Q.Optical isomerism in 2-chlorobutane.

Step 1. Identify the chiral carbon. Listing the four groups attached to each carbon of 2-chlorobutane, CH3-CH(Cl)-CH2-CH3, shows that C-1, C-3 and C-4 each carry at least two identical attached groups, but C-2 carries four completely different groups: -H, -Cl, -CH3 and -CH2CH3 (an ethyl group). A carbon bonded to four different groups is called a chiral carbon (marked with an asterisk: CH3-*CHCl-CH2CH3).

Step 2. State the consequence for the whole molecule. A molecule containing one chiral carbon cannot be superimposed perfectly onto its own mirror image -- it is a chiral molecule. 2-Chlorobutane and its mirror image are therefore two distinct, non-superimposable stereoisomers, called a pair of enantiomers (Fig. 10.1).

Step 3. State how this is detected experimentally. Each enantiomer, in solution, rotates the plane of plane polarised light passed through it -- one enantiomer rotates it clockwise (dextrorotatory, d- or +), the other rotates it anticlockwise by the exact same angle (laevorotatory, l- or -). This measurable rotation is optical activity, and the phenomenon -- isomers differing only in this optical property -- is called optical isomerism. …

Working systematically through the four groups attached to each carbon of 2-chlorobutane, identifying the one chiral centre, and then building up chirality -> enantiomers -> optical activi …

• Checking only C-2 and stopping, without first showing that the OTHER three carbons are NOT chiral -- a complete explanation should make clear why C-2 specifically is special. …