Optical isomerism in halogen derivatives

This section introduces optical isomerism using 2-chlorobutane, CH3-CHCl-CH2-CH3, as the running example. Listing out the four groups attached to each carbon of 2-chlorobutane shows that C-1, C-3 and C-4 each carry at least two identical groups (for example C-1 carries three hydrogens), but C-2 carries four completely different groups: -H, -Cl, -CH3, and -CH2CH3. A carbon atom in a molecule that carries four different groups or atoms is called a chiral carbon atom (marked with an asterisk, *, as in CH3-*CHCl-CH2-CH3), and C-2 of 2-chlorobutane is exactly such a chiral carbon. A molecule containing one chiral atom acquires the unique property that it cannot be superimposed perfectly on its own mirror image -- such a molecule is called a chiral molecule, and a chiral molecule together with its mirror image is a running example carried through the rest of section 10.5: chiral-atom identification (10.5.1), plane polarised light (10.5.2), optical activity (10.5.3), enantiomers (10.5.4), and ways of drawing three-dimensional configuration on paper (10.5.5). Isomers that share the same bond connectivity (the same structural formula) are called stereoisomers in general, and optical is …