Hydrogenation mechanism comparison

Virk and co-workers (24b,c) and King and Stock (35b) have reported rates for H2-transfer to anthracene and phenanthrene in solution containing 1,2- and 1,4-dihydronaphthalene and tetralin. Comparisons between reported rate constants and estimated rate constants for bimolecular disproportionation are given in Table VI. In agreement with Stock, this data does not provide evidence for a concerted H2-transfer mechanism. Our calculations indicate that molecular disproportionation may be a major hydrogenation mechanism in these reaction systems. 

We shall examine the simplest possible molecular orbital problem, calculation of the bond energy and bond length of the hydrogen molecule ion Hj. Although of no practical significance, is of theoretical importance because the complete quantum mechanical calculation of its bond energy can be canied out by both exact and approximate methods. This pemiits comparison of the exact quantum mechanical solution with the solution obtained by various approximate techniques so that a judgment can be made as to the efficacy of the approximate methods. Exact quantum mechanical calculations cannot be carried out on more complicated molecular systems, hence the importance of the one exact molecular solution we do have. We wish to have a three-way comparison i) exact theoretical, ii) experimental, and iii) approximate theoretical. 

This mechanism does not require a decision as to the question of whether the association of imidazole occurs through hydrogen bonding or by ionization. - However, if the methylation with diazomethane is considered together with methylations with dimethyl sulfate, dimethyl sulfate and alkali, and methyl iodide and the silver derivative of the imidazole, then such a comparison is best done using the hydrogen-bonded association model. 

Thermogravimetric data indicate that the structure of a polymer affects stability in a neutral environment (HI). A polymer such as Teflon, with carbon-carbon bonds which are (by comparison) easily broken, and with strong carbon-fluorine bonds, is quite stable thermally. However, polyethylene, also with carbon-carbon bonds but containing carbon-hydrogen bonds which are broken relatively easily in comparison with the carbon-fluorine bond, is less stable than Teflon. In turn, polyethylene is more stable than polypropylene. This difference in stability is probably caused by tertiary carbon-hydrogen bonds in polypropylene. Polypropylene is more stable than polyisobutylene or polystyrene, which decompose principally by unzipping mechanism. 

On this basis Cr(V), not Cr(IV), is the kinetically important intermediate such that k = 3 k4 and k = k Jk. The hydrogen-ion dependence of the reaction rate has been discussed. Furthermore, comparisons are drawn with the rate of the Cr(VI)+Fe(phen)3 reaction, and Sullivan has speculated on the intimate nature of both mechanisms in the light of Marcus theory... 

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