Carbonium Ions, Electron-deficient N and O Atoms
seems probable that the rearrangement occurs simultaneously with
loss of the anion as a concerted process reminiscent of the Hofmann
reaction (p. 94). When an unsymmetrical ketone is oxidised it is
usually the more nucleophilic group that migrates, as in the pinacol/
pinacolone rearrangement (p. 91), but as in the latter reaction steric
effects may also be involved and can have the effect of markedly
changing the expected order of relative migratory aptitude of a series
of groups based on their electron-releasing abilities alone.
(ii) Rearrangements of peroxides s
A rather similar rearrangement is observed during the acid-catalysed
decomposition of a number of peroxides. Thus the hydroperoxide
(XXXV) obtained by the air oxidation (cf. p. 252) of isopropylbenzene
(cumene) is used on the commercial scale for the production of
phenol + acetone by treatment with acid:
Me
(XXXV)PhOH PhO
JI°/H,0 L
0=C—Me <—:— HO—C—Me < ' ®C—Me
| | 2 )-H« |
Me Me Me
(XXXIX) (XXXVIII)Here again it seems likely that the species (XXXVII) has no separate
existence and that loss of HaO from (XXXVI) and migration of the
phenyl group with its electron pair occur as a concerted process to
yield the acetone hemiketal (XXXIX), which then undergoes ready
hydrolysis to yield the end-products, phenol and acetone.
In these examples we have been considering the heterolytic fission
of peroxide linkages, — 0:0—> — 0® + :0® —, and though this
takes place in more polar solvents, the linkage may also undergo
homolytic fission to yield free radicals, -0:0—> — 0- + - O —, as
we shall see below (p. 240).