Q.Arrange the following :
a. In decreasing order of the b.p. — C₂H₅-OH, C₂H₅-NH₂, (CH₃)₂NH
b. In increasing order of solubility in water: C₂H₅-NH₂, C₃H₇-NH₂, C₆H₅-NH₂

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. …

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 …