Mare Piirsalu

Prediction of ortho substituent effect in alkaline hydrolysis of phenyl esters of substituted benzoic acids in aqueous acetonitrile

The second-order rate constants k for the alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C6H4CO2C6H5, in aqueous 50.9% acetonitrile have been measured spectrophotometrically at 25°C. The log k values for meta and para derivatives correlated well with the Hammett σm,p substituent constants. The log k values for ortho-substituted phenyl benzoates showed good correlations with the Charton equation, containing the inductive, σI, resonance, σ○R, and steric, EsB, and Charton υ substituent constants. For ortho derivatives the predicted (log kX)calc values were calculated with equation (log kortho)calc = (log kHAN)exp + 0.059 + 2.19σI + 0.304σ○R + 2.79EsB − 0.0164ΔEσI — 0.0854ΔEσ○R, where DE is the solvent electrophilicity, ΔE = EAN — EH20 = −5.84 for aqueous 50.9% acetonitrile. The predicted (log kX)calc values for phenyl ortho-, meta- and para-substituted benzoates in aqueous 50.9% acetonitrile at 25°C precisely coincided with the experimental log k values determined in the present work.The substituent effects from the benzoyl moiety and aryl moiety were compared by correlating the log k values for the alkaline hydrolysis of phenyl esters of substituted benzoic acids, X-C6H4CO2C6H5, in various media with the corresponding log k values for substituted phenyl benzoates, C6H5CO2C6H4-X.

17O NMR studies of ortho‐substituent effects in substituted phenyl tosylates

17O NMR spectra for 35 ortho‐, para‐, and meta‐substituted phenyl tosylates (phenyl 4‐methylbenzenesulfonates), 4‐CH3‐C6H4SO2OC6H4‐X, at natural abundance in acetonitrile at 50 °C were recorded. The 17O NMR chemical shifts, δ(17O), of the sulfonyl (SO2) and the single‐bonded phenoxy (OPh) oxygens for para and meta derivatives correlated well with dual substituent parameter treatment using the Taft inductive, σI, and resonance, σºR, constants. The influence of ortho substituents on the sulfonyl oxygen and the single‐bonded phenoxy oxygen chemical shifts, δ(17O), was found to be nicely described by the Charton equation: δ(17O)ortho = δ(17O)H + ρIσI + ρRσ°R + δEsB when the data treatment was performed separately for electron‐donating +R substituents and electron‐attracting −R substituents. Electron‐attracting meta and para substituents in the phenyl moiety caused deshielding while the electron‐donating meta, para and ortho +R substituents produce shielding effects on the sulfonyl (SO2) and single‐bonded phenoxy (OPh) oxygens. The influence of ortho inductive and resonance effects in the case of +R substituents was found to be approximately twice higher than the corresponding influence from the para position. Due to the steric effect of ortho substituents a decrease in shielding of the oxygens at the sulfonyl group (δEsB > 0, EsB < 0) was detected. Copyright