ortho-Effect on the acid-catalyzed hydration of 2-substituted α-methylstyrenes

α-Methylstyrene and nine ortho-substituted analogs have been synthesized and the kinetics of their acid-catalyzed hydration in aqueous solutions of sulfuric acid at 25 °C have been investigated. The kinetic acidity function H_S has been constructed from the dependence of the observed rate constants k_obs on the sulfuric acid concentration. The catalytic rate constants of the acid-catalyzed hydration k_ortho have been calculated as well. The identical shape of the kinetic acidity functions for ortho- and para-derivatives confirms what the consistent mechanism A-SE2 of the acid-catalyzed hydration has already proved for the corresponding para-derivatives. The A-SE2 mechanism involves a rate-determining proton transfer of the hydrated proton to the substrate. From the dependence of the catalytic rate constants of the ortho-derivatives on the catalytic rate constants of the para-derivatives, it is seen that the logarithm of the catalytic rate constant for hydrogen as a substituent is markedly out of the range of the other substituents and, simultaneously, that the ortho-derivatives react significantly slower than the corresponding para-derivatives. In correlation with the substitent constants σ_p⁺, a reaction constant of ρ⁺ = –1.45 have been found. The constant is, in absolute value, considerably smaller than that for para-derivatives (ρ⁺ = –3.07). In parallel, the steric effects are enforced more significantly for the monoatomic substituents (slope of the Charton’s constants 3.92) than for substituents including more atoms (slope of the Charton’s constants 2.09). A small value of the reaction constant ρ⁺ has been elucidated due to the lower conjugation between the reaction centre and the benzene ring as a consequence of the geometric twist of the reaction centre out of the main aromatic plane accompanied by fading mesomeric interaction between the reaction centre and the substituents attached to the benzene ring. The isopropyl group in the carbocation is twisted less out of the aromatic plane for the monoatomic substituents and, therefore, also a small difference in the bulk of substituents has considerable steric influence on the conjugation between the carbocation and the benzene ring bearing substituents. On the contrary, the isopropyl group in the carbocations with polyatomic substituents is twisted to such a degree that changes in the bulk of substituents affect the resonant stabilization negligibly. Similar conclusions were also deduced from the correlations of the substitution constants σI and σ_R⁺.