A design to prevent floating within the N scale of nucleophilicity

Two nucleophilicity scales (N' and N '') including 96 nucleophiles, spanning 10(17) in rate constant, have been constructed directly from experimental rate constants for reactions of nucleophiles with dianisyl (log k = N') or dimethylaminobenzhydrylium (log k = N '') cations in dichloromethane at 20 degrees C. The two scales are linked/unified by the formula: N' - N '' = 6.6. In contrast, published procedures (H. Mayr et al. J. Am. Chem. Soc. 2001, 123, 9500, and later work) involve 23 benzhydrylium cations and over 200 adjustable parameters obtained from the equation log k = s(E + N), where k is the rate constant, E is an electrophilicity parameter and s is a nucleophile-specific sensitivity parameter. The definitions of N' and N '' avoid a floating N scale, i.e. having less than the usual attachment to fixed reference points. New equations, log k = (E + sN') or log k = (E + sN ''), where s is now an electrophile-specific parameter, are then used to correlate over 350 second-order rate constants for reactions of other benzhydrylium cations with nucleophiles. For weaker, less polar, nucleophiles, s similar to 1 and the new E and N' parameters are in satisfactory agreement with published data for E and N. For more polar nucleophiles, s not equal 1 and values of N - N '' vary by five log units. Copyright (C) 2010 John Wiley & Sons, Ltd.