But in some cases enantiomers of amines can be resolved. For example, methyl ethyl isopropyl anilinium salt (a quaternary ammonium salt) can be resolved into enantiomers as there is no lone pair to undergo nitrogen inversion. Chiral aziridines may also be resolved into enantiomers. Such a compound has higher activation energy for inversion and cannot attain the sp 2 hybrid transition state (Figure 5).
Figure 5
5.9 Physical Properties
Amines are strongly polar because they have large dipole moments. Primary and secondary amines do form hydrogen bonding but tertiary amines do not, because they have no N-H bonds. While comparing alcohols, amines form weaker hydrogen bond as nitrogen is less electronegative than oxygen. Tertiary amines have lower boiling points than primary and secondary amines of similar molecular weight. Amines, including tertiary amines, form hydrogen bonds with hydroxylic solvents such as water and alcohols. Therefore, amines tend to be soluble in alcohols, and the lower-molecular weight amines (up to six carbon atoms) are soluble in water.
Amines, like ammonia, are strong enough bases that they are completely protonated in dilute acid solutions. Basicity can be increased by increasing the availability of lone pair and stabilizing the resultant positive charge. When one hydrogen of ammonia replaced by methyl group, it stabilizes the resultant positive charge formed on protonation. Alkyl groups have the tendency to stabilize charge through a polarization effect. This effect increases on increasing numbers of the alkyl groups. There is another stabilizing effect called a solvent effect.
Every hydrogen attached directly to nitrogen can form hydrogen bond with solvent water and this also stabilizes the charge. This effect decreases with increasing numbers of alkyl groups. So the observed basicity is the combination of these two effects. Most of tertiary amines are less basic than that of primary amine because bulky groups in the tertiary amines make the lone pair less available. Lone pair held in a sp 2 or a sp orbital is much harder to protonate. So the basicity decreases from amine to imine to nitrile (Fig. 5.22).
Figure 6
While comparing cyclohexanamine and aniline, aniline is a weak base because the lone pair can be delocalized into the benzene ring (Figure 7). In order for the lone pair to be fully conjugated with the benzene ring, the nitrogen would have to be sp 2 hybridized with the lone pair in the p -orbital. This is not found to be the case. Instead, the plane of the NH2 group is about 40° away from the plane of the ring, which means the lone pair is partially conjugated into the ring.
Figure 7