Enantiomers behave in the same way in ordinary test tube reactions. Physical properties like melting, density and solubility are also identical. But enantiomers behave differently when in the presence of other chiral molecules. For example, enantiomers can taste differently due to the chiral nature of the taste bud receptors in the tongue. One enantiomeric form of amino acids all taste sweet, the other isomer is sometimes bitter. Chemicals with different enantiomeric forms can smell different. One isomer of limonene (Figure 17) smells of oranges, the other of lemons.

Figure 17 - Limonene enantiomers

Enantiomers have an opposite effect on plane polarised light. This is why they are said to be optically active. An instrument called a polarimeter ( ) can detect this effect.

The polarimeter

Many medicines are produced as a mixture of enantiomers, only one of which is pharmacologically active, as it can prove costly to separate the isomers. One of the enantiomers of salbutamol used in the treatment of asthma, is 68 times more effective than the other (see Unit 3, Topic10, section 4). Great care has to be taken when using drugs with enantiomeric forms as this has led to tragedy in the past, specifically in the case of thalidomide (Figure 19). A mixture of the isomers was used to treat nausea during pregnancy, and one enantiomer, which was thought to be inactive, turned out to cause damage to the unborn child. Many handicapped babies were born before the drug was recognised as being responsible. Screening of pharmaceuticals has to be very thorough. Regulations were tightened significantly after the thalidomide tragedy to ensure that both enantiomeric forms of chiral drugs are tested.

Figure 19 - Thalidomide

Mechanisms for substitution reactions

In Unit 3, Topic 2, the S_N2 mechanism for nucleophilic substitution reactions of halogenoalkanes was described (Figure 20). The strongest evidence for this mechanism comes from the use of chiral compounds.

Figure 20 - SN2 mechanism

If this mechanism occurs with a chiral compound, then the product will also be chiral but with the configuration inverted (a bit like an umbrella being blown inside out by a strong wind).

Figure 21 - SN1 mechanism, step1

Figure 21 shows the mechanism for the S_N1 reaction which occurs in two steps.

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