Assigning R And S Configuration

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Order the substituents coming off the stereogenic carbon atom using the Cahn-Ingold-Prelog rules. Rotate the molecule until the lowest priority (number 4) substituent is in the back 3.

Draw a curve from number 1 to number 2 to number 3 substituent.

When I get to the ring, I like to 'walk' my fingers around one carbon at a time until I find a difference. Notice the methyl that makes the group labeled 1 higher priority. Note that there are two chiral carbon atoms in this molecule.

The second is on the ring indicated by the green arrow.

Bromine has the highest molecular weight of the atoms, so this gets the highest priority.

Chlorine is next, followed by fluorine, so we number the compounds as shown below.

Take a peek at the two organic molecules below; they are mirror images, but if you tried to lay them on top of each other, the groups would never line up perfectly.

This is what is meant by 'non-superimposable.' This is a tough concept, so let's consider a real life example. Now, try to put one hand on top of the other with the palms facing down. In fact, 19 out of 20 of the amino acids that make up your body are chiral!

A protein may recognize one enantiomer of an organic molecule as a drug, and the enantiomer of that same organic molecule may have little to no effect.

Since chirality is so critical in biological systems, it is important to be able to assign the absolute configuration of a chiral carbon. There are three steps to assign R or S, which is called the absolute configuration of the chiral carbon atom. If the arrow points in a counterclockwise direction, the chiral carbon is S.


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