Investigating chirality

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Chemistry Review Vol. 23, No. 1
Investigating chirality

• This is a model of a molecule of the amino acid
  alanine.
• It can exist in two forms, which are mirror
  images of each other.
• Its central carbon atom has four different groups
  attached to it.
  
  

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Chirality

• A carbon atom with four different groups attached
  to it is referred to as a chiral centre.
• The molecule is chiral, it has no plane of
  symmetry.
• The two mirror image forms of a chiral molecule
  are called enantiomers.
  
  

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Alanine
This is what a molecule of alanine looks like if
we draw it on paper.
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Naming enantiomers

• We need a way of identifying which form of a
  molecule is which.
• There are different systems for naming molecules
  with a chiral centre.
• Biologists tend to use L and D to label amino
  acids, as this makes most sense in a biological
  context.
• The standard system used by chemists for all
  molecules with a chiral centre is the R/S system.
• To identify whether a molecule is in the R form
  or the S form we use the CahnIngoldPrelog rules.

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Identifying a molecule using the
CahnIngoldPrelog rules

1. Look at the carbon atom that is highlighted here,
   it has four different groups attached to it. You
   need to assign priorities to each of these four
   groups.
2. Look at the atoms directly attached to this
   carbon. The higher the atomic number, the higher
   the priority.
3. If two atoms have the same atomic number, you
   need to look at the atoms attached to these atoms
   and compare their atomic numbers.

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Identifying a molecule using the
CahnIngoldPrelog rules

1. Number the groups so that the highest priority is
   1 and the lowest priority is 4.
2. Imagine that you are viewing the molecule so that
   you are looking at the highlighted carbon with
   the lowest priority group (4) pointing away from
   you and then trace around the other groups from
   highest (1) downwards (i.e. 1, 2, 3).

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Identifying a molecule using the
CahnIngoldPrelog rules

• If you traced out a clockwise circle, then the
  molecule is the R form (R for rectus, or right).
• If you went anticlockwise it is the S form (S for
  sinister, Latin for left)

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Identifying a molecule using the
CahnIngoldPrelog rules

• What form is this?

This is the R form