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

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Title:ortho-Effect on the acid-catalyzed hydration of 2-substituted α-methylstyrenes
Creators:
Prusek, Ondřej
Bureš, Filip
Pytela, Oldřich
Journal or Publication Title:
Collection of Czechoslovak Chemical Communications, 74, 1, pp. 85-99
Uncontrolled Keywords:Kinetics, <i>ortho</i>-Effect, Styrene, Hydration, Acidity function, Substituent effects

Abstract

α-Methylstyrene and nine <i>ortho</i>-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 <i>H</i><sub>S</sub> has been constructed from the dependence of the observed rate constants <i>k</i><sub>obs</sub> on the sulfuric acid concentration. The catalytic rate constants of the acid-catalyzed hydration <i>k</i><i><sub>ortho</sub></i> have been calculated as well. The identical shape of the kinetic acidity functions for <i>ortho</i>- and <i>para</i>-derivatives confirms what the consistent mechanism A-SE2 of the acid-catalyzed hydration has already proved for the corresponding <i>para</i>-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 <i>ortho</i>-derivatives on the catalytic rate constants of the <i>para</i>-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 <i>ortho</i>-derivatives react significantly slower than the corresponding <i>para</i>-derivatives. In correlation with the substitent constants σ<sub>p</sub><sup>+</sup>, a reaction constant of ρ<sup>+</sup> = –1.45 have been found. The constant is, in absolute value, considerably smaller than that for <i>para</i>-derivatives (ρ<sup>+</sup> = –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<sup>+</sup> 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<sub>R</sub><sup>+</sup>.

Title:ortho-Effect on the acid-catalyzed hydration of 2-substituted α-methylstyrenes
Creators:
Prusek, Ondřej
Bureš, Filip
Pytela, Oldřich
Uncontrolled Keywords:Kinetics, <i>ortho</i>-Effect, Styrene, Hydration, Acidity function, Substituent effects
Divisions:Life and Chemical Sciences > Institute of Organic Chemistry and Biochemistry > Collection of Czechoslovak Chemical Communications
Journal or Publication Title:Collection of Czechoslovak Chemical Communications
Volume:74
Number:1
Page Range:pp. 85-99
ISSN:0010-0765
E-ISSN:1212-6950
Publisher:Institute of Organic Chemistry and Biochemistry
Related URLs:
URLURL Type
http://dx.doi.org/10.1135/cccc2008115UNSPECIFIED
ID Code:4088
Item Type:Article
Deposited On:02 Mar 2009 10:29
Last Modified:02 Mar 2009 09:29

Citation

Prusek, Ondřej; Bureš, Filip; Pytela, Oldřich (2009) ortho-Effect on the acid-catalyzed hydration of 2-substituted α-methylstyrenes. Collection of Czechoslovak Chemical Communications, 74 (1). pp. 85-99. ISSN 0010-0765

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