The mode of transmission of electrical effects

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 Delta G(acid) values for Y = CO2H and OH, and proton affinities for NR2H+, proton transfer reaction rates for XGCO(2)H with Ph2CN2, and hydrogen bonding equilibria for XGCN (pK(HB)). MI's include rates of base catalyzed ester hydrolysis, nucleophilic substitution of PhCOCH2Br by XGCO(2)(-), and protodetritiation of T-substituted arenes. Mi reactions were solvolyses of XGCHLgMe (Lg is a leaving group) and XGCMe(2)Cl. 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 (Y-1). Correlations of L with 1/n(2) 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 theta, the angle between the X-G bond and the line joining X and Y-1, 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/n(2). 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.