Physical Properties of Amines

PHYSICAL STATE AND SMELL: the lower aliphatic amines (C1-C2) are colourless gases with an ammonia-like smell; those with four or more carbons are volatile liquids with a fish-like smell. Aniline and other arylamines are usually colourless but darken on exposure to air due to oxidation. BOILING POINT: primary and secondary amines can hydrogen-bond intermolecularly through the lone pair on nitrogen, but tertiary amines (no N-H) cannot -- so boiling point order for isomeric amines is 1 degree > 2 degree > 3 degree, e.g. CH3NH2 > (CH3)2NH > (CH3)3N. Amines have LOWER boiling points than alcohols of comparable molecular mass, since nitrogen is less electronegative than oxygen, making the N-H bond less polar than O-H and its hydrogen bonds correspondingly weaker -- e.g. propan-2-ol (mol. mass 60, bp 355 K) boils well above the isomeric-mass amines n-propylamine (mol. mass 59, bp 321 K), ethylmethylamine (mol. mass 59, bp 308 K) and trimethylamine (mol. mass 59, bp 277 K); n-butane (mol. mass 58, bp 272.5 K), with no hydrogen bonding at all, boils lowest of the set. SOLUBILITY: lower aliphatic amines are water-soluble (hydrogen bonding with water), but solubi …