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A design to prevent floating within the N scale of nucleophilicity
Author(s) -
Bentley T. William
Publication year - 2010
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1670
Subject(s) - nucleophile , chemistry , electrophile , reaction rate constant , dichloromethane , polar , order (exchange) , computational chemistry , medicinal chemistry , stereochemistry , kinetics , organic chemistry , physics , solvent , catalysis , quantum mechanics , economics , finance
Abstract 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.