Reductions of Carbonyl Compounds
Diol reactions
Pinacol rearrangement
The pinacol rearrangement is a way to turn a diol, a glycol, into a carbonyl compound. Pinacol is a glycol. Under acidic conditions, one of the hydroxyl groups of the glycol is protonated forming a good leaving group, water. Water leaves forming a carbocation. A 1,2-alkyl shift occurs which is the rearrangement. Resonance forms are drawn moving the lone pair of electrons on an oxygen atom to form a carbonyl. A proton is removed to make the carbonyl of the pinacolone. The pinacol rearrangement gets its name from the reaction turning pinacol into pinacolone, but the pinacol rearrangement can happen with many glycols under acidic conditions. The rearrangement happens whenever the 1,2-alkyl shift can make a more stable carbocation. In the reaction of pinacol, the new carbocation formed after resonance forms stabilize the 1,2-alkyl shift with the lone pair of electrons on the oxygen atom.
The Pinacol Rearrangement
Periodic Acid
The C-C bond of a vicinal diol, a glycol, can be oxidatively cleaved with periodic acid (pronounced purr-eye-oh-dik-acid), HIO4 to make two carbonyl compounds. This is very similar to how alkenes are oxidatively cleaved in an ozonolysis reaction.
