Minegishi Shinya, Kobayashi Shinjiro, Mayr Herbert
Department Chemie der Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13 (Haus F), D-81377 München, Germany.
J Am Chem Soc. 2004 Apr 28;126(16):5174-81. doi: 10.1021/ja031828z.
The rates of the reactions of benzhydrylium ions (diarylcarbenium ions) with solvent mixtures of variable composition (water/acetonitrile, methanol/acetonitrile, ethanol/acetonitrile, ethanol/water, and trifluoroethanol/water) have been determined photometrically by conventional UV-vis spectroscopy, stopped-flow methods, and laser flash techniques. It has been shown that the first-order rate constants follow the previously published relationship log k(20 degrees C) = s(N + E), where E is an empirical electrophilicity parameter, N is an empirical nucleophilicity parameter, and s is a nucleophile-specific slope parameter. From plots of log k versus E of the benzhydrylium ions are derived the solvent nucleophilicity parameters s and N, the latter of which are designated as N1 to emphasize that their use in the quoted correlation equation gives rise to first-order rate constants. A linear correlation between N1 and Kevill's solvent nucleophilicity NT based on S-methyldibenzothiophenium ions is reported, which allows one to interconvert the two sets of data. Because the N1 values are directly comparable to the previously reported nucleophilicity parameters N for pi-systems (www.cup.uni-muenchen.de/oc/mayr/), the systematic design of Friedel-Crafts reactions with solvolytically generated carbocations becomes possible.
通过传统紫外可见光谱法、停流法和激光闪光技术，以光度法测定了二苯甲基正离子（二芳基碳正离子）与组成可变的溶剂混合物（水/乙腈、甲醇/乙腈、乙醇/乙腈、乙醇/水和三氟乙醇/水）的反应速率。结果表明，一级速率常数符合先前发表的关系log k(20℃)=s(N + E)，其中E是一个经验亲电参数，N是一个经验亲核参数，s是一个亲核试剂特异性斜率参数。从二苯甲基正离子的log k对E的图中得出溶剂亲核参数s和N，其中后者被指定为N1，以强调它们在引用的相关方程中的使用会产生一级速率常数。报道了基于S-甲基二苯并噻吩鎓离子的N1与凯维尔溶剂亲核性NT之间的线性相关性，这使得两组数据可以相互转换。由于N1值可直接与先前报道的π体系亲核参数N（www.cup.uni-muenchen.de/oc/mayr/）进行比较，因此利用溶剂解产生的碳正离子进行傅里德-克拉夫茨反应的系统设计成为可能。
