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OPTICAL ISOMERISM • All molecules have a mirror image – but for most molecules it is the same molecule. H H H C C H F H F fluoromethane H • For some molecules the mirror image is a different molecule (the mirror image is non-superimposable). H OH OH C C COOH CH3 (-) lactic acid in sour milk HOOC H3C H (+) lactic acid in muscles • Left and right hands are an example of non-superimposable mirror images. • This usually happens when a molecule contains a C atom with four different groups attached (chiral / asymmetric C). • Such molecules are said to be chiral or optically active. b a a C C d c d c b • The optical isomers are called enantiomers. • These are distinguished by +/-, D/L or more correctly R/S. • A 50/50 mixture of the two enantiomers is called a racemic mixture or a racemate. TASK Which of the following molecules are optically active? 1) 2) 3) 4) propan-2-ol 2-chlorobutane 1-chlorobutane 3-methylhexane 5) 6) 7) 8) butanone 2-methylbutanoic acid butan-2-ol 1-chloro-3-methylpentane propan-2-ol CH3 CH CH3 OH NOT OPTICALLY ACTIVE 2-chlorobutane CH3 CH CH2 CH3 Cl H CH2CH3 CH2CH3 C C CH3 Cl H3C Cl OPTICALLY ACTIVE H 1-chlorobutane CH2 CH2 CH2 Cl NOT OPTICALLY ACTIVE CH3 3-methylhexane CH3 CH2 CH CH2 CH2 CH3 CH3 H CH2CH2CH3 CH2CH2CH3 C C CH3 CH2CH3 CH3 CH3CH2 OPTICALLY ACTIVE H O butanone CH3 C CH2 CH3 NOT OPTICALLY ACTIVE propan-2-ol CH3 CH CH3 OH NOT OPTICALLY ACTIVE 2-methylbutanoic acid CH3 CH3 CH2 CH3 O CH C CH2CH3 CH2CH3 C C H COOH H HOOC OPTICALLY ACTIVE OH CH3 OH butan-2-ol CH3 CH3 CH2 CH CH3 CH2CH3 CH2CH3 C C H OH H HO OPTICALLY ACTIVE CH3 1-chloro-3-methylpentane CH3 CH3 CH3 CH2 CH Cl CH2 CH2 CH2CH3 CH2CH3 C C H CH2CH2Cl H CH2ClCH2 OPTICALLY ACTIVE CH3 • Molecules that are optical isomers are called enantiomers. • Enantiomers have identical chemical and physical properties, except: • Their effect on plane polarised light; • Their reaction with other chiral molecules • Light is a form of electromagnetic radiation. • The wave vibrations are perpendicular to the direction of travel of the wave. normal light (w aves vibrate in all directions) plane-polarised light (vibrates in only one direction) plane-polarised light after clockw ise rotation • Optical isomers rotate the plane of plane polarised light. (-)-enantiomer (anticlockw ise rotation) (+)-enantiomer (clockw ise rotation) (±)-racemate (no overall effect) • Chiral molecules often react differently with other chiral molecules. • This is like the idea that a right hand does not fit a left handed glove – the molecule must be the correct shape to fit the molecule it is reacting with. • Many natural molecules are chiral and most natural reactions are affected by optical isomerism. • For example, most amino acids (and so proteins) are chiral, along with many other molecules. • In nature, only one optical isomer occurs (e.g. all natural amino acids are rotate polarised light to the left). • Many drugs are optically active, with one enantiomer only having the beneficial effect. • In the case of some drugs, the other enantiomer can even be harmful, e.g. thalidomide. • In the 1960’s thalidomide was given to pregnant women to reduce the effects of morning sickness. • This led to many disabilities in babies and early deaths in many cases. O NH O O O H2C NH O C C N CH2 H O S thalidomide (effective drug) The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. O N H2C CH2 H O R thalidomide (dangerous drug) • Thalidomide was banned worldwide when the effects were discovered. • However, it is starting to be used again to treat leprosy and HIV. • Its use is restricted though and patients have to have a pregnancy test first (women!) and use two forms of contraception (if sexually active). CH3 CH3 O O H C CH2 H3C S carvone (caraway seed) Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed overtones. H2C C H CH3 R carvone (spearmint) CH3 CH3 CH2 C H H C CH2 CH3 H3C S limonene (lemons) R limonene (oranges)