Optical isomerism and chiralityAQA A-Level Chemistry: Mind map

Exam questions on Optical isomerism and chirality

1. Lactic acid, CH₃CH(OH)COOH, builds up in muscle tissue during hard exercise and is also the acid that makes milk taste sour. Its molecules show optical isomerism.
   Explain why lactic acid exists as a pair of optical isomers.2 marks
2. A chemist has two samples, X and Y, of the same chiral compound. When each is placed separately in a polarimeter, X rotates the plane of plane-polarised light by 12° clockwise and Y rotates it by 12° anticlockwise. When equal masses of X and Y are dissolved together in the same volume of solvent, the solution produces no rotation at all.
   Explain why the mixture of X and Y produces no rotation of plane-polarised light.2 marks
3. The amino acid alanine, CH₃CH(NH₂)COOH, is found in proteins. Glycine, H₂NCH₂COOH, is the simplest amino acid. A student synthesises alanine in the laboratory from starting materials that are not chiral.
   Alanine is chiral. Identify the chiral centre in alanine and the four groups bonded to it, and state how its two enantiomers differ in their effect on plane-polarised light.3 marks