JOURNAL OF CHEMICAL EDUCATION
NOTE ON THE SYMMETRY BETWEEN ELECTRON AND PROTON TRANSFER
a
R. 5. BRADLEY The University of Leeds, Leeds, England
Tms note is an attempt to underlme the fundamental similarities between electron and proton transfer by current ideas on the separate -gathering- tozether aspects.' As is well known, the equations A*B H + and FGO e, where A and B are conjugate acids and bases. R and 0 coniucrate reducing and oxidizing agents. and H+ and e the free proton and electron, resp&t&ely; are formally similar, giving the equilibrium constants,
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Neither equilibrium can be realized in practice, since all that can be studied is the interaction of two conjugate acid-base pairs, or two conjugate redox systems, giving observable equilibrium constants,
With acids and bases the second conjugate pair is often the solvent system, when K A gives a measure of the
' MICEAEUS,L., AND (1938).
M. P. SCHUBEET, Chem. Rev., 22, 437
so-called dissociation constant of A, B' being omitted (for water [B'] = [HzO] = constant app.); KAclearly depends on k~'. Since common solvents such as water do not usually show redox characteristics the second redox system usually interacts with the first in virtually a "nonelectronic" solvent, corresponding with the interaction of two acid-base conjugate pairs in an aprotic solvent such as benzene; of course the solvent always exerts a solvation effect on the equilibrium even if no particles are exchanged. Very powerful oxidizing agents, however, such as Fzor Co(HnO)s+++, or very powerful reducing agents such as CO(CN)~---interact with water giving phenomena somewhat similar to those covered by the Hantzsch leveling effect for acids and bases, except that in the redox system equilibrium is not usually achieved. Redox systems differ from acid-base systems in that it is possible t o use platinum as a potential mediator, and study two conjugate pairs as halves of a cell. If one-half is a hydrogen electrode H 2 0 '/zH2=OHa+ e, this system may be allowed to come t o equilibrium with any redox conjugate pair, R, 0,when
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APRIL, 1950
209
and [OH3+] adjusts itself to equilibrium. Thus K , may be referred to the system Hz0 1/zH2SOH3+ Putting e, and clearly depends on K. for this system. This device of a two-compartment cell gives the same equilibrium asif redoxinteracted with redox prime in the same phase. With acids and bases an analogous device would be a two-compartment cell with palladium electrodesjoined by a palladium wire. where KE is the hydrogenation constant, deducible Further analogies are provided: from bond-free energies. (a) The rule that electrons are transferred one a t a Although not strictly covered by the title, the G. N. time, analogous to the rule of successive dissociation Lewis acids and bases may be discussed here for of a polybasic acid (cf. Michaelis and Schubert, lac. purposes of comparison. In this system cit.). As an illustration of the rule for electron transfers it is probable that the reaction between the watercoordinated ions 2Fe+++aq. Sn++aq+ZFe++aq. Sn++++aq. proceeds via the Sn+++ ion, Sn++ where the "base" B, donates an electron pair to the reacting in the form of a complex anion coordinated "acid" A,. Hence with chlorine, and giving the intermediate Sn+++ similarly co~rdinated.~ (b) Arnphoteric molecules in equilibrium, such as NHzCHr CO*H+NH3+CH2CO~-may be said to have usually called the instability constants of the coordinathe proton nonlocalized, with a preference for the tion complexes, and nitrogen end of the molecule. In the crystal the proton may be exchanged between the COO- of one molecule . .. . and the NHe o f another. In solution exchange is possible via the intervention of water. In this sense i. e., K gives a measure of the "strength" of & relative the proton may be said to be nonlocalized, and possibly to A, with respect to the "base" B,. Clearly K depends best represented in terms of standing waves. This on the base used. There are no conjugate acid-base may be compared with the nonlocalized electron dis- pairs. The color changes produced by the usual acidtribution in compounds such as semiquinones, where base indicators, e. g., thymol blue in acetone with Agone end carries an "excess" of electrons (R) and the Clod, are obscure, and presumably depend on a shift in the I*+IB equilibrium due to coordination or to other a deficiency (0). color changes produced by coordination. Moreover, (c) When an indicator is added to an acid-base there seems little point in extending the concept reducsystem in water there are three conjugate pairs in tion to cover receipt of electron pairs in coordination. mutual equilibrium, giving constants such as These remarks are su5cient to show that G. N. Lewis' system stands apart from that due to Lowry and Bronsted, and that a special place is occupied in chemwhere IAand IBare the acidic and basic members of istry by molecules undergoing prototropic change.3 In so far as the acid-base systems of Jander and the indicator conjugate pair (intramolecular rearrangeothers involve electron rather than proton transfers i t ment is neglected). In the same way might be better to regard them as redox systems, e. g., Ca would be a reducing agent in the reaction COCl+ Ca+CO C1Ca++. It is possible Sometimes the same molecule may be studied from that solvents such as SOn, COC12, etc., would play a the viewpoint of electron or proton transfer, e. g., part in redox chemistry, act.ively engaging in electron transfer, resembling water in acid-base chemistry. C&OrHa CSHIOI-- 2H+
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WEISS,J., J. Chem. Soc., 1944, 309.
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"BELL,
R. P.,Qumt. Rev. Chem. Soe., 1, 113 (1947).
