19.5:
Structure of Amines
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.
19.5:
Structure of Amines
The hybridized nitrogen atom in amines possesses a lone pair of electrons and is bound to three substituents with a bond angle of around 108°, which is less than the tetrahedral angle of 109.5°. However, the C–N–H bond angle is slightly larger at 112°, with a carbon–nitrogen bond length of 147 pm. This carbon–nitrogen bond length of of amines is longer than the carbon–oxygen bond of alcohols (143 pm) but shorter than alkanes’ carbon–carbon bond (154 pm). These aspects are illustrated in Figure 1.
Figure 1. The influence of the lone pair on the bond angle and bond lengths in amines.
Aliphatic amines have trigonal pyramidal geometry, where the pyramid’s apex is occupied by nitrogen, and the three substituents extend toward the triangular base of the pyramid. The nonbonded electron pair is present above the apex of the nitrogen atom. The nitrogen atom in amines with its lone pair and three different substituents becomes a stereogenic center. This results in two nonsuperposable mirror images of enantiomers. The configuration of these enantiomers can be assigned by giving the lowest priority to the unshared electron pair on the nitrogen atom.
Figure 2. Pyramidal inversion in amines with a lone pair.
As shown in Figure 2, there is a rapid interconversion via the pyramidal inversion of one enantiomer to the other through an sp2-hybridized transition state, making the resolution of enantiomers difficult. As a result, such chiral amines are optically inactive. For most simple amines, the energy barrier of interconversion is about 25 kJ mol−1.
However, this is not the case with quaternary ammonium salts. As depicted in Figure 3, quaternary ammonium salts, with four different substituents and no electron pair, don’t undergo pyramidal inversion. Hence, they can be easily resolved. Besides quaternary ammonium salts, amines with chiral carbons and amines which cannot attain an sp2-hybridized transition state can also be separated into a pair of enantiomers.
Figure 3. The chiral enantiomers of quaternary ammonium salts.