O

(S)-(+)-carvone spearmint odour lower threshold

(fl)-(-)- carvone caraway odour higher threshold

Figure 2.21 Some examples of enantiomers which we are able to distinguish by taste and odour

The natural juvenile hormone of insects (+)-JH I (Chapter 6) is about 10,000 times more active than its enantiomer (-)-JH I. Cholesterol (Chapter 7) has nine asymmetric centres, yet exists naturally as only one enantiomer. The usual cause of chirality in organic molecules is an asymmetric carbon atom, one with four different groups attached (Figure 2.22). The rules for describing the arrangement of groups about an asymmetric carbon atom, the Cahn-Ingold-Prelog rules, can be found in any textbook of organic chemistry. The priority of atoms or groups are assigned in descending order of atomic mass (Figure 2.22).

(S)-bromochloro-fluoromethane

H Br

(f?)-bromochloro-fluoromethane

Figure 2.22 Some simple examples of chiral molecules. The numbers denote priority of groups according to the Cahn-Inglod-Prelog rules. (S)-(+)-alanine and (R)-(-)-lactic acid are naturally occurring compounds. Note there is no relation between the assignment of S and R to a chiral centre and optical rotation

2.4.1 Asymmetric Induction

The formation of a chiral substance from an achiral starting material is called asymmetric induction. Glycerol has a symmetric molecule, but when it is held on the surface of an enzyme and one hydroxyl group is

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