摘要:
350 CHEMICAL COMMUNICATIONS The Gas-phase Hydrogen Bromide-catalysed Decomposition of Trimethylacetic Acid By J. T. D. CROSS and V. R. STIMSON (University of New England, Armidale, N.S. W., Australia) CARBOXYLATION of olefins is achieved with carbon monoxide at high pressure in the presence of strong acid catalysts, doubtless via the acylium ion, and, in view of the mechanisms suggested for the hydrogen halide-catalysed decompositions of alcohols1 and ethers,, the reverse reaction in the gas phase has been sought. For trimethylacetic acid and hydrogen bromide reaction does occur at measurable rates at 343-462"c for 21-184 and 12-341 mm. initial pressures of the reactants, respectively. Isobutene, carbon monoxide, and water are produced quantitatively and hydrogen bromide is not lost.Me,CCO,H + HBr --f Me,C : CH, + CO + H,O + HBr Individual runs followed the first-order rate law accurately to 50% reaction in general, and the first-order constants were proportional to the initial pressures of hydrogen bromide, e.g., at 422" for variations of 9 and 12 times in the reactants'NUMBER 12, 1966 351 pressures. The rates fit well the Arrhenius equation k,(sec.-1 C.C. mole-') = exp(- 31,70O/RT) Addition of cyclohexene (7-13 cm.) to the reaction or increase in the surface: volume ratio of the vessel by 8 times did not affect the rate. Preliminary measurements under comparable conditions indicate that the acid itself decomposes at above ca. 450°, probably by a radical mechanism, to a number of breakdown products including C,-hydrocarbons ; this reaction was not significant compared with the catalysed reaction. Formation of olefin via the alkyl halide (I) (cf. Case I, ref. 1) is not here kinetically reasonable. Concerted electron movements in a seven-mem- bered ring (11), a type not usually favoured, is required for a ring transition state analogous to Case I1 of ref. 1. The extreme case of gas-phase heterolysis analogous to Case 111, ref. 1, may be represented by (111), and this implies the reverse of the synthesis of the acid in solution; acid- catalysed loss of carbon monoxide is well known for tertiary acids in s~lution.~ L -I (Received, April 13th, 1966; Corn. 234.) A. Maccoll and V. R. Stimson, J . Chem. SOG., 1960, 2836. V. R. Stimson and E. J. Watson, Austral. J . Chem., 1966, 19, 75. 3 E. Rothstein and R. W. Saville, J . Chem. SOG., 1949, 1950 and 1961.
ISSN:0009-241X
DOI:10.1039/C19660000350
出版商:RSC
年代:1966
数据来源: RSC