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Cambridge Pre-U Chemistry
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Figure S35.7 Inverting just one chiral centre at a time produced two new compounds. They are not mirror images of the original (2R,3R)-2,3-dichloropentane.
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These two new compounds in Figure S35.7 are not mirror images of the original (2R,3R)2,3-dichloropentane. Stereoisomers that are not mirror images are called diastereoisomers. Diastereoisomers have different physical and chemical properties, while the pair of enantiomers differ only in the direction in which they rotate polarised light. Figure S35.8 shows how the four possible stereoisomers of 2,3-dichloropentane are linked to one another.
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Figure S35.8 The four stereoisomers of 2,3-dichloropentane. There are two pairs of enantiomers. The pairs of stereoisomers which are not enantiomers are diastereoisomers.
In the example above, a molecule with two chiral centres has four stereoisomers and two pairs of enantiomers. For a molecule with n chiral centres, the maximum number of stereoisomers can be calculated by simply calculating 2n. For example, a molecule with three chiral centres will have a maximum of 23 = 8 possible stereoisomers, four pairs of enantiomers: R,R,R
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S,S,S S,S,R S,R,R S,R,S The structure and stereoisomers of tartaric acid are shown in Figure S35.9. Two chiral centres are present, so a maximum of four stereoisomers should be possible. However, two of the potential isomers are in fact the same molecule and although this molecule contains chiral centres, it is achiral because it is superimposable with its mirror image. R,S-tartaric acid becomes the S,R-tartaric acid when the molecule is rotated by 180o. Compounds which contain chiral centres but are themselves achiral are called meso compounds. This means
Original material Š Cambridge University Press 2016
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