Couloamperometry. A fast kinetic technique for halogenations. I. Bromination rate constants of highly reactive enol ethers

Abstract

AbstractA new couloamperometric apparatus has been designed to extend the range of this kinetic technique to the measurement of very high rate constants, 108M−1s−1, by using TFCR‐EXSEL conditions (TFCR—very low reactant concentration; EXSEL—salt excess), which give half‐lives of a few seconds for very fast second‐order reactions. Very low faradaic currents, in the nanoampere range for halogens, corresponding to very low reactant concentrations of 10−8–10−9M, are measured selectively by compensating the eddy currents, principally the residual and the induced currents. When the electroactive species is bromine, the concentration is demonstrated to be linearly related to the limiting reduction current in the very low concentration range. The upper limit of this technique for bromination is at present 3 × 108M−1s−1. The method is applied to the kinetic study of highly reactive enol ethers EtO‐C(R) = CH‐R′, where R and R′ are H or Me. A value of 2.2 × 108M−1s−1 is obtained for k, the rate constant for free bromine addition to EtO‐CH = CH2, by extrapolating the kinetic bromide ion effects to [Br−] = 0. An α‐methyl effect (kα‐Me/kH)EtO of 15 is found; this is a small decrease in the methyl effect compared to the marked increase in the double bond reactivity. For the enol acetate MeCOO‐CH = CH2, whose rate constant is 6 × 102M−1s−1, (kα‐Me/kH)OCOMe is 21. The dependence of substituent effects on reactivity is discussed in terms of the Hammond effect on the transition state position and of charge delocalization by group G of olefins G‐CH = CH2.