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© A Jul-12
All molecules have a mirror image – but for most molecules it is the same molecule. © A Jul-12
For some molecules the mirror image is a different molecule (the mirror image is non-superimposable). © A 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. © A 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. © A 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 b) 2-chlorobutane c) 1-chlorobutane d) 3-methylhexane e) butanone f) 2-methylbutanoic acid g) butan-2-ol h) 1-chloro-3-methylpentane © A Jul-12
propan-2-ol NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
2-chlorobutane OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
1-chlorobutane NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
3-methylhexane OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
butanone NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
propan-2-ol NOT OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
2-methylbutanoic acid OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
butan-2-ol OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A Jul-12
1-chloro-3-methylpentane OPTICALLY ACTIVE Click here to go back to the optical isomerism task © A 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 © A Jul-12
Light is a form of electromagnetic radiation.
The wave vibrations are perpendicular to the direction of travel of the wave. © A Jul-12
Optical isomers rotate the plane of plane polarised light. © A 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. © A 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). © A 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. © A 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:
S thalidomide (effective drug) The body racemises each enantiomer, so even pure S is dangerous as it converts to R in the body. R thalidomide (dangerous drug) © A 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). © A Jul-12
S carvone (caraway seed)R carvone (spearmint) Caraway Seed has a warm, pungent, slightly bitter flavour with aniseed overtones.
S limonene (lemons)R limonene (oranges)
© A Jul-12
GEOMETRIC ISOMERISM CANNOT ROTATE AROUND C=C X © A Jul-12
GEOMETRIC ISOMERISM (E) but-2-ene(Z) but-2-ene E entgegen (opposite)Z zusammen (together) © A Jul-12
GEOMETRIC ISOMERISM E 2-bromo-3-fluorobut-2-ene PRIORITY: H < CH 3 < CH 2 CH 3 Look at atomic numbers of atoms attached to C=C (or subsequent atoms if the same). Highest values opposite = E Highest values together = Z E 3-methylpent-2-ene , 6 © A Jul-12
GEOMETRIC ISOMERISM trans but-2-ene (E) but-2-ene BOTH C ATOMS OF C=C MUST HAVE TWO DIFFERENT GROUPS cis but-2-ene (Z) but-2-ene © A Jul-12
TASK methylpropene 1-chloropropene NO YES Z E © A Jul-12
TASK 2-methylpent-2-ene 2-ethylpent-1-ene NO © A Jul-12
TASK 3-methylpent-2-ene but-1-ene YES NO Z E © A Jul-12
TASK 3,4-dimethylhex-3-ene 2-bromo-3-chlorobut-2-ene YES Z Z E E © A Jul-12
OPTICAL ISOMERISM. All molecules have a mirror image – but for most molecules it is the same molecule.
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