12201 13 Thalidomide was first marketed in 1957 as a sedative to reduce morning sickness during pregnancy. It contains a chiral centre and exists as two optical isomers. O O O N NH O thalidomide (a) Circle the chiral centre in the structure above. [1] (b) Define the term optical isomers. [1] (c) The original use of thalidomide led to babies being born with deformed limbs. It was found that one optical isomer of thalidomide caused these birth defects. Suggest how the drug action may be determined by the stereochemistry of the drug. [1] (d) Thalidomide, in the sedative, was used as a racemic mixture. Define the term racemic mixture and explain why a racemic mixture would be optically inactive. [2]