The pinacol rearrangement
When the 1,2-diol pinacol is treated with acid, a rearrangement takes place.
Whenever you see a rearrangement, especially in acid, you should now think ‘carbocation’. Here, protonation of one of the hydroxyl groups allows it to leave as water, giving the carbocation.
You now know that carbocations rearrange by alkyl shifts to get as stable as they can be— but this carbocation is already tertiary, and there is no ring strain, so why should it re arrange? Well, here we have another source of electrons to stabilize the carbocation: lone pairs on an oxygen atom. We pointed out early in the chapter that oxygen is very good at stabilizing a positive charge on an adjacent atom, and somewhat less good at stabilizing a positive charge two atoms away. By rearranging, the first-formed carbocation gets the positive charge into a position where the oxygen can stabilize it, and loss of a proton from oxygen then gives a stable ketone.
You can view the pinacol as a rearrangement with a ‘push’ and a ‘pull’. The carbocation left by the departure of water ‘pulls’ the migrating group across at the same time as the oxygen’s lone pair ‘pushes’ it. A particularly valuable type of pinacol rearrangement forms spirocyclic ring systems. You may find this one harder to follow, although the mechanism is identical with that of the last example. Our ‘top tip’ of numbering the atoms should help you to see what has happened: atom 2 has migrated from atom 1 to atom 6.
When drawing the mechanism it doesn’t matter which hydroxyl group you protonate or which adjacent C–C bond migrates—they are all the same. One five-membered ring expands to a six-membered ring but the reason this reaction happens is the formation of a carbonyl group, as in all pinacol rearrangements.
