Question

If a molecule contains `n` chiral centres, how many stereoisomers are possible?

Answer 1

If there are `n` chiral centres, there are `2^n` stereoisomers......

Explanation:

There is of course a catch, and it is illustrated by a simple organic molecule, `"2,3-butandiol...."`

`H_3stackrel(1)C-stackrel(2)CH(OH)stackrel(3)CH(OH)stackrel(4)CH_3`

We look at the molecule and `C2` and `C3` ARE POTENTIALLY CHIRAL (do you agree?), and each carbon has 2 optical antipodes, so that we could potentially form `"RR"` and `"SS"` and `"RS"` and `"SR"` stereoisomers (i.e. in accord with the rule proposed). `"RR"` and `"SS"` share an enantiomeric relationship, but share a DIASTEREOMERIC relationship with `"RS"` and `"SR"`.

However, for this example, the `"RS"` and `"SR"` stereoisomers are SUPERPOSABLE mirror images; these are example of optically inactive `"meso"` isomers.

Mind you, a compound with more than 2 chiral centres will be rather difficult to synthesize in optically pure forms. You should not have to consider structures with MORE than 2 stereocentres. Why not? Because these questions are as difficult to mark as they are to answer. And please don't forget the utility of using a model set to gain understanding of stereochemistry.