Rearrangements of Cationic Oxygen

Protonation of the divalent oxygen atom of alcohols and ethers by strong acids produces a tricoordinate oxonium cation. Because the oxygen of an oxonium ion has a valence shell octet, it does not constitute an electron deficient site and cannot serve as a rearrangement terminus. To induce rearrangement in the same manner as a tricoordinate carbocation, oxygen must be converted to a unicoordinate oxacation, as noted in the following diagram. A 1,2-alkyl or aryl shift then transforms a relatively unstable oxacation into a more stable carbocation.

The simplest precursor of an oxa cation is a peroxide or equivalent derivative (e.g. R-O-OH or R-O-X). Removal of hydroxide anion from a hydroperoxide is energetically unfavorable, unless it is initially converted to a better leaving group in a manner similar to that used to facilitate substitution reactions of alcohols. By protonating the hydroxyl group, the leaving group becomes water, thus generating an oxacation. A useful industrial procedure for preparing phenol (and acetone) is based on this strategy.