Strong Protic Acid Research Articles

Homogeneous catalysis of the water gas shift reaction using a platinum chloride-tin chloride system

An efficient homogeneous catalyst consisting of K/sub 2/PtCl/sub 4/, SnCl/sub 4/, H/sub 2/O, and HCl dissolved in glacial acetic acid was developed for the water gas shift reaction, H/sub 2/O + CO reversible H/sub 2/ + CO/sub 2/. Requirements and features of the catalyst are: (1) no reaction takes place in the absence of the platinum chloride salt; (2) the omission of tin chlorides greatly diminishes catalytic activity, leading to decomposition and formation of a brown precipitate; (3) large excesses of the tin chlorides inhibit catalysis- e.g., at a tin/ platinum ratio of 300/1, no reaction occurs; (4) if acetic acid is replaced by water, only a trace of carbon dioxide is produced and a dark brown precipitate is obtained; (5) no catalysis occurs in the absence of either a strong protic acid or a chloride source. A mechanism involving the tin(II)/tin(IV) couple and a platinum species is proposed.

Metal catalysis in oxidation by peroxides. Part 2. Molybdenum catalysed oxidation of organosulphur compounds by t-butyl hydroperoxide

The reaction of di-n-butyl sulphide with t-butyl hydroperoxide in dry ethanol at 25°, catalysed by dioxomolybdenum(VI) acetylacetonate, affords di-n-butyl sulphoxide in quantitative yield. The kinetics conform to a rate law which is first order in sulphide and hydroperoxide; in the range of catalyst concentrations from 1.0 × 10–7 to ca. 200 × 10–7M, it is first order in the molydbdenum chelate, allowing a catalytic third-order rate constant (k3) of 162 l2 mol–2 s–1 to be estimated. This is ca. 2 × 104 higher than the k3 value determined for catalysis by methanesulphonic acid, a typical strong protic acid, in the same solvent. At a given sulphide and metal catalyst concentration, dependence of rates on the initial concentration of ButO2H suggests that a Michaelis–Mententype rate law holds. A Lineweaver–Burk type treatment of data allows values of 0.56 mol l–1 for K′(a constant related to the dissociation constant of a postulated intermediate hydroperoxide–metal ion complex) and 103 l mol–1 s–1 for kII(the limiting specific rate constant for rate-determining nucleophilic attack by the sulphide on the metal ion-activated peroxide molecule) to be estimated. The relative rates of oxidation of some representative substrates were found to decrease in the order Bun2S (100) > Bun2SO (0.15) cyclohexene (0.01), indicating that the molybdenum-catalysed reaction is quite sensitive to substrate nucleophilicity.

The Acid-catalyzed Isomerization of the Adducts of Azodiacyls with Cyclopentadiene and 1,3-Cyclohexadiene

The isomerization of the Diels–Alder adducts of azodiacyls with cyclopentadiene and 1,3-cyclohexadiene is powerfully catalyzed by strong protic acids and by Lewis acids. In adducts from p-substituted azodibenzoyls with cyclopentadiene the catalytic effect increases with electron release by the substituent.