Physical Properties of Amines

Amines show intermediate physical properties, positioned chemically between

hydrocarbons (no hydrogen bonding at all) and alcohols (strong hydrogen bonding), because

nitrogen is less electronegative than oxygen and so an $\text{N}-\text{H}$ bond is less

polarised than an $\text{O}-\text{H}$ bond.

Boiling point. A primary or secondary amine can both donate a hydrogen bond (via its

$\text{N}-\text{H}$) and accept one (via its lone pair), so $1^\circ$ and $2^\circ$

amines have noticeably higher boiling points than a hydrocarbon of similar molar mass,

which has no hydrogen bonding available at all. But because $\text{N}-\text{H}\cdots \text{N}$ hydrogen bonds are weaker than $\text{O}-\text{H}\cdots\text{O}$ hydrogen bonds

in an alcohol, amines boil at a LOWER temperature than the corresponding alcohol of

comparable molar mass. A tertiary amine, having NO $\text{N}-\text{H}$ at all, cannot

hydrogen-bond to another tertiary amine molecule (it can still accept a hydrogen bond

from water or from an $\text{O}-\text{H}/\text{N}-\text{H}$-bearing partner), so a

tertiary amine boils at a noticeably lower temperature than an isomeric primary or

secondary amine.

Solubility. The lower members of the aliphatic amine series ($\text{C}_1$–$\text{C}_4$,

roughly) are freely miscible with water because every amine, regardless of class, can

accept a hydrogen bond from water via its lone pair (even a tertiary amine, with no

$\text{N}-\text{H}$ of its own, still accepts one this way). Solubility falls off steadily

as the non-polar hydrocarbon portion of the molecule grows, exactly as it does for

alcohols. …