Figure 11. Enantiomers of (+) and ( ) Methylallylphenyl-benzyl ammonium Iodide

A somewhat spectacular example is contrasting interactivity and physiological effect of the different enantiomers of carvone. Carvone ( ) is the substance that imparts the characteristic aroma to Spearmint oil whereas the (+) carvone is the essential characteristic constituent of Caraway seed oil.

A mixture of the (+) and ( ) enantiomers in equal proportions is a racemic modification called the racemate and is optically inactive. The optical inactivity results from the opposite rotation of each isomer canceling each other out. The racemic mixture is designated as ( ) (e.g. ( ) lactic acid). As enantiomers are chemically identical they cannot be easily separated by the usual separation techniques such as distillation. Hence special separation techniques need to be employed and one of the more popular is chiral chromatography.

The structure and nature of the substituent groups associated with the chiral atom determines the extent of the rotation. Groups of very similar types, although different, will make very small contributions to the amount of rotation. An example of this small contribution of similar groups to the overall rotation is demonstrated by ethyl-n-propyl-n-butyl-n-hexyl methane the structure of which is shown in figure 12. It should be noted that all the substituent groups are simple hydrocarbons but each substituent has a different chain length.

Figure 12. The Structure of Ethyl-n-propyl-n-butyl-n-hexyl Methane

The hydrocarbon, although optically active, has a very small rotation 0.00001 which has to be calculated as it is too small to be experimentally measured.