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Nucleophilicity—Periodic Trends and Connection to Basicity
Author(s) -
Uggerud Einar
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200500639
Subject(s) - nucleophile , chemistry , exothermic reaction , kinetic energy , computational chemistry , organic chemistry , catalysis , quantum mechanics , physics
Abstract The potential energy profiles of 18 identity S N 2 reactions have been estimated by using G2‐type quantum‐chemical calculations. The reactions are: X − + CH 3 X → XCH 3 + X − and XH + CH 3 XH + → + HXCH 3 + XH (X = NH 2 , OH, F, PH 2 , SH, Cl, AsH 2 , SeH, Br). Despite the charge difference, the barrier heights and the geometrical requirements upon going from the reactant to the transition structure are surprisingly similar for X − and XH. The barrier heights decrease on going from left to right in the periodic table, and increasing ionization energy (of X − and XH) is correlated with decreasing barrier. The observed trends are explained in terms of substrates with stronger electrostatic character giving rise to lower energetic barriers due to decreased electron repulsion in the transition structure. On the basis of this study, the relationship between the kinetic concept of nucleophilicity and the thermodynamic concept of basicity has been analyzed and clarified. Since the trends in intrinsic nucleophilicity (only defined for identity reactions) and basicity are opposite, overall nucleophilicity (defined for any reaction) will be determined by the relative contribution of the two factors. Only for strongly exothermic reactions will basicity and nucleophilicity be matching.