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How to do the R-S configuration with 2 chiral carbons !

How to do the R-S configuration with 2 chiral carbons !


1 Answers

Aman Bansal
592 Points
9 years ago

Dear Karan,

The method for determining the R-S configuration of a compound with 2 chiral carbons is the same as determining the R-S configuration for a compound with only 1 chiral carbon. You assign a priority to the atoms attached to each carbon based on the molecular weight and then you see Prority 1 - 2 - 3 -4 groups are arranged clockwise or anti-clockwise. If the groups are arranged in the clockwise direction, then the configuration is R and otherwise the configuration is S. You do this for both the chiral carbons.

Let''s take an example by considering the compound 2-methylamino-1-phenylpropanol.

Consider the first diagram on the left. If you consider the carbon numbered 1, then it has 2 carbon atoms, one oxygen atom and one hydrogen atom attached to it. Oxygen has the highest priority in this case as it has the highest atomic number out of carbon, oxygen and hydrogen. The carbon that has CH3, H and NHCH3 attached to it has 2nd priority and the carbon in the ring has the third priority. Hydrogen of course, has 4th priority. When you arrange the compound such that hydrogen is inside the plane and curl your fingers from priority 1-2-3-4, you see that your fingers get curled in the clockwise direction. Therefore, the first carbon in the leftmost picture has a configuration R.

If you repeat the same process for the second carbon, then you will see that nitrogen gets 1st priority in that case as it has the highest atomic weight. Then the chiral carbon attached to the second carbon gets second priority and the carbon of the methyl group gets third priority. and the hydrogen gets 4th priority. If you curl your fingers from priority 1 - 2- 3- 4 such that the lowest priority atom hydrogen is below the plane, you get the direction of curling as anti-clockwise. Thus, this second carbon has S configuration. Note that before deciding the direction as clockwise or anti-clockwise, you must make sure that the lowest priority atom is below the plane.

The same process can be applied to calculate the R, S configuration for all the other stereoisomers of 2-methylamino-1-phenylpropanol.

Now that we have discussed the R, S configuration in detail, let us come to D-L configuration.

In the D-L configuration, the only carbon that matters is the carbon that is furthest away from the COH group. If this carbon has the hydroxyl group on the left in its fischer projection, then it has L configuration and if it has the hydroxyl group on the right in its fischer projection, then it has D configuration. Take a look at the picture below.

In the picture above, the carbon that is furthest away from the COH group has OH on the right in the D configuration and on the left in the H configuration.

Here is a picture of fisher projections of some other compounds that have similar configurations.

The star in the above pictures indicates the carbon that is farthest away from the COH group.

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Aman Bansal

Askiitian Expert

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