Premium
Brønsted β Nu Values and Leaving Group Effects in S N 2 Reactions. Tests of the Reactivity—Selectivity Postulate
Author(s) -
Bordwell Frederick G.,
Branca John C.,
Cripe Thomas A.
Publication year - 1985
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198500120
Subject(s) - chemistry , reactivity (psychology) , carbanion , steric effects , selectivity , reaction rate constant , conjugate acid , bromide , solvation , medicinal chemistry , chloride , solvent , inorganic chemistry , stereochemistry , ion , organic chemistry , catalysis , kinetics , pathology , quantum mechanics , medicine , physics , alternative medicine
The rate constants for reactions of a family of 19 carbanions, ArCHSO 2 Ph − (derived from benzyl phenyl sulfones) with n ‐butyl chloride have been measured in Me 2 SO solution. A plot of log k vs. the pK a of the conjugate acids for 12 of these carbanions give a linear plot ( R 2 = 0.999) with a Brønsted coefficient of β Nu = 0.402. Points for para electron‐withdrawing substituents, SPh, SOPh, SO 2 Ph. COPh, CN and NO 2 , deviate substantially from the plot. The deviations are attributed to the enhanced solvation of these remote substituents in the anion which leads to rate retardation. A curved Brønsted plot can be drawn through all the points, which would be consistent with the predictions of the Hammond–Leffler postulate (HLP) and the reactivity–selectivity postulate (RSP), but this interpretation is rejected. Instead, it is suggested that the apparent curvature in Brønsted plots for acid–base reactions – upon which HLP and RSP are based – is also caused by deviations due to solvent effects, donor atom effects in the bases, mechanistic changes and/or the failure to keep electronic and steric effects constant. A reactivity–selectivity plot for reactions of 9 ArCHSO 2 Ph − ions with n ‐butyl bromide and n ‐butyl chloride indicated constant selectivity. A similar plot for 4 carbanions derived from α‐methylbenzyl phenyl sulfones reacting with n ‐butyl bromide and n ‐butyl chloride also showed constant selectivity. Based on these results and an examination of the literature, it is concluded that there is no firm experimental basis for HLP and RSP.