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Encyclopedia of Physical Science and Technology EN005F-213 June 15, 2001 20:32

354 Electron Transfer Reactions

electron transfer occurring without any eventual change The neutral alcohol does not appear to react. Hydride
in the two coordination spheres as radioactive iron equili- transfer from neutral R 2 CH(OH) would involve charge
brates between the two oxidation states. separation of H − from R 2 C (OH). Oxidation of C–H
+
−
bonds in hydrocarbons by MnO has been found to gen-
4
erally involve hydrogen atom (H·) transfer.
3. Inner Sphere One-Electron Transfer
b. Solvated electron. Hydrated electrons have sta-
These processes were described above in connection with
chlorine atom transfer from a cobalt(III) complex to Cr 2+ bility enough to enable studies of the reduction of hun-
dreds of other aqueous species. The e (aq) has a hy-
−
and with the electron transfer from Cr 2+ to IrCl 2− with-
6
out accompanying chlorine atom transfer. Activated states dration energy of −40 kcal/mol, an oxidation potential
of ∼2.5 V, and a k of only 16 M −1 sec −1 for reaction with
probably contain Cl bridges in each case.
−
H 2 O. Diffusion-controlled reactions are observed with
most paramagnetic species except alkyl radicals. Corre-
lation between reactivity and redox potentials of electron
acceptors is not likely. The availability of empty orbitals
on transition metal ions and the energy gain on adding
the electron are primary factors. Mn 2+ is unreactive while
Cu 2+ reacts at about one-tenth the rate of diffusion control.
4. Inner Sphere Two-Electron Transfer

The oxygen atom transfer reaction described above has
been shown as a two-electron inner sphere transfer. Oxi-
−
dationofprimaryandsecondaryalcoholsbyMnO andby
4
HCrO have been rationalized as two-electron transfers
−
4
that require removal of a proton from the carbon attached
tooxygen.Manganese(V)hasbeendetectedasaninterme-
diate product, and chromate ester precursors to reactions
suggest the following activated states:
Chromium(VI) is an extraordinarily efficient radical
trap even though its potential for forming chromium(V)
does not appear to be particularly high. Empty t 2g orbitals
as opposed to high potential make it a diffusion-controlled
acceptor of hydrated electrons as well. It is notable that
alkyl radicals are inert to hydrated electrons, which would
form carbanions, and reactive to Cr(VI), which oxidizes
them to carbonium ions.

Tertiary alcohols without protons on the carbon are not B. Two-Path Systems
oxidized. It is tempting to consider that removal of the
proton in the activated states “frees” a pair of electrons to 1. Two-Electron, Two-Path Transfers
be transferred through the oxygen bridge to the metal ion. a. Single donor—Two acceptors.
In the case of chromium(VI) oxidation of secondary alco-
hols there is general acid catalysis and a large deuterium
Reaction:
isotope effect.
Activated state:
a. Hydride (H: ) transfer. The reaction of alcohols
−
with MnO in basic solution is found to involve the reac-
−
4
−
tion of alcoholate ion (R 2 CHO ) with MnO and is prob- N 2 H 4 is normally a discriminator between one- and two-
−
4
ably an example of direct transfer of hydride ion from C electron acceptors. In this case its two-electron product
2−
to form HMnO . (N 2 ) is observed and two one-electron acceptors appear in
4
the activated state and are reduced. One can imagine the
O − immediate result of two one-electron transfers from N to
/ − 2−
+
[R 2 C H ··· OMnO ] → R 2 C O + HOMnO 3 V(V) to be [V(IV) NH NH V(IV)] + 2H .
3