© www.chemsheets.co.uk A2 011 12-Jul-12 • All molecules have a mirror image – but for most molecules it is the same molecule. H H H C C H F H F H fluoromethane © www.chemsheets.co.uk A2 011 12-Jul-12 • 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 © www.chemsheets.co.uk A2 011 12-Jul-12 • 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 © www.chemsheets.co.uk b A2 011 12-Jul-12 • 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. © www.chemsheets.co.uk A2 011 12-Jul-12 TASK Some of the following molecules are optically active. For each one, click its name below and decide whether it is optically active or not. Click again to see if you are correct. a) propan-2-ol e) butanone b) 2-chlorobutane f) 2-methylbutanoic acid c) 1-chlorobutane g) butan-2-ol d) 3-methylhexane h) 1-chloro-3-methylpentane © www.chemsheets.co.uk A2 011 12-Jul-12 propan-2-ol CH3 CH CH3 OH NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 2-chlorobutane CH3 CH CH2 CH3 Cl H CH2CH3 CH2CH3 C C CH3 Cl H3C Cl H OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 1-chlorobutane CH2 CH2 CH2 CH3 Cl NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 3-methylhexane CH3 CH2 CH CH2 CH2 CH3 CH3 H CH2CH2CH3 CH2CH2CH3 C C CH3 CH2CH3 CH3 CH3CH2 H OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 O butanone CH3 C CH2 CH3 NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 propan-2-ol CH3 CH CH3 OH NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 CH3 O 2-methylbutanoic acid CH3 CH3 CH2 CH C OH CH2CH3 CH2CH3 C C H COOH H HOOC CH3 OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 OH butan-2-ol CH3 CH3 CH2 CH CH3 CH2CH3 CH2CH3 C C H OH H HO CH3 OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 CH3 1-chloro-3-methylpentane CH3 CH3 CH2 CH Cl CH2 CH2 CH2CH3 CH2CH3 C C H CH2CH2Cl H CH2ClCH2 CH3 OPTICALLY ACTIVE Click here to go back to the optical isomerism task © www.chemsheets.co.uk A2 011 12-Jul-12 • 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 © www.chemsheets.co.uk A2 011 12-Jul-12 • Light is a form of electromagnetic radiation. • The wave vibrations are perpendicular to the direction of travel of the wave. normal light (waves vibrate in all directions) plane-polarised light (vibrates in only one direction) © www.chemsheets.co.uk plane-polarised light after clockwise rotation A2 011 12-Jul-12 • Optical isomers rotate the plane of plane polarised light. (-)-enantiomer (anticlockwise rotation) (+)-enantiomer (clockwise rotation) © www.chemsheets.co.uk (±)-racemate (no overall effect) A2 011 12-Jul-12 POLARIMETERS can be used to analyse the effect optical isomers have on plane polarised light: • 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. © www.chemsheets.co.uk A2 011 12-Jul-12 • 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). © www.chemsheets.co.uk A2 011 12-Jul-12 • 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. © www.chemsheets.co.uk A2 011 12-Jul-12 • 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. The photographs are both from ‘Molecule of the Month’ at Bristol University: http://www.chm.bris.ac.uk/motm/thalidomide/start.html O NH O O O H2C NH O C C N CH2 N H H2C CH2 H O S thalidomide (effective drug) O O R thalidomide (dangerous drug) The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. © www.chemsheets.co.uk A2 011 12-Jul-12 • 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). © www.chemsheets.co.uk A2 011 12-Jul-12 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 CH3 S limonene (lemons) H C CH2 H3C R limonene (oranges) © www.chemsheets.co.uk A2 011 12-Jul-12 GEOMETRIC ISOMERISM CH3 X C H H C CH3 CANNOT ROTATE AROUND C=C © www.chemsheets.co.uk A2 011 12-Jul-12 GEOMETRIC ISOMERISM CH3 H C CH3 C H CH3 C CH3 (E) but-2-ene E entgegen (opposite) C H H (Z) but-2-ene Z zusammen (together) © www.chemsheets.co.uk A2 011 12-Jul-12 GEOMETRIC ISOMERISM PRIORITY: H < CH3 < CH2CH3 Look at atomic numbers of atoms attached to C=C (or subsequent atoms if the same). Highest values opposite = E Highest values together = Z 9 F CH3 C 6 12 CH3 6 12 1 6, 612 CH2CH3 12, H C C Br 35 E 2-bromo-3-fluorobut-2-ene 12 6 C CH3 CH3 12 6 E 3-methylpent-2-ene © www.chemsheets.co.uk A2 011 12-Jul-12 GEOMETRIC ISOMERISM CH3 H C CH3 C H CH3 C CH3 C H H trans but-2-ene cis but-2-ene (E) but-2-ene (Z) but-2-ene BOTH C ATOMS OF C=C MUST HAVE TWO DIFFERENT GROUPS © www.chemsheets.co.uk A2 011 12-Jul-12 TASK CH3 H C NO C CH3 H methylpropene H H C CH3 H C C Z Cl Cl CH3 YES C E H 1-chloropropene © www.chemsheets.co.uk A2 011 12-Jul-12 TASK CH2 CH3 C CH3 NO C H CH3 2-methylpent-2-ene H CH2 C H CH2 CH3 NO C CH2 CH3 2-ethylpent-1-ene © www.chemsheets.co.uk A2 011 12-Jul-12 TASK CH3 CH3 C C E H CH3 H CH2 CH3 CH3 YES C C Z CH2 CH3 3-methylpent-2-ene H H C H NO C CH2 CH3 but-1-ene © www.chemsheets.co.uk A2 011 12-Jul-12 TASK CH3 CH3 C CH3 CH2 CH3 C Z CH2 C CH2 CH3 CH3 CH2 CH3 YES C E CH3 3,4-dimethylhex-3-ene CH3 CH3 CH3 Cl YES C Br C C Z Cl Br C E CH3 2-bromo-3-chlorobut-2-ene © www.chemsheets.co.uk A2 011 12-Jul-12