No abstract

The second-order rate constants k2 for alkaline hydrolysis of phenyl esters of meta-, para- and ortho-substituted benzoic acids, X-C6H4CO2C6H5 (X = H, 3-Cl, 3-NO2, 3-CH3, 4-NO2, 4-Cl, 4-F, 4-CH3, 4-OCH3, 4-NH2, 2-NO2, 2-CN, 2-F, 2-Cl, 2-Br, 2-I, 2-CH3, 2-OCH3, 2-CF3, 2-NH2), and of substituted phenyl esters of benzoic acid, C6H5CO2C6H4-X (X = 2-I, 2-CF3, 2-C(CH3)3, 4-Cl, 4-CH3, 4-OCH3, 4-NH2), have been measured spectrophotometrically in water at 25 °C. The substituent effect in alkaline hydrolysis of phenyl esters of para-substituted benzoic acids, similar to that for ethyl esters of para-substituted benzoic acids, was found to be precisely described by the Hammett relationship (ρ = 1.7 in water). The log k value for alkaline hydrolysis of phenyl and ethyl esters of meta-, para- and ortho-substituted benzoic acids, X-C6H4CO2R, was nicely correlated with log km,p,ortho = log ko + (ρ)m,pσ + (ρI)orthoσI + (ρ°R)orthoσ°R + δorthoEsB where σ, σI, σ°R are the Hammett polar, Taft inductive and Taft resonance (σ°R = σ° - σI) substituent constants, respectively. EsB is the steric scale for ortho substituents calculated on the basis of the log k values for the acid hydrolysis of ortho- substituted phenyl benzoates in water owing to the ortho substituent in the phenyl of phenyl benzoates. In water, the main factors responsible for changes in the ortho substituent effect in alkaline hydrolysis of phenyl and ethyl esters of ortho-substituted benzoic acids, X-C6H4CO2R, were found to be the inductive and steric factors while the role of the resonance term was negligible ((ρ°R)ortho ca. 0.3). In alkaline hydrolysis of substituted benzoates in neat water, the ortho inductive effect appeared to be 1.5 times and steric influence 2.7 times higher than the corresponding influences from the ortho position in the phenyl of phenyl benzoates. The contributions of the steric effects in alkaline hydrolysis of esters of ortho-substituted benzoic acids was found to be approximately the same as in acid hydrolysis of esters of ortho-substituted benzoic and acid esterification of ortho-substituted benzoic acids.

Effect of ortho substituents on carbonyl carbon 13 C NMR chemical shifts in substituted phenyl benzoates Vilve Nummert a , Mare Piirsalu a , Vahur Mä emets a , Signe Vahur a and Ilmar A. Koppel a * 13 C NMR spectra of 37 ortho-, meta-, and para-substituted phenyl benzoates, containing substituents in benzoyl and phenyl moiety, 4 ortho-substituted methyl and 5 ethyl benzoates as well as 9 R-substituted alkyl benzoates have been recorded. The influence of the ortho substituents on the carbonyl carbon 13 C NMR chemical shift, d CO , was found to be described by a linear multiple regression equation containing the inductive, s I , resonance, s-R , and steric, E B s , or y substituent constants. For all the ortho-substituted esters containing substituents in the acyl part as well as the phenyl part, the substituent-induced reverse inductive effect (r I < 0), the normal resonance effect (r R > 0), and the negative steric effect (d ortho < 0) with the E B s were observed. In the case of ortho substituents in the phenyl part, the resonance effect was negligible. Due to inductive effect, the ortho electron-withdrawing substituents showed an upfield shift or shielding of the carbonyl carbon, while the electron-donating substituents had an opposite effect. Because of the sterical consequences, ortho substituents revealed a deshielding effect on the 13 C NMR chemical shift of the carbonyl carbon. For all the meta-and para-substituted esters, the reverse substituent-induced inductive and resonance effects (r I < 0, r R < 0) were found to be significant. In alkyl benzoates, the alkyl substituents showed the reverse inductive and steric effects. The log k values for the alkaline hydrolysis in water, aqueous 0.5 M Bu 4 NBr and 2.25 M Bu 4 NBr, and the IR frequencies, n CO , for the ortho-, meta-, and para-substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding 13 C NMR substituent chemical shifts, Dd CO .ORTHO EFFECT ON CARBONYL CARBON CHEMICAL SHIFTS 1.5) þ (58.4 AE 3.4)s I þ (32.6 AE 4.1)E B s , R ¼ 0.991, s 0 ¼ 0.138, n ¼ 8.

The second order rate constants k 2 (M Ϫ1 s Ϫ1 ) for the alkaline hydrolysis of meta-, para-, and ortho-substituted phenyl benzoates C 6 H 5 CO 2 C 6 H 4 -X (X = H, 3-Cl, 3-CH 2 , 3-NH 2 , 3-NO 2 , 4-F, 4-NO 2 , 2-Cl, 2-F, 2-CH 3 , 2-OCH 3 , 2-NO 2 ) in 2.25 M aqueous n-Bu 4 NBr have been measured spectrophotometrically at various temperatures (15, 25, 30, 40, 50 ЊC). The log k values were treated using both the modified Fujita-Nishioka and the Charton three-parameter correlation equations. The σЊ, σ I , σ R Њ, E s B scales were used as the measure of the ordinary polar, inductive, resonance, and steric components of the substituent effects.where k X and k H are the rate constants of the acidic hydrolysis for ortho-substituted and unsubstituted phenyl benzoates in water at 50 ЊC. The modified Fujita-Nishioka and Charton treatments gave the excellent and identical results. The susceptibility to the induction effect of ortho-substituents was found to be about 0.2 units of the ρ scale smaller [(ρ I ) ortho = 1.80] than that for metaand para-substituents [(ρЊ) m,p = 2.03], in the alkaline hydrolysis of substituted phenyl benzoates in 2.25 M aqueous n-Bu 4 NBr at 50 ЊC. Also in this solution, the variation of the susceptibility to the inductive and the resonance effects with temperature for the ortho-derivatives appeared slightly smaller than that for para-derivatives. During the transition from water to 2.25 M aqueous Bu 4 NBr the susceptibility of the meta-and para-substituent polar effect (ρЊ) m,p was found to increase by about 1.0 units, but the susceptibility to the inductive effect of orthosubstituents was enhanced only by about 0.5 units of the ρ I scale at 50 ЊC. The variation of the inductive term of ortho-substituents with both solvent and temperature was found to be half that for para-substituents. Variation of the inductive term of ortho-substituents with the solvent and temperature by the same extent had earlier been found for the alkaline hydrolysis of substituted phenyl tosylates, though the ratio of the susceptibilities to the polar influence of meta-, para-, and ortho-substituents in water differed two-fold. The steric term of ortho-substituents for 2.25 aqueous n-Bu 4 NBr was found to be independent of temperature and nearly equal to the steric term for water.

Infrared spectra of 25 substituted phenyl esters of benzoic acid C6H5CO2C6H4‐X (XH, 3‐Cl, 3‐F, 3‐CN, 3‐NO2, 3‐CH3, 3‐OH, 3‐NH2, 4‐Cl, 4‐F, 4‐NO2, 4‐CN, 4‐OCH3, 4‐CH3, 4‐NH2, 2‐Cl, 2‐F, 2‐I, 2‐NO2, 2‐CF3, 2‐CN, 2‐CH3, 2‐OCH3, 2‐N(CH3)2, 2‐C(CH3)3), 8 alkyl benzoates C6H5CO2R (XCH3, CH2CH3, CH2Cl, CH2CN, CH2CCH, CH2CH2Cl, CH2CH2OCH3, CH2C6H5), and 22 phenyl esters of substituted benzoic acids X‐C6H4CO2C6H5 (X3‐Cl, 3‐NO2, 3‐CH3, 3‐N(CH3)2, 4‐F, 4‐Cl, 4‐Br, 4‐NO2, 4‐CH3, 4‐C(CH3)3, 4‐OCH3, 4‐NH2, 2‐Cl, 2‐F, 2‐Br, 2‐I, 2‐NO2, 2‐CN, 2‐CF3, 2‐CH3, 2‐OCH3, 2‐NH2) were recorded in tetrachloromethane in the region of 400–4000 cm−1. Carbonyl stretching frequencies νCO for meta‐ and para‐substituted phenyl esters of benzoic acid and phenyl esters of meta‐substituted benzoic acids were shown to correlate with the substituent constants σo. The influence of the through resonance effect on νCO was found to be important in the case of +R para substituents in the benzoyl part of phenyl benzoates as well. The carbonyl stretching frequencies of ortho derivatives in phenoxy part were shown to correlate with the inductive substituent constant σI only. In the benzoyl part of the esters the carbonyl stretching frequencies of cis and trans conformers (relative to the carbonyl group) of ortho derivatives were nicely described by dual parameter equations: (νCO)cis = (νCO)o + c1σI + c3 $E_{\rm s}^{\rm B} $and (νCO)trans = (νCO)o + c1σp+ + c3 $E_{\rm s}^{\rm B} $ (R = 0.99). The trans isomers of phenyl esters of ortho‐substituted benzoic acids showed direct resonance similar to that for para derivatives. The positive steric term found for both the cis and trans conformers could be considered as measure of the steric inhibition of resonance between the phenyl ring and the carboxy‐group caused by bulky ortho substituents. The existence of cis/trans conformations was supported by frequency calculations with Density Functional Theory (DFT) method at B3LYP/6‐311+G** level for the ortho‐substituted benzoates. In the case of alkyl benzoates good correlations of νCO values were obtained when both the Taft σ* and the steric $E_{\rm s}^{\rm B} $ constants were used. For meta‐ and para‐substituted phenyl benzoates s‐trans conformation where the plane of the benzene ring in the benzoyl part of the ester is coplanar with the carbonyl bond plane and the plane of the benzene ring in the phenoxy part is twisted nearly perpendicular relative to the carbonyl bond plane was supported. Copyright © 2006 John Wiley & Sons, Ltd.