82955-01-9Relevant articles and documents
Kinetics and mechanism of acid catalysed hydration of α- methylstyrenes
Pytela, Oldrich,Trlida, Bronislav
, p. 1025 - 1036 (2008/09/19)
Twelve para-substituted α-methylstyrenes with substituents H, CH 3, CF3, CH3O, CH3S, F, Cl, Br, CH3CO, CH3SO2, CN a NO2 were synthesised; additionally, the acid catalysed hydration kinetics of these compounds were measured in sulfuric acid in a concentration range c from 0.017 to 9.58 mol l-1, at 25.0°C. The observed rate constants obtained were used to construct the kinetic acidity function and calculate the catalytic rate constants. Based on the evaluation of the acidity function kinetic dependence on acid medium concentration, and the substituent effects of acid catalysed hydration of α-methylstyrenes on the catalytic rate constants, the mechanism of acid catalysed hydration was verified. The mechanism involves the addition of a proton to the double bond of α-methylstyrene in the rate-limiting reaction step denoted as A-SE2. No evident difference was found between the effects of the acid medium on the acid catalysed hydration of styrenes and α-methylstyrenes, which indicates very similar activity coefficients of the reactants, and of the transition state of both substrates. The substituent effects evaluation shows that the rate-limiting step of the reaction consists in the addition of a proton to the substrate. The carbocation formation in the transition state of this reaction step proceeds roughly half-way compared with the extent of the carbocation formation by cumyl chloride hydrolysis. The obtained carbocation is in particular stabilised by the substituents with +M effect, while the influence of the substituents with -M and I effects is significantly smaller.
A comparison of substituent effects on the stability of α,α-dimethylbenzyl carbocations in aqueous solution and in the gas phase: How significant is nucleophilic solvation?
Richard, John P.,Jagannadham, Vandannapu,Amyes, Tina L.,Mishima, Masaaki,Tsuno, Yuho
, p. 6706 - 6712 (2007/10/02)
Rate and equilibrium constants for conversion of ring-substituted cumyl alcohols in acidic solutions of 50:50 (v/v) trifluoroethanol/water (I = 0.50, NaClO4) to an equilibrium mixture of the corresponding cumyl alcohol, cumyl trifluoroethyl ether, and α-methylstyrene and the fractional yields of cumyl trifluoroethyl ether obtained from partitioning of the cumyl carbocation intermediates of these reactions between capture by water and by trifluoroethanol have been determined. These data and estimates of absolute rate constants for the reaction of ring-substituted cumyl carbocations with water in 50:50 (v/v) trifluoroethanol/water30 have been used to calculate equilibrium constants KR and Kp respectively for conversion of ring-substituted cumyl carbocations to the corresponding cumyl alcohols and α-methylstyrenes and the changes in Gibbs free energy (△Gx)sol for deprotonation of ring-substituted cumyl carbocations by α-methylstyrene. A plot of (△Gx)sol against (△Gx)gas for the corresponding reactions in the gas phase is linear with a slope of 0.70, in contrast to the previously reported unitary slopes of correlations of substituent effects on carbocation stability in solution and in the gas phase. We conclude that there is a modest increase in the stabilization of ring-substituted cumyl carbocations by solvation as their stability is decreased, but that this is much smaller than the change in stabilization by solvation with the changing stability of pyridinium and anilinium ions. The possible relevance of these data to the stabilization of carbocations by nucleophilic solvation is discussed.
