Sequence rules and R, S configurations.

Optically active molecules rotate plane polarised light. One enantiomer will rotate plane polarised light anti-clockwise while the other enantiomer will rotate plane polarised light clockwise. The enantiomer that rotates plane polarised light anti-clockwise is said to be laevorotatory while the enantiomer that rotates plane polarised light clockwise is said to be dextrorotatory. By convention anti-clockwise rotation is also give the minus sign (-) while clockwise rotation is given the positive sign (+), e.g. Glucose is an optically active active molecule, (+)-glucose is dextrorotatory and will rotate plane polarised light clockwise, while (-)- glucose is laevorotatory and will rotate plane polarised light anti-clockwise.

As a chemist it would be helpful to be able to describe the actual arrangement of the atoms and groups in 3d-space around a chiral carbon atom in each of the enantiomers present. However it is quite difficult to actually describe the arrangement of atoms around a chiral centre in any two enantiomers, since each of the optically active molecules contains the same groups, they are just arranged differently in 3d-space around the chiral carbon atom. So rather than trying to describe the arrangement of the bonded groups or atoms around the chiral centre a pictorial representation is often used to describe the arrangement. To describe the arrangements of the atoms around the chiral carbon atom we use the Cahn-Ingold-Prelog rules to put the attached groups in order of priority. If you cannot recall the rules simply click the link above to refresh your memory.

Example 1

The molecule shown below shows a chiral carbon atom bonded to four different atoms (it could also be groups), so this molecule will be optically active and consist of a pair of enantiomers. We can place a sample of each enantiomer in a polariser and find out which of the enantiomers rotates plane polarised light clockwise and which enantiomer rotates plane polarised light anti-clockwise. However this will not give any indication of how the attached atoms are actually arranged around the chiral carbon atom. This is where we use the Cahn-Ingold-Prolog rules to help use assign orders of priority for each of the attached groups. Once the orders of priorities of the groups attached to the chiral carbon have been assigned then we can determine the relative configuration of the attached groups. These relative configurations are labelled simply as R and S. It is very easy and straightforward to assign a R and S configuration to an optically active molecule. Simply follow these simple rules:

• Assign the orders of priorities of each of the atoms or groups attached to the chiral carbon atoms using the Cahn-Ingold-Prolog rules. In this case we simply rank the attached atoms in order of atomic number, iodine having the largest atomic number has the highest priority and hydrogen having the lowest atomic number is ranked fourth. This is shown in the diagram below.
• Next orientate the molecule in such a way that the atom or group of lowest priority is pointing backwards as shown in the diagram below. Next imagine travelling from priority group 1 to priority group 2 and then to priority group 3, it may be easier to imagine looking through a car steering wheel and asking yourself which way would you turn the steering wheel when you travel from priority 1 to priority group 2 and then to priority group 3; if the wheel needs to be turned clockwise the configuration around the chiral carbon is labelled as the R-configuration and if the wheel needs to be turned anti-clockwise then the configuration is labelled as the S-configuration.

The R-configuration

The S-configuration

The image below shows the mirror image or the other enantiomer of the molecule above. You can see that this time when we travel from high priority atoms to lower priority atoms attached to the central chiral carbon atom we travel in an anti-clockwise direction. This tells us that this molecule has a S-configuration.

You will no doubt see molecules labelled with both the direction of rotation of plane polarised light and also with their relative configuration e.g. (R)(-)-lactic acid, here we should be able to draw the structure of (R)-lactic acid based on the method discussed above and from this work out the structure of (S)(+)-lactic acid.

It is worth noting that there is NO method which will enable you to work out whether one enantiomer of any particular molecule will be laevorotatory or dextrorotatory. The only way to know is to put a sample of the molecule in a polariser and measure the direction and size of the rotation of plane polarised light. Just because in the example I have given, namely (-)-lactic acid, the laevorotary enantiomer has a R-configuration does not mean that all enantiomers which are laevorotary will have a R-configuration. There is no link between the direction of rotation of plane polarised light and the relative configuration around a chiral carbon atom.

Key Points

• By using the Cahn-Ingold-Prolog rules it is possible to assign orders of priorities to atoms or groups attached to chiral carbon atoms.
• By placing an optically active molecule with the lowest priority group pointing to "the rear" we can determine the relative configuration of the enantiomer by simply deciding whether a clockwise or anti-clockwise direction of travel is needed when moving from high priority groups to lower priority groups.
• There is no known connection or method which will enable you to predict in advance whether a molecule will rotate plane polarised light clockwise or anti-clockwise. There is no link between the direction of rotation of plane polarised light and the assignment of R or S configurations.

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