Twelve para-substituted α-methylstyrenes with substituents H, CH3, 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.
十二种带有取代基H、CH
3、CF
3、CH
3O、CH
3S、F、Cl、Br、CH
3CO、CH
3SO
2、CN和NO
2的α-
甲基苯乙烯被合成;此外,在浓度范围从0.017到9.58摩尔/升的
硫酸中,测量了这些化合物的酸催化
水合动力学。所得到的观察速率常数被用来构建动力学酸度函数并计算催化速率常数。通过评估酸性函数动力学对酸性介质浓度的依赖性,以及α-
甲基苯乙烯的酸催化
水合反应的取代基效应对催化速率常数的影响,验证了酸催化
水合的机理。该机理涉及在速率限制的反应步骤中向α-
甲基苯乙烯的双键加入质子,被标记为A-
SE2。在酸性介质对
苯乙烯和α-
甲基苯乙烯的酸催化
水合的影响之间没有明显差异,这表明反应物和两种底物的过渡态的活性系数非常相似。取代基效应评估显示,反应的速率限制步骤在向底物加入质子时发生。与叔丁基
氯化物
水解形成的碳正离子相比,该反应步骤的过渡态中的碳正离子形成大致进行了一半。所得到的碳正离子特别受到具有+M效应的取代基的稳定作用,而具有-M和I效应的取代基的影响则明显较小。