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 °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.