Explain optical isomerism in 2-chlorobutane.

Answer:

2-Chlorobutane is CH₃–CH(Cl)–CH₂–CH₃. Its C-2 carries four different groups: –H, –Cl, –CH₃ and –C₂H₅. C-2 is therefore a chiral (asymmetric) carbon and the molecule is chiral (no plane of symmetry).

Hence the four groups can be arranged in space in two different ways, giving two stereoisomers that are non-superimposable mirror images of each other, i.e. a pair of enantiomers:

[Figure: two tetrahedral (wedge–dash) structures of C-2 drawn as mirror images; in one H, Cl, CH₃, C₂H₅ are arranged clockwise and in the other anticlockwise]

The two enantiomers have identical physical and chemical properties except the direction in which they rotate plane-polarised light: one is dextrorotatory (+)-2-chlorobutane and the other laevorotatory (−)-2-chlorobutane, by equal angles. An equimolar mixture of the two is optically inactive and is called a racemic mixture (±).