2; electron-donating effect of alkyl groups).

Both amines act as Lewis bases by donating the lone pair on N to the H of HCl. Alkyl groups are electron-donating. Diethylamine has two ethyl groups on N, while ethylamine has only one, so in diethylamine:
• the electron density on N is higher, and its lone pair is more available for donation to H of HCl;
• the positive charge on the product ion, \((\text{C}_2\text{H}_5)_2\text{NH}_2^+\), is dispersed more, so this ion is more stable.
In the gas phase there are no solvent effects, so diethylamine is the stronger Lewis base and equilibrium 2 lies further to the right.

• A is wrong: alkyl groups are not electron-withdrawing, and equilibrium 1 does not lie further to the right.
• B is wrong: the electron-donating effect is the correct reason, but it makes the amine with more alkyl groups (diethylamine) the stronger base, so it is equilibrium 2, not equilibrium 1, that lies further to the right.
• C is wrong: equilibrium 2 is correct, but the reason is not: alkyl groups are electron-donating, and electron-withdrawing groups would make the lone pair less available, making the amine with more of them the weaker base.