The polarographic reduction of conjugated hydrocarbons: IV. The mechanism of the reduction of various alternant and non‐alternant hydrocarbons

Abstract

The current-voltage curves of various alternant and non-alternant hydrocarbons were measured in 75% and %% dioxane-water. In many cases the limiting current of the first reduction wave corresponds to the overall addition of one electron. From the effect of the increase of the percentage of water and the addition of hydrogen iodide on the limiting current and on the half-wave potential of the first and the second wave, evidence was obtained about the reduction mechanism. The primary steps of the reduction are: R + e e R' and R' + e % R"; the standard potential determined by the first electron addition is less negative than that of the second step.When the rate of proton addition to R' is low with respect to the rate of diffusion of R'. the first wave is a one-electron wave and the second wave involves the reduction to R". When R' reacts with a proton in the electrode surface layer, the reduction proceeds according to:The reduction mechanism is discussed in detail and compared with the experimental results reported in this paper or found by other investigators.

Stanley Wawzonek and the Introduction of Polar Aprotic Solvents into Organic Electrochemistry

“…G. J. Hoijtink and coworkers had been studying the polarographic reduction of unsaturated hydrocarbons in 96% dioxane simultaneously with Wawzonek (the Hoijtink papers were published in 1954 and Wawzonek's publication was submitted to JES in August 1954). 18,19 They found that many such substances behave in this medium similarly to their behavior in DMF and AN, i.e., exhibiting two one-electron polarographic waves. Because of the lower concentration of water in 96% dioxane compared to 75% dioxane 7 protonation of 3 is sufficiently slow on the polarographic time scale to permit observation of the second wave for reduction of it to the diianion 5.…”

Polarographic Reduction of the Phenyl-Substituted Ethenes

“…Therefore, current-ratio plots and fl values would be expected to remain constant. Chemical reactions involving hydrogen ions preceded by reversible electrochemical reactions.---The following mechanism has been thoroughly discussed (19) with regard to the types of log plots obtained:…”

“…4 In both of the mechanisms of Eq. [19] and [20], it is difficult to predict current-ratio slopes and fl values, although behavior approximating that expected for a single slow electrochemical reaction is most likely.…”

A Combined Experimental and Theoretical Study on the Activity and Selectivity of the Electrocatalytic Hydrogenation of Aldehydes

“…The distribution and H-bonding behavior of the solvent molecules will strongly vary with the electrode potential and alter the interactions and rate of charge transfer between the target organic molecules and the electrode surface. Several previous studies have investigated the effect of the solvent on the electroreduction process of aromatic aldehydes. − In aqueous and aquo-alcoholic media, two successive one-electron waves during reduction are observed in the presence of acid: the first wave corresponding to the formation of a pinacol (fast process k ≈ 0.1 cm s –1 ), with the second wave corresponding to the formation of the alcohol. − The proton transfer mechanism, whether concerted or antecedent, was correlated to the solution pH. In aprotic solvents, including dimethylformamide (DMF) or acetonitrile, the acid serves to protonate the aromatic radical species formed via one-electron reduction, making subsequent additions of electrons easier. − Unlike protic solvents, ECH of aromatic aldehydes using aprotic solvents leads to dimerization products such as pinacol. ,,− These studies offer robust evidence that the solvent strongly influences the ECH reaction mechanism and selectivity.…”