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Isomers with atoms having the same order of bonding, but different spatial arrangements are called stereoisomers. Stereoisomers which are non-superimposable mirror images are called enantiomers. Diastereomers are stereoisomers that are not mirror images.

image\Ch 19 fig 19-2.png

In glycine, the carbon indicated by the arrow is achiral because the carbon has two hydrogen (two of the same substituents) attached to it. That is not the case in alanine, and thus the carbon indicated by the arrow is chiral.
Figure 19-2

A carbon which is sp3 hybridized (tetrahedral structure) with four different substituents is called a chiral carbon. If the carbon doesn't have four different substituents or say at least the carbon has two of the same substituents, then it is an achiral carbon.


Enantiomers are compounds with same molecular formula and are nonsuperimposable mirror images. Look at the example shown in Figure 19-3.

image\Ch 19 sec B.png

Figure 19-3


Diastereomers have the same molecular formula, but they do not have a mirror-image relationship to each other. Study the examples in Figure 19-4.

image\Ch 19 fig 19-4.png

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