Chemistry · Amines · NEET
The nitrogen atom of an amine has a lone pair and the N-H bond is polar. So amine molecules form hydrogen bonds with water: water H bonds to the amine N, and amine N-H bonds to water O. This attraction pulls the small amine into the water. Since methylamine, ethylamine, and similar small amines have only a tiny hydrocarbon part, hydrogen bonding wins and they dissolve fully.
An amine has two parts: the polar -NH2 (or N) head that likes water, and the hydrocarbon chain (-CnH...) that hates water. As the chain grows, the water-hating (hydrophobic) part becomes the larger share of the molecule. The single -NH2 head can no longer pull such a big oily body into water, so solubility drops. Amines with more than about 6 carbons are practically insoluble in water.
A tertiary amine has no N-H bond, so it cannot donate a hydrogen bond. But its nitrogen lone pair can still accept a hydrogen bond from water, so small tertiary amines are still fairly soluble. Overall, for the same molecular size, primary and secondary amines (which have N-H to donate) tend to dissolve a bit more than tertiary amines. All are still less soluble than the matching alcohol.
Amines are usually a little less soluble in water than alcohols of comparable size. Nitrogen is less electronegative than oxygen, so N-H...O hydrogen bonds are weaker than O-H...O hydrogen bonds. Weaker hydrogen bonding means slightly lower water solubility for amines than for alcohols.
Aromatic amines such as aniline are only very slightly soluble (sparingly soluble) in water. The large benzene ring is hydrophobic, and the lone pair on nitrogen is partly pulled into the ring by resonance, so hydrogen bonding with water is weak. Aniline dissolves readily in organic solvents and in dilute acid (forming a salt).
Organic solvents like alcohol, ether, and benzene are non-polar or weakly polar and have low polarity/no strong hydrogen bonds to break. The rule 'like dissolves like' applies: the hydrocarbon part of the amine mixes freely with the organic solvent. So even big amines that will not dissolve in water dissolve easily in organic solvents.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Roughly up to 6 carbon atoms. Amines smaller than this are soluble; larger ones become practically insoluble in water.
The amine, by far. Alkanes have no polar group, so they are essentially insoluble in water, while the amine's -NH2 forms hydrogen bonds.
Only slightly. Aniline is sparingly soluble in water but dissolves well in organic solvents and in dilute acids as its salt.
Nitrogen is less electronegative than oxygen, so N-H...O hydrogen bonds are weaker than the O-H...O hydrogen bonds of alcohols, giving amines slightly lower water solubility.
Yes. Their hydrocarbon part mixes freely with non-polar organic solvents like ether, alcohol, and benzene, so even water-insoluble amines dissolve in them.
