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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.
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.
Alanine
This is what a molecule
of alanine looks like if
we draw it on paper.
O
C
H3C
OH
CH
NH2
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 Cahn–Ingold–Prelog rules.
Identifying a molecule using the
Cahn–Ingold–Prelog 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.
H3C
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.
O
OH
C
C
H
NH2
Identifying a molecule using the
Cahn–Ingold–Prelog rules
4. Number the groups so that the
highest priority is 1 and the lowest
priority is 4.
5. 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).
O
2
OH
C
4
3
H3C
H
C
1
NH2
Identifying a molecule using the
Cahn–Ingold–Prelog rules
•
•
If you traced out a clockwise circle, then
the molecule is the R form (R for rectus,
or right).
O
3
1
1
C
C
R
2
2
S
H3C
OH
C
If you went anticlockwise it is the S form
(S for sinister, Latin for left)
C
H
NH2
3
Identifying a molecule using the
Cahn–Ingold–Prelog rules
•
What form is this?
3
2
1
This is the R form