stereochemistry


 * Stereochemistry** in organic chemistry has a few unique terms that are used to describe the differences in structure between two seemingly identical (but differrent) compounds known as //stereoisomers//.

We have seen structural isomers before, such as all of the [|isomers of hexane].

Stereoisomers are different than structural isomers - the order in which the atoms in the isomers are put together is identical. The only difference lies in one type of carbon, called a [|chiral] carbon.

A //chiral// atom has four different attachments. The way in which these four attachments are oriented is what makes a stereoisomer (also known as an enantiomer).

For example, 2-butanol (a.k.a. butan-2-ol) has a chiral carbon.



The carbon having the hydroxy functional group also has a methyl branch, an ethyl branch, and a hydrogen (four different attachments) and is therefore chiral.

Limonene is another molecule having a chiral carbon:



Every chiral carbon has two possible stereo configurations, which are distinguished using R or S (see that section below).



The orientation of the chiral carbon is shown using either a dark wedge-shaped bond (R, above) or a dashed wedge-shaped bond (S, above). This is trying to convey the hydroxy functional group as either pointing out towards you or back away from you (the butane backbone is lying flat in the plane of the screen).

One thing to remember - a darkened wedge is not always R - it could also be an S configuration.


 * __R vs. S__**

A chiral carbon is either an "R" (latin - rectus, right-handed or clockwise) or an "S" (latin - sinister, left-handed or counterclockwise) configuration.

The two stereoisomers of 2-butanol shown above are actually mirror images of one another. If you hold the R isomer up to a mirror, the image in the mirror would be the S isomer. However, these two molecules are not exactly identical, and cannot be overlaid (what we call superimposed) on one another so that all the atoms are in the same spot. These two molecules are therefore called //enantiomers//.

Two enantiomers are non-superimposable mirror images of one another. But if a molecule has more than one chiral carbon, not all stereoisomers are going to be enantiomeric pairs, so the ones that are not mirror images are called //diastereomers//.

The total number of stereoisomers is determined by the formula **2^n**, where n is the number of chiral centers.

Go here to find out how to determine R versus S.