Structuur van aminen

Amines consist of an sp³ hybridized nitrogen atom exhibiting a trigonal pyramidal molecular geometry with nitrogen at the apex and the three bonded groups forming the pyramid base.

If the nonbonding electrons are considered as the fourth group, the geometry of amine nitrogen is approximately tetrahedral, with 108° bond angles and a C–N bond length of 147 pm.

Amines bearing three different substituents are chiral with nitrogen as the stereogenic center. They exist as enantiomers, and the nonbonding electrons are assigned the lowest priority while naming.

The two enantiomers can interconvert rapidly via pyramidal inversion.

During inversion, the enantiomer passes through an sp² hybridized planar transition state, followed by rehybridization to give an inverted tetrahedral configuration.

Due to the low energy barrier, this interconversion can take place at room temperature, and the two enantiomers are difficult to resolve, resulting in a racemic mixture.

On the contrary, chiral quaternary ammonium ions—with no lone pairs for inversion—are easily resolved.