Breaking S-H Bonds

The initial step of the adsorption of thiols on Mo(100) surface is the formation of adsorbed thiolate groups. Phenyl thiolate is formed upon the adsorption of benzenethiol at 120 K on a clean Mo(110) surface.49 The thiolate intermediate subsequently undergoes competing C—S bond hydrogenolysis to form benzene, or C—S and C—H bond scission to form surface benzyne. The adsorption of benzenethiol was also studied on a sulfur-covered Mo surface.50 Phenyl disulfide is formed via S—H bond scission and S-S bond formation. The S-S linkage is oriented perpendicular and the phenyl ring parallel to the surface. 

dissertation, Effects of Solvent and Surface Structure on Catalytic Hydrogenation of Olefins, Southern Illinois University at Carbondale, 1982, pp. 30-31. 

Summaries are given in Rylander, R N., Hydrogenation Methods, Academic Press, New York, 1985, p. 67 and Rylander, P. N., Catalytic Hydrogenation in Organic Syntheses, Academic Press, New York, 1979, pp. 72-73. 

Rylander, P. N., Hydrogenation Methods, Academic Press, New York, 1985, p. 67. 

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This reaction is exothermic by a significant amount and is driven by the strong S-H bond formation while only breaking a relatively weak C-S bond. From DFT calculations, AG = -29.7 kcal moTh... 

The hydroxide must do something, and since it is negatively charged, a reasonable starting point is going to be to use it as a nucleophile to break the S—H bond. Hydroxide is after all a base it likes to remove protons. So here s the first step ... 

The theory of electrolytic dissociation was not immediately recognized universally, despite the fact that it could qualitatively and quantitatively explain certain fundamental properties of electrolyte solutions. For many scientists the reasons for spontaneous dissociation of stable compounds were obscure. Thus, an energy of about 770kJ/mol is required to break up the bonds in the lattice of NaCl, and about 430kJ/mol is required to split H l bonds during the formation of hydrochloric acid solution. Yet the energy of thermal motions in these compounds is not above lOkJ/mol. It was the weak point of Arrhenius s theory that this mismatch could not be explained. 

It is seen that the values of kd are very close. Hence, the reaction of POOH with the C—H bond is not the main initiation reaction. If the breakdown is a monomolecular process, the rate of O—O bond homolysis in polymer must be close to that in the gas phase. 2,2-Dimethylethyl hydroperoxide breaks down in the gas phase with a rate constant of 1.6 x 1013 exp(— 158/i 7) = 5.3 x 10 x s 1 (398 K, [4]), that is, by four orders of magnitude more slowly than in polymer. Hence, the decomposition reactions in the polymers are much faster than the monomolecular homolysis of peroxide. Decomposition reactions may be of three types (see Chapter 4), such as the reaction of POOH with a double bond... 

This all suggests slow, rate-limiting breaking of the C—H bond to form the stabilised carbanion intermediate (54), followed by fast uptake of D from the solvent D2O. Loss of optical activity occurs at each C—H bond breakage, as the bonds to the carbanion carbon atom will need to assume a planar configuration if stabilisation by delocalisation over the adjacent C=0 is to occur. Subsequent addition of D is then statistically equally likely to occur from either side. This slow, rate-limiting formation of a carbanion intermediate, followed by rapid electrophilic attack to complete the overall substitution, is formally similar to rate-limiting carbocation formation in the S i pathway it is therefore referred to as the SeI pathway. 

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