Nakata Kazuhide, Fujio Mizue, Nishimoto Kichisuke, Tsuno Yuho
Science Research Center, Hosei University, 2-17-1 Fujimi, Chiyoda-ku, Tokyo 102-8160 (Japan), Fax: (+81) 3-3264-9427.
Institute for Materials Chemistry and Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581 (Japan).
Chempluschem. 2013 Sep;78(9):1099-1108. doi: 10.1002/cplu.201300167. Epub 2013 Jul 26.
The relative gas-phase acidities of 25 ring-substituted benzoic and phenylacetic acids were theoretically determined with proton-transfer reactions. The energies and geometries of the acids and their corresponding anions, which were involved in the reactions, were calculated at the B3LYP/6-311+G(2d,p) level of theory. The obtained substituent effects were compared with each other and those of phenols. The acidities of the benzoic and phenylacetic acids were governed by three kinds of electronic effects (inductive, resonance, and saturation effects), which confirmed that the acidities were reflected in the nature of the benzoate and phenylacetate anions, respectively. Substituent effect analyses with an extended Yukawa-Tsuno equation, ${ - {\rm{\Delta }}E_X = \rho \left( {\sigma ^0 + r^ - {\rm{\Delta }}\bar \sigma _{\rm{R}}^ - + s{\rm{\Delta }}\bar \sigma _{\rm{S}} } \right)}$, gave excellent linear correlations for both anionic systems. The degrees of through-resonance and saturation effects in the phenylacetate anion, as reflected by the resultant r and s values, were unexpectedly larger than those in the benzoate anion, which were mainly attributed to hyperconjugation and through-space interactions between the anionic moiety and the benzene π-electron system, respectively. The acidities of the benzoic acids (or stabilities of the benzoate anions) constituted a better system than the acidities of phenylacetic acids (or stabilities of the phenylacetate anions) for a standard of the normal substituent constants (σ ) for anions in the gas phase, in contrast to solution-phase results.
通过质子转移反应从理论上测定了25种环取代苯甲酸和苯乙酸的相对气相酸度。在B3LYP/6-311+G(2d,p)理论水平下计算了反应中涉及的酸及其相应阴离子的能量和几何结构。将得到的取代基效应相互比较，并与酚类的取代基效应进行比较。苯甲酸和苯乙酸的酸度受三种电子效应（诱导效应、共振效应和饱和效应）的支配，这证实了酸度分别反映在苯甲酸根阴离子和苯乙酸根阴离子的性质上。用扩展的汤川-津野方程${ - {\rm{\Delta }}E_X = \rho \left( {\sigma ^0 + r^ - {\rm{\Delta }}\bar \sigma _{\rm{R}}^ - + s{\rm{\Delta }}\bar \sigma _{\rm{S}} } \right)}$进行取代基效应分析，结果表明两种阴离子体系均具有良好的线性相关性。由所得的r和s值反映出，苯乙酸根阴离子中的贯穿共振效应和饱和效应程度出乎意料地大于苯甲酸根阴离子中的相应效应程度，这主要分别归因于阴离子部分与苯π电子体系之间的超共轭作用和空间相互作用。与溶液相结果相反，对于气相中阴离子的标准取代基常数(σ)而言，苯甲酸的酸度（或苯甲酸根阴离子的稳定性）构成了一个比苯乙酸的酸度（或苯乙酸根阴离子的稳定性）更好的体系。
