3.2. Refresher and Expansion: Geometry and Stereogenicity of Amines and Ammonium Cations
Recall that Valence Shell Electron Pair Repulsion (VSEPR) theory states that the most stable three-dimensional structure for a molecule is the one in which valence electron pairs are as far apart as possible. In practical terms VSEPR theory amounts to keeping groups (attached atoms or lone pairs) as far apart from each other as possible. All amines have four attached groups: three atoms/groups of atoms and one lone pair. As long as the amine cannot participate in conjugation (resonance), having four groups means the three-dimensional structure they adopt is tetrahedral (Figure 3.5). Alternatively, the hybridization of the nitrogen is described as sp3 hybridized.
Figure 3.5 – Examples of Generic Amines Highlighting Geometry Around the Nitrogen.
If the nitrogen of an amine can participate in delocalization/resonance then the geometry is less intuitive. Recall that depending on which resonance form is depicted the hybridization [geometry] of atoms may appear to change. For example, the nitrogen in N,N-dimethylprop-1-en-2-amine appears to be sp3 hybridized [tetrahedral] in the (a) form but sp2 hybridized [trigonal planar] in the (b) form (Figure 3.6).
Figure 3.6 – Apparent Hybridization of Nitrogen in Resonance Forms of N,N– Dimethylprop-1-en-2-amine.
Recall that resonance cannot move atoms, only electrons. The geometry cannot be changing, it must be one or the other. In this case the nitrogen has a trigonal planar geometry and/or is described as being sp2 hybridized. Recall that resonance is highly stabilizing. The nitrogen requires a p orbital to form a delocalized electron cloud. It is not possible to have an available p orbital and be sp3 hybridized. The nitrogen adopts sp2 hybridization to be able to participate in resonance. This sets its three-dimensional geometry as trigonal planar.
This is true of any molecule that has multiple resonance forms. Technically, each atom has the hybridization with the highest s character from all its resonance forms. This wording is often confusing. An alternative phrasing is “each atom has the hybridization made from the fewest p orbitals of all its resonance forms”. For example, the nitrogen above appears to be either sp2 or sp3. The sp2 orbitals are made from an s orbital and two p orbitals. The sp3 orbitals are made from an s orbital and three p orbitals. Two is smaller than three, so the hybridization on nitrogen is sp2 instead of sp3. The hybridization is a synonym for the geometry.
Conversely, the geometry of the nitrogen in any ammonium cation is always tetrahedral (sp3 hybridized). None of the ammonium cations have a lone pair on nitrogen to participate in resonance (Figure 3.7). This means their geometry will always be straightforward.
Figure 3.7 – Examples of Generic Ammonium Cations Highlighting Geometry Around the Nitrogen.
Recall that nitrogen atoms with a lone pair on them are not (usually) stereogenic. This is because the lone pair on the nitrogen is able to ‘move’ from one side to the other, causing the two isomers to interconvert (Figure 3.8). This process is sometimes referred to as (pyramidal) inversion. Why lone pairs on nitrogen are able to do this is beyond the scope of this text. This means that almost all amines will not be stereogenic at the nitrogen.
Figure 3.8 – Interconversion of Stereoisomers Due to Inversion at Nitrogen Atoms with Lone Pairs.
Nitrogen atoms with lone pairs can be stereogenic if they are part of highly rigid systems and thus cannot undergo inversion. Usually this requires a geometric constraint such as a ring system (Figure 3.9). These types of stereogenic centres are rare.
Figure 3.9 – Example of Stereogenic Nitrogen Atoms with Lone Pairs.
Again, the nitrogen in an ammonium cation does not have a lone pair. If it meets the other conditions required, the nitrogen of an ammonium cation can be stereogenic (Figure 3.10). In general, these types of stereocentres are transient (e.g. an ammonium cation intermediate in an acid-base mechanism) and/or uncommon in nature (e.g. stereogenic quaternary ammonium cations). However, in recent years they have begun to be explored in synthetic organic chemistry for a variety of applications.
Figure 3.10 – Examples of Ammonium Cations Highlighting Stereogenicity at the Nitrogen.
