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Ionizing Power of Aprotic Solvents
Author(s) -
Streidl Nicolas,
Mayr Herbert
Publication year - 2011
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201001700
Subject(s) - chemistry , heterolysis , nucleophile , acetonitrile , reaction rate constant , solvent , solvent effects , dimethyl sulfoxide , ion , chloride , medicinal chemistry , photochemistry , inorganic chemistry , computational chemistry , organic chemistry , kinetics , catalysis , physics , quantum mechanics
Rate constants for the heterolysis reactions (S N 1) of a series of chloro‐diarylmethanes in aprotic solvents (dimethyl sulfoxide (DMSO), acetonitrile, carboxamides, etc.) have been determined conductometrically in the presence of amines or triphenylphosphane, which trap the intermediate ion‐pairs and suppress ion recombination. The operation of S N 2 mechanisms can be excluded because the observed first‐order rate constants become almost independent of the nature of the nucleophilic additive when a certain concentration of nucleophile is exceeded. The heterolysis rate constants are used to calculate nucleofugality parameters N f and s f for chloride in aprotic solvents according to the linear free‐energy relationship lg k (25 °C) = s f ( N f + E f ). Ionizing powers Y BnCl of these solvents were calculated according to the Winstein–Grunwald equation. Because the heterolysis rate constants in aprotic solvents correlate only poorly with dielectric constants or empirical solvent parameters such as Gutmann's acceptor numbers or E T N values, the most common solvent polarity parameters do not reliably predict ionization rates in aprotic solvents.