# How can you calculate the number optical isomers a molecule will have?

It can be up to ${2}^{n}$, where $n$ is the number of chiral centres.....
When I say $\text{it can be up to}$, of course there is a catch. Sometimes a given set of stereoisomers are the same compound, which I will attempt to demonstrate with the one example.
Consider, $\text{2,3-butanediol} ,$ ${\text{H"_3"CCH(OH)CH(OH)CH}}_{3}$. The carbons attached to the hydroxyl groups are potentially chiral in that, clearly, they are each attached to 4 different residues. Now the $\text{R, R}$, and the $\text{S, S}$ compounds are enantiomeric; however, the $\text{R, S}$ and the $\text{S, R}$ isomers are IDENTICAL; i.e. they are symmetric mirror images, and one stereocentre would map onto the other upon reflection in a mirror plane. These so-called $\text{meso}$ isomers are not optically active.