The mode of transmission of electrical effects†

Abstract

The dependence of substituent electrical effect transmission on substituent–reaction site distance and on the charge on reactant and product or transition state has been studied in the systems X–G–Y and X–Y where X is a variable substituent, Y a reaction site, and G a skeletal group. Reaction types studied were molecule–molecule (MM), molecule–ion (MI), and molecular ionization (Mi). MM reactions include proton transfer equilibria (pK_a’s) of compounds with Y = CO₂H, OH, SO₂NH₂, NR₂H⁺, azarenes, PO₂(OH)^–, and SH; gas phase ΔG_acid values for Y = CO₂H and OH, and proton affinities for NR₂H⁺, proton transfer reaction rates for XGCO₂H with Ph₂CN₂, and hydrogen bonding equilibria for XGCN (pK_HB). MI’s include rates of base catalyzed ester hydrolysis, nucleophilic substitution of PhCOCH₂Br by XGCO₂^–, and protodetritiation of T-substituted arenes. Mi reactions were solvolyses of XGCHLgMe (Lg is a leaving group) and XGCMe₂Cl. The measure of electrical effect magnitude used was L, the coefficient of the localized (field and/or inductive) effect obtained from correlation of appropriate data sets with linear free energy relationships. The substituent–reaction site distance was parameterized by n, the number of bonds between the substituent and the nearest atom of the reaction site undergoing bond change (Y¹). Correlations of L with 1/n² and 1/n; and of log |L| with log n by simple linear regression analysis determined the dependence of L on n. Data sets with very large values of θ, the angle between the X–G bond and the line joining X and Y¹, were excluded. Data in aqueous–organic solvent mixtures can be combined into a single data set regardless of the solvent composition, probably due to preferential solvation by water. The results do not agree with the Kirkwood–Westheimer model for MI, Mi, and some MM reactions all of which show a dependence on 1/n rather than 1/n². They support a modified field effect as the mode of transmission. This model differs from the Kirkwood–Westheimer model m seeming to depend on the charge difference between initial and final states.