Question

Write the reason of the following: Ethylamine is soluble in water, while aniline is not soluble in water.

Hint:

Compare the size of the carbon part and the availability of the nitrogen lone pair for hydrogen bonding with water.

Answer 1

Step 1: Solubility of ethylamine. Ethylamine \( (C_2H_5NH_2) \) has a small hydrocarbon part and an \(-NH_2\) group. The nitrogen and hydrogen of the amino group form hydrogen bonds with water molecules. Because the ethyl group is small, this hydrogen bonding is strong enough to dissolve ethylamine in water.
Step 2: Insolubility of aniline. In aniline \( (C_6H_5NH_2) \) the \(-NH_2\) group is attached to a large hydrophobic benzene ring. This bulky non-polar phenyl group cannot form hydrogen bonds with water and it dominates the molecule.
Step 3: Effect of lone pair delocalisation. In aniline the lone pair of electrons on nitrogen is delocalised into the benzene ring (resonance). This reduces the availability of the nitrogen lone pair for hydrogen bonding with water, further lowering its solubility.
Step 4: Conclusion. The large hydrophobic aromatic ring plus reduced hydrogen bonding make aniline almost insoluble in water, whereas the small ethylamine, with a freely available lone pair, hydrogen bonds well and dissolves.