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

Electron Transfer Reactions 353

SCHEME 1

us assume that an individual orbital pathway may con- The attack of a radical (R·) on carbon C-1 of butadiene
duct one or two electrons and that more than one orbital appears to transfer the odd electron on the radical to either
pathway may lead to or from a given redox center. It is C-4 or C-2, which creates a new radical. This radical can
then possible for us to classify proposed electron trans- add to another monomer. The delocalized π-orbital in bu-
fer process in terms of combinations of numbers of path- tadiene can “conduct” the intruding electron from C-1 to
ways and numbers of electrons. Equations for overall re- any other C, since it has a significant probability of being
actions and formulas for postulated activated states that in valence orbitals associated with any participating car-
provide models for the structures in which electron trans- bon. It will end up most often on carbon atoms in which
fers are thought to occur will be used to illustrate each it can reside in the most stable half-filled nonbonding or-
classification. bital, characterizing a new radical. One can also imagine
that one of the two pairs of π electrons is “transferred”
A. One-Path Systems to the R–C bond and the delocalization of the second pair
lowered from an orbital covering all four carbon atoms
1. Direct Transfer—Atom to Atom to one “localized” on C-2 and C-3. Seemingly, one might
just as well talk of electron “scrambling” as of electron
a. One-electron transfers. transfer; certainly one must be aware that an “electron
+
Reaction: 2Na + Cl 2 → 2Na + 2Cl − transfer” process involves a change in the condition of
all the valence electrons in the system connecting oxidant
Activated state(s): [Na ··· Cl Cl]‡
with reductant and reactants with products. One net result
([Na ·· Cl]‡ + Cl·) of all this electronic activity is the loss of electron(s) by the
reductant and gain by the oxidant which we label electron
Direct transfer of electrons from sodium atoms to chlorine
transfer.
molecules or atoms in the reacting systems results in the
formation of [Na Cl ] ion pairs.
+
−
b. Two-electron transfers. Direct two-electron tra-
+
Reaction: 2Fe 2+ + HNO 2 + H → nsfers presumably occur in the extreme case of a Lewis
base’s donating an electron pair to an acid, which forms
[FeNO] 2+ + H 2 O + Fe 3+ a bond so polar that the electrons become a lone pair
on the once acidic atom. Two-electron donors are often
Activated state: [Fe(II) ··· (NO )]‡
+
nonmetal atoms in complexes in which they exhibit ox-
idation numbers of n − 2, where n is the number of the
+
Electron transfer occurs by attack of Fe 2+ on NO . The
group in the periodic table in which they are found. Ex-
product NO forms a detectable complex with a second
2−
−
+
amples are :SnCl , :SO , and Tl . :SnCl − is the ac-
Fe 2+ ion. The electron transfer step in most processes— 3 3 3
tive species when Sn(II) in aqueous hydrochloric acid
especially in solution and even at electrodes—must be
is used as a reductant. It reduces the diazo group in
sorted out by intuition from a complexity of reactions lead- −
+
ing to formation of the active state for electron transfer and methyl orange (2:SnCl + RN NR + 6H + 6Cl − →
3
2−
+
−
+
also those leading from the activated state to formation of 2SnCl 6 + 2RNH ). H and :SnCl attack the double-
3
3
eventual products. Often it is clear that the activated state bonded nitrogen atoms to produce the activated state in
for the slow step in a reaction is not the state in which Scheme 1.
electron transfer occurs.
Polymerization: 2. Outer Sphere One-Electron Transfer
∗
∗
Fe (CN) 4− + Fe(CN) 3− → Fe (CN) 3− + Fe(CN) 4−
6 6 6 6
The activated state is a collision complex with struc-
tures close enough in energy to enhance the probability of