Schaller Heike F, Tishkov Alexander A, Feng Xinliang, Mayr Herbert
Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), 81377 München, Germany.
J Am Chem Soc. 2008 Mar 12;130(10):3012-22. doi: 10.1021/ja0765464. Epub 2008 Feb 19.
Rates and equilibria of the reactions of highly stabilized amino-substituted benzhydrylium ions (Ar2CH+) with carboxylate ions have been determined photometrically in acetone and acetonitrile solutions. Treatment of covalent benzhydryl carboxylates (Ar2CH-O2CR) with aqueous acetone or acetonitrile leads to the regeneration of the colored amino-substituted benzhydrylium ions Ar2CH+, which do not undergo subsequent reactions with the solvent. One can, therefore, directly measure the first step of S(N)1 reactions. The electrofugality order, i.e., the relative ionization rates of benzhydryl esters Ar2CH-O2CR with the same anionic leaving group, does not correlate with the corresponding electrophilicity order, i.e., the relative reactivities of the corresponding benzhydrylium ions Ar2CH+ toward a common nucleophile. Thus, benzhydrylium ions which are produced with equal rates by ionization of the corresponding covalent esters may differ by more than 2 orders of magnitude in their reactivities toward nucleophiles, e.g., carboxylate ions. Variable intrinsic barriers account for the breakdown of the rate-equilibrium relationships. Complete free-energy profiles for the ionization of benzhydryl carboxylates Ar2CH-O2CR are constructed, which demonstrate that the transition states of these ionizations are not carbocation-like. As a consequence, variation of the solvent-ionizing power Y has only a small effect on the ionization rate constant (m = 0.35 to 0.55) indicating that small values of m in the Winstein-Grunwald equation do not necessarily imply an S(N)2 type mechanism.
在丙酮和乙腈溶液中，通过光度法测定了高度稳定的氨基取代二苯甲基鎓离子（Ar2CH+）与羧酸根离子反应的速率和平衡。用含水丙酮或乙腈处理共价二苯甲基羧酸盐（Ar2CH-O2CR）会导致有色的氨基取代二苯甲基鎓离子Ar2CH+再生，该离子不会与溶剂发生后续反应。因此，可以直接测量单分子亲核取代反应（S(N)1）的第一步。离去倾向顺序，即具有相同阴离子离去基团的二苯甲基酯Ar2CH-O2CR的相对电离速率，与相应的亲电顺序，即相应的二苯甲基鎓离子Ar2CH+对常见亲核试剂的相对反应活性，并不相关。因此，由相应共价酯电离以相同速率产生的二苯甲基鎓离子，其对亲核试剂（如羧酸根离子）的反应活性可能相差超过2个数量级。可变的内在势垒导致了速率-平衡关系的破坏。构建了二苯甲基羧酸盐Ar2CH-O2CR电离的完整自由能剖面图，结果表明这些电离的过渡态不像碳正离子。因此，溶剂电离能力Y的变化对电离速率常数的影响很小（m = 0.35至0.55），这表明温斯坦-格伦瓦尔德方程中较小的m值并不一定意味着是双分子亲核取代（S(N)2）型机制。
