Optical isomerism and chirality
A chiral centre is a carbon with four different groups attached. Molecules with one chiral centre exist as two enantiomers: non-superimposable mirror images. Enantiomers have identical physical properties except that they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture (50 : 50) is optically inactive. Reactions through planar intermediates (carbonyl addition, SN1 carbocations) give racemic products; SN2 gives a single enantiomer with inversion. Drugs are often made as single enantiomers because only one fits the biological target (thalidomide is the classic warning).
≈ 40 min