Cyclohexane, from benzene reduction stability

Benzene is reduced in 95% current yield to a mixture of 23% cyclohexadiene, 10% cyclohexene and 67% cyclohexane. HMPTA as a solvent additive seems to play a dual role. Firstly it is selectively adsorbed at the cathode surface, thereby preventing hydrogen evolution from the protic solvent. Thus it permits the attainment of a potential sufficiently cathodic for the generation of the solvated electron. It secondly stabilizes the solvated electron, thus suppressing its reaction with protic solvents (eq. (130) ). With decreasing HMPTA concentration in the electrolyte the current efficiency for reduction decreases and hydrogen evolution dominates. In pure ethanol the current efficiency is less than 0,4%. 

The various metals shovv significant differences in the stability of adsorbed layers with respect to oxidative and reductive attacks.Benzene is completely oxidized to CO2 on Pt at potentials where the oxygen adsorption takes place, whereas in the case of Rh and Pd a part of the adsorbed benzene desorbs from the surface without oxidation at potentials where the oxide layer is formed. A reductive treatment of benzene adsorbed on Pt and Pd leads to a desorption in the form of benzene and/or cyclohexane while from Rh only 15 [)ercent of the adsorbate is desorbable. 

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