Richard John P, Williams Kathleen B
Department of Chemistry, University at Buffalo, SUNY, Buffalo New York 14260-3000, USA.
J Am Chem Soc. 2007 May 30;129(21):6952-61. doi: 10.1021/ja071007k. Epub 2007 May 9.
Rate and equilibrium constants were determined for protonation of ring-substituted -methoxystyrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted alpha-methyl alpha-methoxybenzyl carbocations at 25 degrees C and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by 14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Brønsted coefficient alpha for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents, and the Brønsted coefficient beta for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in these Brønsted coefficients with changing reaction driving force, (inverted theta)alpha/ (inverted theta) deltaG(av) degrees=(inverted theta)beta/(inverted theta)delta G(av) degrees= 1/8lambda = 0.011, is used to calculate a Marcus intrinsic reaction barrier of lambda= 11 kcal/mol which is close to the barrier of 13 kcal/mol for thermoneutral proton transfer between this series of acids and bases. The value of alpha= 0.66 for thermoneutral proton transfer is greater than alpha= 0.50 required by a reaction that follows the Marcus equation. This elevated value of beta may be due to an asymmetry in the reaction coordinate that arises from the difference in the intrinsic barriers for proton transfer at the oxygen acid reactant and resonance-stabilized carbon acid product.
在25℃和离子强度I = 1.0（KCl）条件下，测定了环取代的对甲氧基苯乙烯被水合氢离子和羧酸质子化形成相应的环取代α-甲基α-甲氧基苄基碳正离子的速率常数和平衡常数。当苯环取代基从4-MeO-变为3,5-二-NO₂-，以及羧酸从二氯乙酸变为乙酸时，碳正离子形成的热力学势垒增加了14.5 kcal/mol。在该苯环取代基范围内，羧酸质子化的布朗斯特系数α从0.67增加到0.77，并且当羧酸从二氯乙酸变为乙酸时，质子转移的布朗斯特系数β从0.63增加到0.69。这些布朗斯特系数随反应驱动力的变化，（倒三角）α/（倒三角）ΔG（av）° =（倒三角）β/（倒三角）ΔG（av）° = 1/8λ = 0.011，用于计算马库斯本征反应势垒λ = 11 kcal/mol，这与该系列酸碱之间热中性质子转移的13 kcal/mol势垒相近。热中性质子转移的α值为0.66，大于遵循马库斯方程的反应所需的α = 0.50。β值的升高可能是由于反应坐标的不对称性，这是由氧酸反应物和共振稳定的碳酸产物中质子转移的本征势垒差异引起的。
