Impact of Chirality Enantiomers

Chirality is an important aspect of aroma chemicals since enantiomers of the same compound may possess different organoleptic characters. Chirality means the occurrence of one or more asymmetric carbon atoms in an organic molecule. Such molecules exhibit optical activity and therefore have the ability to rotate plane-polarised light by equal amounts but in opposite directions. In other words, two stereoisomers which are mirror images of each other are said to be enantiomers. If two enantiomers exist in equal proportions, then the compound is called racemic. Enantiomers can be laevorotatory (, I, -, S), meaning rotating the plane of the polarised light to the left or dextrorotatory (°, d, -f, R), that is. 

Z)-2-methylbatanoic acid 201 (5)-2-methylbutanoic acid Fruity, sweet 

Many natural compounds originating from essential oils which are used in perfumes, flavours and fragrances are optically active. Each enantiomer may display entirely different organoleptic properties. Each enantiomer may be characteristic for a particular essential oil source. Some examples are given in Fig 4.9, illustrating frequently used compounds. 

The pattern of distribution of enantiomers may serve as fingerprints to prove the authenticity of a certain essential oil or its adulteration. As high ratio of 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

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