Benzene, carbon formation, hydrogen effect

In this paper it is shown that the growth of carbon deposits can be maintained for long periods of time in the presence of hydrogen. The effect of hydrogen on the kinetics of carbon formation from benzene in hydrogen has been studied in experiments using nickel and iron foils. The results are presented below. 

The same workers have more recently used a similar catalyst system to produce formic acid (formate ion) from hydrogen and carbon dioxide (160). The catalyst system is again Pd(diphos)2 and Et3N in benzene solution under 25 atm each of C02 and H2. The reaction is run at room temperature or higher. A small amount of water dramatically accelerates the rate of the reaction, and a mechanism is proposed to account for this effect. Control experiments are stated to rule out the initial reduction of C02 to CO by H2, followed by reaction with water to yield the formic acid. 

Effect of Hydrogen on the Iron- and Nickel-Catalyzed Formation of Carbon from Benzene... 

In some instances, particularly in hydroxyla-tions meta to a halide substituent, the hydrogen on the hydroxylated carbon is quantitatively lost (i.e., there is no NIH shift), and a small deuterium kinetic isotope effect is observed These hydroxylations could result from direct oxygen insertion into the C-H bond, as in a true hydrox-ylation mechanism, but they are more likely to result from oxidation of the aromatic ring without the formation of a discrete epoxide intermediate. Isotope effect studies with deuterated benzenes bearing a variety of substituents have shed some light on this process A small, normal isotope effect is observed for weta-hydroxylation when deuterium is located meta- to the halogen in chlorobenzene = 1.1-1.3), but a small,... 

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