Meso Compouds

Remember way back when I said this?

A molecule that contains more than one stereogenic center might be chiral, but it might not. More on this later.

Well, it's later now. This post is on meso compounds. Please, no jokes about the name. Mostly because I am lazy, I will just start with the exact same example as this textbook: 2,3-dibromobutane.
And of course you spotted the tetrahedral stereogenic centers, right? We have a carbon attached to a methyl group, a hydrogen, a bromine, and another, identical carbon. So that's two chiral centers.

I don't think that I've mentioned it so far, but a molecule with two chiral centers can have, at most, four stereoisomers. I suppose that at this point I should introduce wedges and dashes to make three-dimensional interpretations of these stereoisomers, but I won't, so there. Actually, I should have done that a while ago. Fine, I'll get around to it at some point. Moving on...

Anyway, I'll probably just make a video to explain this, because it's even worse than trying to explain what a tetrahedral stereogenic center is using just words. But the short version is that we can arrange the bonds around both centers to have one version of the molecule, take its mirror image and have a pair of enantiomers, then take one of those forms and switch two bonds to have a third stereoisomer that is neither superimposable on either of the previous two molecules nor a mirror image of either of them. But this molecule is superimposable on its mirror image, so it has no enantiomers. It is achiral, even though it has two stereoisomers that are chiral. And that makes it a meso compound.

Also, the term for the relationship between stereoisomers that are not enantiomers is diastereomers.