OXIDATIONOF FORMATE IONBY HALOGENS IN THE DARK
Aug., 1935
1437
Studies on the Relationship between the Rates of Reactions a d Oxidation-Reduction Potentials. 1. Oxidation of Formate Ion by Halogens in the Dark BY BACONF. CHOW' Introduction I t is generally believed that there is no apparent relationship between the kinetic rate constant and the thermodynamic equilibrium constant. However, there is now collected a mas3 of data to question such a belief. Conant and Pratt,2 in their study of apparent oxidation potentials of aminophenols and phenols, found that the rate of oxidation is related to the potential of oxidants employed. They postulated that the first step in the reaction is a reversible equilibrium between the oxidant and phenols ROH Phenol
+
B F R O - + B H Oxidant
and the second step RO-
--+
R=O
I
is a relatively slow one. On the basis of this postulate they were able to formulate a relationship between rates and potentials from ordinary electrochemical and the kinetic equations. Considering the difficulties of the experiments and uncertainties of the course of reactions, they obtained a satisfactory agreement between the theory and the experimental results. It was also found that the rate constant of mutarotation3 of glucose is proportional to the basicity of various catalysts, and that the rate of alkylation4 of trimethylamine by the methyl ester of carboxylic acid is related to the strength of the acid. Barrens found that the rate of autoxidation of various indophenols is dependent on their oxidation-reduction potentials. These and numerous other facts,6 when combined, seem to show that when one or more reactants form a labile equilibrium in the solution either in the form of addition or loss of proton or electron, the rate of reaction is also dependent on that equilibrium. In our previous study' of oxygen absorption of oleic acid and linseed 'oil, as catalyzed by ferri(1) Address: Peiping Union Medical College, Peiping, China. (2) J. B. Conont and M. F. Pratt, THIS JOURNAL, 48, 3178, 3220 (1926). (3) J, N.Bronsted and Guggenheim, ibid., 49, 2554 (19273. (4) L.P. Hammett and H. L. PBuger, ibid., 66, 4079 (1933). (5) E.S. Guzman Barron, J. B i d . Ckcm., 91, 287 (1932). (6) V. K. La Mer and J. W. Temple, Proc. Nul. A c n d . Sci., 15, 191 (1Y29);0.Dirnroth, 2. angnu. Chcm., 46,571 (1933). (71 B. F. Chow, ' T i m JOURNAL, 66, 894 (1934); B . F. Chow and S. 73. Kamerling, J . R i d . Chcm., 104,69 (1934).
cyanide, it was found that the rate of oxygen absorption is not only dependent upon the concentration of ferricyanide but also the ratio of ferri- to ferrocyanide. An empirical relationship which is somewhat similar to Conant and Pratt's was suggested and expressed by Rate
(1)
ae(nE'E/3HT)
where e is the base of natural logarithm and E is the oxidation-reduction potential of the system. n, F , R and T retain their usual meanings in electrochemistry. The purpose of this paper is to show that the same relationship will also hold true in the oxidation of formate ion by halogens, iodine and bromine. The mechanism of the oxidation of formate by bromine8 or iodinee has been studied in great detail by numerous investigators. The reaction was found to be of tbe second order with respect to formate ion and halogen. The rate of osidation is greatly decreased by the addition of the corresponding halides which increase the formation of tri-halides, according to the equilibrium (X*)(X-)/(Xa-) = K
(2)
Now according to empirical equation (1) the rate of reduction of the halogen by formate ion is given by -d(X2)/dt = k(r)(X2)e(nFE/3'
