Eduqas Chemistry for A Level Year 2: Student Bk

117 OA1.1 Stereoisomerism Optical activity Optical activity occurs in molecules with chiral centres. Light consists of waves that are vibrating in all planes. If light is passed through a polarising filter (P) (e.g. a piece of Polaroid), the light that emerges is vibrating in one plane only. This light is called plane polarised light. If a solution of an enantiomer (R) is placed in a beam of plane polarised light, the beam is rotated. The instrument used to measure the amount of rotation is called a polarimeter. The extent to which the plane of polarised light is rotated depends on: The particular enantiomer The concentration of the enantiomer in the solution The length of the tube containing the solution through which the light passes The frequency of the light source used and the temperature also need to be considered. An enantiomer may rotate the plane of polarised light to the right (+) or to the left (–). Equal amounts of each enantiomer in a solution produce no overall rotation since the rotation effects cancel each other out. This mixture results from external compensation, as the effect is caused by two different compounds. This equimolar mixture is called a racemic mixture. The painkiller ibuprofen was first made in 1961. OH O Ibuprofen contains a chiral centre and is sold as a racemic mixture. Since biological systems usually respond to one enantiomer in the required way rather than the other, there is a danger that the ‘unwanted’ form could have serious and unwanted side effects. Separation of the two enantiomers (called resolution) would be difficult and expensive. When ibuprofen is taken, one of the enantiomers is much more biologically active than the other but fortunately the less active form is converted by an enzyme in the body into the other, more active enantiomer. 3 Knowledge check Explain why a racemic mixture can be described as an equimolar mixture or as a mixture containing equal amounts of both enantiomers. 4 Knowledge check Malic acid occurs in unripe apples. Its systematic name is 2-hydroxybutanedioic acid. Write the displayed formula for malic acid and identify any chiral centre present in its formula. 5 Knowledge check Explain why the unsaturated acid that has the formula CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH exists as E-Z isomers.