Chirality and optical isomerismEdexcel A-Level Chemistry: Mind map

Exam questions on Chirality and optical isomerism

1. A student examines four alcohols: propan-2-ol, butan-2-ol, 2-methylpropan-2-ol and pentan-3-ol. Only one of them exists as a pair of optical isomers.
   Explain why butan-2-ol shows optical isomerism.2 marks
2. A polarimeter measures the rotation of plane-polarised monochromatic light by solutions of 2-chlorobutane. Sample P is a pure single enantiomer and rotates the plane of polarised light by +20°. Sample Q is the other enantiomer of 2-chlorobutane. Sample R is made by mixing equal amounts of P and Q, with all solutions at the same total concentration.
   Explain why Sample R shows no optical activity.2 marks
3. Lactic acid, CH₃CH(OH)COOH, exists as two optical isomers. In a polarimeter, a solution of one pure enantiomer of lactic acid at a concentration of 0.10 mol dm⁻³ rotates plane-polarised monochromatic light by +6.0°. The other enantiomer rotates it by the same angle in the opposite direction. A student prepares a second solution, also with total concentration 0.10 mol dm⁻³, containing 75% of the (+) enantiomer and 25% of the (−) enantiomer by amount.
   Identify the chiral centre in lactic acid and explain why this molecule has two enantiomers. State how the two enantiomers differ in their effect on plane-polarised light.3 marks