Basicity and physical properties

Boiling points. Primary and secondary amines have N–H bonds, so they form hydrogen bonds with each other. Tertiary amines have no N–H, so they cannot. For amines of the same mass, the order is 1° > 2° > 3°. Since N is less electronegative than O, these hydrogen bonds are weaker than in alcohols, so amines boil lower than alcohols of similar mass.

Solubility. Small amines dissolve in water, forming hydrogen bonds with it. As the carbon chain grows, the oily, water-hating (hydrophobic) part takes over and solubility falls. Aniline and other higher amines hardly dissolve.

Colour. Pure aryl amines such as aniline are colourless, but they slowly turn coloured when stored, because air oxidises them.

Basic strength. The lone pair on nitrogen can pick up an H⁺, so amines are bases. The more available the lone pair, the stronger the base. Basic strength is often compared using pK_b: the smaller the pK_b, the stronger the base.

• Alkyl groups push electrons towards the nitrogen (+I effect), making the lone pair more available. So aliphatic amines are stronger bases than ammonia. In the gas phase, where nothing else interferes, the order is simply 3° > 2° > 1° > NH₃.
• In water, two more things matter: how well the ion formed is stabilised by hydrogen bonds with water (solvation), and how crowded the nitrogen is (steric effect). These mix up the order. For the ethylamines in water, it is (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃; for the methylamines, (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃.

Aryl amines are much weaker bases than ammonia. In aniline, the nitrogen's lone pair is spread into the benzene ring by resonance, so it is less available to grab an H⁺. Electron-withdrawing groups on the ring, such as –NO₂, make the base weaker still; electron-releasing groups, such as –CH₃ or –OCH₃, make it a little stronger.