The kinetics of the hydrogenation of certain unsaturated organic compounds.
thesisposted on 19.11.2015, 08:48 by Dennis Edward. Webster
The hydrogenation of a number of unsaturated compounds containing carbon-oxygen bonds has been studied. In most cases the reactions were performed at 30C and one atmosphere pressure of hydrogen, over Adams platinum oxide catalyst, in 50ml. of a solvent, usually containing some acid. It has been concluded from the results obtained that :- (1). Hydrogenolysis of the carbon-oxygen bond is always prevalent when acid or acidified solvents are used with a substrate having a centre of unsaturation close to the carbon-oxygen link. Thus under these conditions phenols, benzyl alcohols, allylic alcohols and cyclohexanones cleave readily. Removal of the centre of unsaturation from the vicinity of the carbon-oxygen bond renders it less liable to hydrogenolysis, as shown by the considerably smaller cleavage of ?-phenyl ethyl alcohol. The kinetics of the hydrogenolysis of phenols and benzyl alcohols show marked similarities with the kinetics of the hydrogenolysis of the corresponding phenyl and benzyl chlorides and quaternary ammonium salts. In the case of phenols only small amounts of aromatic hydrocarbons are observable in the liquid phase during hydrogenolysis, in contrast to the case of chlorobenzene and trimethylphenyl ammonium chloride where cleavage leads to large amounts of benzene appearing in the liquid phase. In all the examples studied very little hydrogenolysis takes place in neutral or weakly basic solutions, and the proportion of hydrogenolysis is affected by the amount and acid strength of the acid used. (2). Addition of hydrogen to the unsaturation, producing the saturated alcohol, is also observed for all the oxygenated compounds tried, and in the absence of acid this appears to be the major reaction. Of particular interest is the case of phenols where the reduction of the aromatic ring is shown to be, to some extent, a two stage process. Cyclohexanone has been shown to be an intermediate in the reaction. An attempt has been made to estimate the extent of this two stage mechanism by interception of the cyclohexanone with semicarbazide, and by examination of the proportions of cis- and trans- methylcyclo- hexanols from ortho-, meta- and para-cresols and from the corresponding methylcyclohexanones. Addition of other aromatic molecules (e.g. benzene, toluene) and cyclohexene reduces the rate of the reduction of phenol, benzyl alcohol and cyclohexanones. Substituents in the ring of these compounds also led to smaller rates of reduction, the effect being qualitatively similar for both hydrogenolysis and addition reactions. The reduction of phenols and cyclohexanones in acidic methanol leads to the formation of cyclo-hexyl methyl ethers. The overall reactions are first order with respect to hydrogen pressure and weight of catalyst, and zero order with respect to the substrate concentration.