Question

What is the difference between a stereogenic center and a chiral center?

Answer 1

Chiral centres are a subset of a stereogenic centres.

Explanation:

Many texts say they are the same, but there is a subtle difference.

A chiral centre is an atom that has four different groups attached.

Thus, in ribitol, carbons 2 and 4 are chiral centres.

A stereogenic centre is any atom for which exchanging two groups creates a different stereoisomer.

Thus, all chiral centres are stereocentres.

However, not all stereocentres are chiral centres.

Note that ribitol is a meso compound.

Carbon 3 is a stereogenic centre, because the two attached carbon chains are mirror images of each other and hence not identical.

If we interchange the `"H"` and `"OH"` on carbon 3, we get xylitol.

Xylitol is a stereoisomer, and it is also a meso compound.

Interchanging `"H"` and `"OH"` gave a different stereoisomer but did not change the chirality of the molecule.

Thus, carbon 3 is a stereogenic center, but not a chiral centre.