A design to prevent floating within the N scale of nucleophilicity

Two nucleophilicity scales (N′ and N″) including 96 nucleophiles, spanning 1017 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 °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 ∼ 1 and the new E and N′ parameters are in satisfactory agreement with published data for E and N. For more polar nucleophiles, s ≠ 1 and values of N − N″ vary by five log units. Copyright © 2010 John Wiley & Sons, Ltd.