2. All molecules have a mirror image
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1. 1 OPTICAL ISOMERISM
3. 3 For some molecules the mirror image is a different molecule (the mirror image is non-superimposable).
5. 5 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.
7. 7 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.
8. 8 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.
9. 9 propan-2-ol
10. 10 2-chlorobutane
11. 11 1-chlorobutane
12. 12 3-methylhexane
13. 13 butanone
14. 14 propan-2-ol
15. 15 2-methylbutanoic acid
16. 16 butan-2-ol
17. 17 1-chloro-3-methylpentane
18. 18 Molecules that are optical isomers are called enantiomers.
19. 19 Light is a form of electromagnetic radiation.
20. 20 The wave vibrations are perpendicular to the direction of travel of the wave.
21. 21 Optical isomers rotate the plane of plane polarised light.
22. 22 POLARIMETERS can be used to analyse the effect optical isomers have on plane polarised light:
25. 25 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.
26. 26 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.
28. 28 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).