Physical and Bond Properties of Amines

In general boiling and melting points of amines are higher than those of the corresponding alkanes. Hydrogen bonding is important between amine molecules having at least one N-H group.

Boiling and melting points of propane, propylamine and propanol, and of trimethylamine, dipropylamine and tripropylamine

Compared with amines, alcohols of similar molecular weight and size have higher physical constants. This means that N–H···N hydrogen bonds are weaker than O–H···O ones.

Secondary amines, and even tertiary ones where hydrogen bonding is not possible, have relatively high b.p. because of their size and the increase in van der Waals attraction among molecules.

Pyramidal structure and nitrogen inversion

Pyramidal geometry of methylamine: N–H 1.01 Å and N–C 1.47 Å distances and angles of 105.9° and 112.9°; the nitrogen is sp3-hybridized

The mechanism of this inversion process goes through a TS where nitrogen is planar and sp² hybridized. The energy barrier to nitrogen inversion is in general so low that the process happens for example in ammonia about 2·10¹⁰ times per second at room temperature!

Inversion of the nitrogen of an amine through a planar transition state, with a barrier of 5-7 kcal/mol

Nitrogen inversion interconverts the two enantiomers of N-methylethylamine

A consequence of this inversion is that, in general, amines are NOT configurationally stable.

That is to say, if object and image are not superimposable, the nitrogen inversion interconverts one another and, inevitably, racemization takes place.

In quinuclidine the bicyclic framework prevents nitrogen inversion

A methoxylated Tröger’s base: some amine nitrogens retain their configuration

Nitrogen cannot invert because it is included in a bicycle.

Nitrogen is a stereogenic center because it bears three different substituents plus the electron lone pair. It cannot invert and the molecule is chiral: there is a pair of enantiomers that in this particular structure cannot interconvert

In ammonium salts, nitrogen is tetrasubstituted and, like a carbon, it maintains its configurational integrity and becomes a stereogenic center.

The two (S) and (R) enantiomers of a quaternary ammonium salt with four different substituents