Q.Which one of the following compounds has the highest boiling point ?
a. n-Butylamine b. sec-Butylamine c. isobutylamine d. tert-Butylamine

Both amines can in principle hydrogen-bond with water through N–H and the nitrogen lone pair, but ethylamine's small ethyl group barely interferes with this, while aniline's bulky, non-polar benzene ring dominates the molecule and makes it largely water-insoluble.

Primary amines can act as hydrogen-bond donors (via N–H) and acceptors (via the N lone pair) with water molecules, which is what allows small amines to dissolve.

• Ethylamine, $C_2H_5NH_2$: the alkyl part is just a small ethyl group. The polar –NH$_2$ group's ability to hydrogen-bond with surrounding water molecules is not significantly hindered, and the small hydrocarbon part is easily accommodated within the hydrogen-bonded water structure. As a result ethylamine is completely miscible with water in all proportions.

• Aniline, $C_6H_5NH_2$: the amino group is attached to a large, flat, hydrophobic benzene ring. Although the –NH$_2$ group itself could hydrogen-bond with water, the bulky non-polar aromatic ring cannot be accommodated by water's hydrogen-bonded structure, and the lone pair on nitrogen is also partly delocalised into the ring (reducing its availability to water). The hydrophobic ring dominates the molecule's overall behaviour, so aniline is only sparingly soluble in water (dissolves better in organic solvents).

Solubility of amines in water depends on their ability to hydrogen-bond with water molecules through the N–H and lone pair; a small alkyl group favours this, a large aromatic ring hinders it.

Ethylamine ($\text{C}_2\text{H}_5\text{NH}_2$) has a small, polar ethyl group that does not obstruct hydrogen bonding between its $-\text{NH}_2$ group and water molecules, so it is completely miscible with water in all proportions (lower amines are always water-soluble). Aniline ($\text{C}_6\text{H}_5\text{NH}_2$) has a large, bulky, hydrophobic benzene ring attached to nitrogen. This bulky non-polar ring (i) sterically hinders the amino group from approaching water molecules to form hydrogen bonds and (ii) the lone pair on N is partly delocalised into the ring by resonance, making it less available for hydrogen bonding. As a result aniline is only very sparingly soluble in water.