OPTICAL ISOMERISM

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OPTICAL ISOMERISM
R W Grime Ripon Grammar School
Molecular mirror images
Optical isomerism and chirality
Enantiomers and racemates
Self-Test recognising optical isomers from
structural formulae
Polarimetry recognising optical isomers by
experiment
Examples of optical isomers and their importance
thalidomide
carvone
limonene
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• All molecules have a mirror image but for most
  molecules it is the same molecule.

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• For some molecules the mirror image is a
  different molecule (the mirror image is
  non-superimposable).

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• Left and right hands are an example of
  non-superimposable mirror images.

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• 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.

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• 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.

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• 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
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• propan-2-ol

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
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• 2-chlorobutane

Click here to go back to the optical isomerism
task
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• 1-chlorobutane

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
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• 3-methylhexane

Click here to go back to the optical isomerism
task
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• butanone

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
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• propan-2-ol

NOT OPTICALLY ACTIVE
Click here to go back to the optical isomerism
task
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• 2-methylbutanoic acid

Click here to go back to the optical isomerism
task
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• butan-2-ol

Click here to go back to the optical isomerism
task
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• 1-chloro-3-methylpentane

Click here to go back to the optical isomerism
task
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• 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

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• Light is a form of electromagnetic radiation.

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• The wave vibrations are perpendicular to the
  direction of travel of the wave.

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• Optical isomers rotate the plane of plane
  polarised light.

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• POLARIMETERS can be used to analyse the effect
  optical isomers have on plane polarised light

Heriot Watt University has a web page with an
interactive tutorial and self-test questions
about this topic http//scholar.hw.ac.uk/site/che
mistry/activity5.asp?outline
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• 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.

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• 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).

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• 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.

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• In the 1960s 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
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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)
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• 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).

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S carvone (caraway seed)
R carvone (spearmint)
Caraway Seed has a warm, pungent, slightly bitter
flavour with aniseed overtones.
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S limonene (lemons)
R limonene (oranges)