OPTICAL ISOMERISM

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OPTICAL ISOMERISM
R W Grime Ripon Grammar School
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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 Which of the following molecules are
  optically active?

• propan-2-ol
• 2-chlorobutane
• 1-chlorobutane
• 3-methylhexane

• butanone
• 2-methylbutanoic acid
• butan-2-ol
• 1-chloro-3-methylpentane

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• propan-2-ol

NOT OPTICALLY ACTIVE
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• 2-chlorobutane

OPTICALLY ACTIVE
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• 1-chlorobutane

NOT OPTICALLY ACTIVE
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• 3-methylhexane

OPTICALLY ACTIVE
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• butanone

NOT OPTICALLY ACTIVE
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• propan-2-ol

NOT OPTICALLY ACTIVE
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• 2-methylbutanoic acid

OPTICALLY ACTIVE
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• butan-2-ol

OPTICALLY ACTIVE
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• 1-chloro-3-methylpentane

OPTICALLY ACTIVE
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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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• http//scholar.hw.ac.uk/site/chemistry/activity5.a
  sp?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.

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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)