Polar radicals. 15. Interpretation of substituent effects on the mechanism of electrolytic reduction of the carbon-halogen bond in series of substituted benzyl halides

Abstract

The electrochemical polarographic reduction of a series of substituted benzyl chlorides and benzyl bromides showed half-wave potentials which gave excellent Hammett linear free-energy correlations for each series. These correlations were obtained using " substituent constants. The correlation with ' suggests that the potential-determining electrochemical process involves carbon-halogen bond breaking or, alternatively, radical anion intermediate formation. The Hammett p values for each correlation were calculated; their magnitudes, Cl > Br, were in the inverse order of their reactivity toward chemical reduction. As in the case of trialkyltin hydride reduction, both p values were positive. The results of the polarographic electroreduction of benzyl iodide and benzylmercuric iodide confirmed the previously reported mechanism for reduction; namely, that the iodide underwent reaction with mercury to form benzylmercuric iodide, which was subsequently reduced, as was benzylmercuric iodide itself, by a process having two distinct polarographic waves. The half-wave potentials for the second wave (the wave at the most negative potential) for series of substituted benzyl iodides and benzylmercuric iodides were both found to give excellent Hammett correlation, having positive p values. The correlations were both obtained by using ' substituent constants. The success of the use of these substituent constants was taken as a confirmation of the suggestion, previously made, that the second step in the electroreduction was the reduction of the benzylmercuryl radical, and suggests that the process is concerted with carbon-mercury bond rupture.

Ab Initio MO and Density Functional Theory Study of Substituent Effects on Electron Attachment to Benzyl Chlorides

“…[1][2][3][4]43 According to our calculations and the literature data, this process appears to include the formation of complexes of benzyl 6a,40,41 or alkyl 44 radicals with halide anions. The formation of such complexes can occur even along the pathway of the so-called dissociative ET.…”

Kinetics and mechanism of the reaction of benzyl bromide with copper in hexamethylphosphoramide

“…The correlation with σ substituent constants gave no correlation (r = 0.77, S ρ = 0.28, r = 0.78, S ρ = 0.14) [32].…”

“…These data are listed in Table III. Polarographic reductions of organic halides are well known to be irreversible [31,32] and the overall twoelectron transfer process leads to carbanion which is Figure 3 Hammett plots of log (k/k o ) vs the substituent constant σ obtained for reaction of tributyltin radical with benzyl bromides [22]. then protonated.…”

Electrode reactions and electroanalysis of organomercury compounds

“…If R 2 Hg compounds formed under these conditions were electroactive, the polarization curve of electroreduction of RHgX (for example, benzylmercury halides 93,163 or alkenylmercury 35 ) demonstrated three waves. If the rate of a dismutation reaction such as (18a) was sufficiently high and R 2 Hg was electrochemically more active than R 2 Hg 2 , the second wave corresponded to the reduction of the overall compound and the third wave corresponded to the reduction of`organic calomel'.…”

“…, the Tafel dependences were much alike at Hg and GC electrodes (the deviations did not exceed 0.1 V) and virtually coincided for Hg and nanostructured carbon. 228 It is generally accepted 141 that OMC are formed most easily from organohalides that are capable of generating relatively stable radicals (allyl, 235 ± 237 benzyl and its derivatives, 73,93,141,163,210,211 etc. ); this is confirmed by numerous experimental data.…”