Rate and product studies in the solvolyses of methanesulfonic anhydride and a comparison with methanesulfonyl chloride solvolyses

The specific rates of solvolysis of methanesulfonic anhydride have been measured conductometrically at ?10 °C in 41 solvents. Use of the extended Grunwald–Winstein equation, with the N_T scale of solvent nucleophilicity and the Y_OTs scale of solvent ionizing power, leads to sensitivity to changes in solvent nucleophilicity (? value) of 0.95 and a sensitivity to changes in solvent ionizing power (m value) of 0.61, with a multiple correlation coefficient (R) of 0.973. Product selectivity values (S) in binary hydroxylic solvents favor alcohol attack in EtOH–H₂O (a value of 1.2 in 90% EtOH rising to 4.0 in 40% EtOH) and in MeOH–H₂O (a value of 3.7 in 90% MeOH rising to 6.0 in 50% MeOH). In 2,2,2,‐trifluoroethanol–H₂O, the S values are much lower at about 0.1. Entropy of activation values are appreciably negative. Literature values for the specific rates of solvolysis of methanesulfonyl chloride have been extended to fluoroalcohol‐containing solvents (titrimetric method) and, at 45.0 °C, for an overall 43 solvents values are obtained (using N_T and Y_C1 scales) of 1.20 for ? and of 0.52 for m (R = 0.969). It is proposed that both substrates solvolyze by an S_N2 pathway. Copyright © 2007 John Wiley & Sons, Ltd.     

Self‐association,tautomerism and E–Z isomerization of isatin–phenylsemicarbazones – spectral study and theoretical calculations

The self‐association and tautomerism of (E)‐isatin‐3‐4‐phenyl(semicarbazone) Ia and (E)‐N‐methylisatin‐3‐4‐phenyl(semicarbazone) IIa were investigated in solvents of various polarity. In weakly interacting non‐polar solvents, such as CHCl₃ and benzene, phenylsemicarbazone concentrations above 1×10^?5 mol dm^?3 result in the formation of dimers or higher aggregates of E‐isomers Ia and IIa . This aggregate formation prevents room temperature E–Z isomerization of Ia and IIa to more stable Z‐isomers. In contrast to the situation in non‐polar solvents, E–Z isomerization from the monomeric form of phenylsemicarbazone Ia and IIa E‐isomers occurs in highly interactive polar solvents including MeOH and DMF only at temperatures above 70 °C. Moreover, decrease in phenylsemicarbazone concentration below 1×10^?4 mol dm^?3 in these highly solute–solvent interacting systems leads to aggregate dissociation, and a new hydrazonol tautomeric form with a high degree of conjugation predominates in these solutions. Theoretical calculations confirm obtained experimental results. Copyright © 2013 John Wiley & Sons, Ltd.     

Lack of the neighboring group rate effect in solvolytic reactions that proceed via participation

In order to investigate the influence of solvent polarity on the rate effect of double bonds in reactions that proceed via an extended π‐participation mechanism, the solvolysis rates (k_U) of the benzyl chloride derivative 1 and tertiary chloride 2 that have doubly unsaturated side chains were measured in absolute ethanol, 80% v/v. aq. ethanol and 97% wt. aq. trifluoroethanol. The rates of the corresponding saturated analogs 1S and 2S (k_S) were measured in 80% aq. ethanol and 97% wt. aq. trifluoroethanol, while those in pure ethanol were calculated according to LFER equation log k = s_f (E_f + N_f). In solvents with moderate ionizing power (ethanol and 80% aq. ethanol) the expected rate effects were obtained (k_U/k_S>1), while in solvent with high ionizing power (2,2,2‐trifluoroethanol) absence of the rate effect was observed (k_U/k_S≈1), indicating that in the k_S process the solvation of the transition state is very important, while in k_Δ process the breaking of the C? Cl bond is not appreciably developed in the transition state and the solvent effect is marginal. Copyright © 2007 John Wiley & Sons, Ltd.     

Correlation of the rates of solvolysis of diphenylphosphinyl chloride using an extended form of the Grunwald–Winstein equation

The specific rates of solvolysis of diphenylphosphinyl chloride ( 1 ) have been measured at 25.0 °C in 30 solvents. For six representative solvents, studies were made at several temperatures and activation parameters determined. These were used to calculate a value at 25.0 °C in 100% 2,2,2‐trifluoroethanol (TFE) from values at higher temperatures. The 31 solvents gave a reasonable extended Grunwald–Winstein plot, correlation coefficient (R) of 0.920, which improved to 0.956 when the four TFE–ethanol points were excluded. The sensitivities (l and m) were similar to those obtained for dimethyl phosphorochloridate and phosphorochloridothionate and for N,N,N′,N′‐ tetramethyldiamidophosphorochloridate. As with the three previously studied solvolyses, an S_N2 pathway is proposed for the solvolyses of 1 . Copyright © 2007 John Wiley & Sons, Ltd.     

Prediction of ortho substituent effect in alkaline hydrolysis of substituted phenyl benzoates in aqueous acetonitrile

The second‐order rate constants k (dm³mol^?1s^?1) for alkaline hydrolysis of meta‐, para‐ and ortho‐substituted phenyl esters of benzoic acid, C₆H₅CO₂C₆H₄‐X, in aqueous 50.9% (v/v) acetonitrile have been measured spectrophotometrically at 25 °C. In substituted phenyl benzoates, C₆H₅CO₂C₆H₄‐X, the substituent effects log k_X ? log k_H in aqueous 50.9% acetonitrile at 25 °C for para, meta and ortho derivatives showed good correlations with the Taft and Charton equations, respectively. Using the log k values for various media at 25 °C, the variation of the ortho substituent effect with solvent was found to be precisely described with the following equation: Δlog k_ortho = log k_ortho ? log k_H = 1.57σ_I + 0.93σ°_R + 1.08E_s^B ? 0.030ΔEσ_I ? 0.069ΔEσ°_R, where ΔE is the solvent electrophilicity, ΔE = E_S ? E_H20, characterizing the hydrogen‐bond donating power of the solvent. We found that the experimental log k values for ortho‐, para‐ and meta‐substituted phenyl benzoates in aqueous 50.9% acetonitrile at 25 °C, determined in the present work, precisely coincided with the log k values predicted with the equation (log k^X)_calc = (log k^H_AN)_exp + (Δlog k^X)_calc where the substituent effect (Δlog k^X)_calc was calculated from equation describing the variation of the substituent effect with the solvent electrophilicity parameter, using for aqueous 50.9% CH₃CN the solvent electrophilicity parameter, ΔE = ?5.84. In going from water to aqueous 50.9% CH₃CN, the ortho inductive term grows twice less as compared with the para polar effect. Copyright © 2013 John Wiley & Sons, Ltd.     

The water‐promoted Diels–Alder reaction in quaternary ammonium salts

Prevailing classification of salts based on their effect in solubility and stability of proteins in aqueous solution predicts that tetraalkylammonium salts, guanidinium chloride (GnCl), LiClO₄ act as salting‐in (S/I) and LiCl, NaCl act as salting‐out (S/O) in aqueous conditions. In the same context the behaviour of GnCl, LiClO₄ and LiCl are contradictory in polar solvents like ethylene glycol and formamide. In these solvents, expected salt effect shows just opposite nature from their usual expectation. However, in the aqueous solution salts like tetraalkylammonium halide (R₄NX, R = alkyl group, X = Br group) behave like salting‐in salts. The physicochemical origin of the salting in effect of R₄NX type of salts has been discussed elaborately in the present work. The role of cations in terms of substitution of various alkyl groups on R₄NX has been systematically presented here on the basis of experimental kinetic and thermodynamic studies. The abnormal behaviour of R₄NX salts in aqueous solution has also been explained by the Setschenov equation (k_s) and Δμ_solvation values, which highlights their individual nature out of common properties of R₄NX. Copyright © 2015 John Wiley & Sons, Ltd.     

Solvolytic reactivity of pyridinium ions

The leaving group abilities of pyridine, 4‐methylpyridine, and 4‐chloropyridine in S_N1 solvolytic reactions have been determined by analyzing the rate constants of X,Y‐substituted benzhydrylpyridinium salts obtained in various solvents. By applying the linear free energy relationship equation, log k = s_f (E_f + N_f), the nucleofuge specific parameters of 4‐substituted pyridine have been extracted. Because of solvation in the reactant ground state, the reactivity (nucleofugality, N_f) of a given pyridine decreases as the polarity of the solvent increases. High slope parameters (s_f > 1) may be due to the spread of the energy levels of the benzhydrylium ion/pyridine pair intermediates in comparison to benzhydrylium ion/chloride pairs (s_f ≈ 1). Because of slow heterolysis step of pyridinium salts in various solvents, some are stable under normal conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Rate and product studies with 2‐adamantyl fluoroformate under solvolytic conditions

The specific rates of solvolysis of 2‐adamantyl fluoroformate have been measured at 25.0 °C in 20 pure and binary solvents. These are well correlated using the extended Grunwald–Winstein equation, with incorporation of the N_T solvent nucleophilicity scale and the Y_Cl solvent ionizing power scale. The sensitivities (l = 2.15 ± 0.17 and m = 0.95 ± 0.07) toward the changes in solvent nucleophilicity and solvent ionizing power, and the k_F/k_Cl values are very similar to those previously observed for solvolyses of n‐octyl fluoroformate, consistent with the addition step of an addition‐elimination pathway being rate‐determining. For aqueous ethanol, measurement of the product ratio allowed selectivity values (S) to be determined. The results are compared with those reported earlier for 2‐adamantyl chloroformate and mechanistic conclusions are drawn. Copyright © 2007 John Wiley & Sons, Ltd.     

Transition from concerted to stepwise processes as a function of leaving group ability: density functional theory and experimental study of lyoxide‐promoted cleavages of phosphorothioate and phosphate triesters in water and methanol

The base‐promoted solvolysis of a series of O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphorothioates (1) as well as O,O‐dimethyl O‐aryl and O,O‐dimethyl O‐alkyl phosphates (2) was studied computationally by density functional theory methods in methanol and water continuum media to determine the transition between concerted and stepwise processes. In addition, an experimental study was undertaken on the solvolysis of these series in basic methanol and water. The computations indicate that the solvolytic mechanism for series 1 involves lyoxide attack anti to the leaving group in a concerted manner with good leaving groups having pK_a^Lg values < 12.3 in methanol and in a stepwise fashion with the formation of a 5‐coordinate thiophosphorane intermediate when the pK_a^Lg > 12.3. A similar transition from concerted to stepwise mechanism occurs with series 2 in methanol as well as with series 1 and 2 in water, although for the aqueous solvolyses with hydroxide nucleophile, the transitions between concerted and stepwise mechanisms occur with better leaving groups than in the case in methanol. The computational data allow the construction of Brønsted plots of log k₂^?OS versus pK_a^Lg in methanol and water, which are compared with the experimental Brønsted plots determined with these series previously and with new data determined in this work. Both the computational and experimental Brønsted data reveal discontinuities in the plots between substrates bearing O‐aryl and O‐alkyl leaving groups, with the gradients of the plots being far steeper than, and non‐collinear with, the O‐aryl leaving groups for solvolysis of the O‐alkyl‐containing substrates. These discontinuities signify that care should be exercised in interpreting breaks in Brønsted plots in terms of changes in rate‐limiting steps that signify the formation of an intermediate during a solvolytic process. Copyright © 2011 John Wiley & Sons, Ltd.     

Variation of the temperature‐dependent substituent effects with solvent in alkaline hydrolysis of substituted phenyl and alkyl benzoates and phenyl tosylates

The second‐order rate constants k for the alkaline hydrolysis of eight substituted alkyl benzoates have been measured spectrophotometrically in aqueous 5.3 M NaClO₄ and 0.5 M n‐Bu₄NBr at various temperatures. Variation of the substituent effect with temperature in alkaline hydrolysis of ortho‐, meta‐, and para‐substituted phenyl benzoates, phenyl tosylates, and alkyl benzoates in various solvents (water, aqueous 0.5 M Bu₄NBr, 80% (v/v) DMSO, 2.25 M Bu₄NBr, and 5.3 M NaClO₄) was studied. The susceptibility to temperature variation of the meta and para polar substituent effect, the ortho inductive effect, and the alkyl polar effect for various media showed good correlation with the solvent electrophilicity, E_S, which characterizes the hydrogen‐bond donating power of the solvent. The variation of the temperature‐dependent ortho inductive effect with solvent hydrogen‐bond donor capacity (electrophilicity) was found to be nearly twice smaller than that for meta and para polar effect. The temperature‐dependent alkyl polar substituent effect was found to vary with E_S nearly by the same extent as the polar effect of meta and para substituents. The dependences of the ρ values (altogether 109 values of ρ) on the (1/T) term for various media were found to cross nearly at the same isosolvent temperature (1/β_isosolv ≈ 2 × 10^?3) for meta‐, para‐, ortho‐, and alkyl‐substituted esters. At T = β_isosolv the difference (ρ)_S ? (ρ)_Water becomes zero for all polar substituent effects in all media considered and the additional inductive effect from the ortho position (compared with para derivatives) disappears for all solvents studied. Copyright © 2007 John Wiley & Sons, Ltd.     

Effect of microheterogeneity of CF3CH2OH–H2O mixed solvent on reactions of carbocations from 1,2,2,3‐tetramethyl‐1,2‐dihydroquinoline

Carbocations are key intermediates in many important organic reactions. The remarkable effect of the solvent composition on the kinetic parameters of the carbocation decay and product composition was found in the photolysis of 1,2,2,3‐tetramethyl‐1,2‐dihydroquinoline ( 1 ) in 2,2,2‐trifluoroethanol (TFE)–H₂O mixtures. The rate constant of the intermediate carbocation decay has a maximum, and the activation energy is minimal in the TFE–H₂O mixture 3 : 7 (v/v). In the steady‐state photolysis, products of oligomerization of 1 with n up to 8 and their adducts with TFE and H₂O were identified at this solvent composition. The results were rationalized in terms of TFE clustering in aqueous mixtures, with the maximum of cluster formation at 30 vol % TFE. The clusters form a pseudo‐phase, in which the molecules of 1 are concentrated and the carbocations are generated. TFE, H₂O and 1 compete in the combination reaction with the photogenerated carbocation to afford the products. This effect was not observed for 1,2,2,4‐tetramethyl‐1,2‐dihydroquinoline ( 2 ), the isomer of 1 , due to steric hindrance at C(4) carbon atom of the heterocycle, the active site of the intermediate carbocation, which makes impossible for the carbocation from 2 to react further with 2 . Thus, the kinetic parameters and the product composition in the photolysis of 1 in TFE–H₂O mixtures reflect the changes in the microstructure of the binary solvent. Copyright © 2013 John Wiley & Sons, Ltd.     

Propanolysis of arenesulfonyl chlorides: Nucleophilic substitution at sulfonyl sulfur

We have studied the mechanism of solvolysis of arenesulfonyl chlorides by propan‐1‐ol and propan‐2‐ol at 303‐323 K. Kinetic profiles were appropriately fit by first‐order kinetics. Reactivity increases with electron‐donating substituents. Ortho‐alkyl substituted derivatives of arenesulfonyl chlorides show increased reactivity, but the origin of this “positive” ortho‐effect remains unclear. Likely, ortho‐methyl groups restrict rotation around the C‐S bond, facilitating the attack of the nucleophile. No relevant reactivity changes have been found with propan‐1‐ol and propan‐2‐ol in terms of nucleophile steric effect. The existence of isokinetic relationships for all substrates suggests a single mechanism for the series. Solvolysis reactions of all substrates in both alcohols show isokinetic temperatures (T_iso) close to the working temperature range, which is an evidence of the process being influenced by secondary reactivity factors, likely of steric nature in the TS. Solvation plays a relevant role in this reaction, modulating the reactivity. In some cases, the presence of t‐Bu instead of Me in para‐ position leads to changes in the first solvation shell, increasing the energy of the reaction (ca. 1 kJ·mol^?1). The obtained results suggest the same kinetic mechanism of solvolysis of arenesulfonyl chlorides for propan‐1‐ol and propan‐2‐ol, as in MeOH and EtOH, where bimolecular nucleophilic substitution (S_N2) takes place with nucleophilic solvent assistance of one alcohol molecule and the participation of the solvent network involving solvent molecules of the first solvation shell.     

Influence of solvent on the ortho substituent effect in the alkaline hydrolysis of phenyl esters of substituted benzoic acids

The second‐order rate constants k (dm³ mol^?1 s^?1) for the alkaline hydrolysis of phenyl esters of meta‐, para‐ and ortho‐substituted benzoic acids in aqueous 5.3 M NaClO₄ and 1.0 M Bu₄NBr were measured by UV/Vis spectrophotometry at 25 °C. The variations in the ortho inductive, ortho resonance, as well as meta and para polar effects with solvent parameters were studied using data for the alkaline hydrolysis of phenyl esters of substituted benzoic acids in various media. The dependence of the ortho substituent effect on solvent can be precisely described with the following equation: Δlog k_ortho = log k_ortho ? log k_H = 0.059 + 2.19σ_I + 0.304σ°_R + 2.79E ? 0.016ΔEσ_I ? 0.085ΔEσ°_R, where ΔE is the solvent electrophilicity, ΔE = E_S ? E_H2O, characterizing the hydrogen‐bond donating power of the solvent. The increase in the meta and para polar substituent effects with decrease in the solvent hydrogen‐bond donor capacity (electrophilicity) was approximately to the same extent (?0.068ΔEσ°_m,p) as the resonance term for the ortho substituents. The steric term of ortho substituents was independent of the solvent parameters. The variations in the ortho inductive, ortho resonance, as well as meta and para polar substituent effects with the solvent electrophilicity were to the same extent as in phenyl benzoates containing the substituents in the phenyl part. The substituent effects in the alkaline hydrolysis of ethyl benzoates appeared to vary with the solvent electrophilicity nearly to the same extent as in the alkaline hydrolysis of substituted phenyl esters of benzoic acids. Copyright © 2009 John Wiley & Sons, Ltd.     

Reactivities of mixed organozinc and mixed organocopper reagents. Part 13 Kinetic study for phosphine‐catalyzed acylation of alkylarylzincs and effect of residual group on the transfer rate of alkyl group

Kinetics of reactions of di‐n‐butylzinc, n‐Bu₂Zn, and mixed n‐butyl(substituted phenyl)zinc reagents and n‐Bu(functional group (FG)?C₆H₄)Zn with benzoyl chloride in the presence of tri‐n‐butylphosphine have been investigated. Reaction rates of transferable n‐butyl group have been determined in tetrahydrofuran at 0 °C to compare the transfer rate of n‐butyl group in homo and mixed diorganozincs. Rate law is consistent with a third‐order reaction, which is first order in diorganozinc, benzoyl chloride, and n‐Bu₃P, and a mechanism was proposed. The lower reaction rate of n‐BuPhZn than that of n‐Bu₂Zn and negative reaction constant in Hammett plot are in accordance with the carbanionic charge of transferable n‐butyl group in the acylation reaction. These findings support the hypothesis that the reaction rate of transferable group, R_T, changes depending upon the residual group, R_R, in R_RR_TZn reagents. Copyright © 2016 John Wiley & Sons, Ltd.     

Spectral Properties of Cyclodextrin Inclusion Complexes with Nonionic Surfactants and 1‐Bromonaphthalene

Abstract

Steady‐state fluorescence and phosphorescence of inclusion complexes of cyclodextrins (CDs) with fluorescent nonionic surfactant and 1‐bromonaphthalene (BN) are described in detail. The inclusion of the hydrophobic moiety of surfactants inside the cavity of CDs led to enhanced monomer‐like fluorescence with a bathochromic shift of λ_ex and a hypsochromic shift of λ_em. ¹H‐NMR provides additional evidence for deep inclusion of the hydrophobic moiety of surfactants. BN can squeeze into more hydrophobic cavity of β‐CD that has accommodated the hydrophobic moiety of a surfactant and show its phosphorescence and remarkable quenching effect on the fluorescence of a surfactant in aerated aqueous solution. Stern–Volmer quenching depends on the inclusion of the phenyl rings of surfactants and BN into the cavity of CDs. Comparison of molecular sizes reveals that further inclusion of BN into the cavity of β‐CD occupied by a surfactant may force the flexible octyl group of a surfactant to deform to a greater extent, and close‐packing complexes were obtained. In the case of heptakis(2,6‐di‐O‐methyl)‐β‐CD, BN only binds to its cavity opening due to the steric hindrance of methyl substituents at the rim of its cavity.     

Ultrasonic detection of hydrophobic interactions: a quantitative approach

Kinetic effects of sonication on ester hydrolysis and tert‐butyl chloride solvolysis, studied in ethanol–water binary solvent, are discussed in terms of quantitative relationships between their magnitude and the hydrophobicity of reagents. A number of conclusions were drawn from the observed linear free‐energy (LFE) relationships. Independent of reaction mechanism, the decrease in reaction rates with increasing ethanol content in the solvent is mainly due to hydrophobic stabilization of the ground state. While hydrophobic species can be hidden in the ethanol clusters present in the region X_EtOH > 0.15, at lower ethanol contents hydrophobic reagents are weakly solvated and the hydrophobic stabilization can be easily overcome by sonication. Copyright © 2008 John Wiley & Sons, Ltd.     

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

DFT computations have been performed in acetone and water solvents in order to investigate the mechanism of hydrolysis of acid chlorides. Acetyl chloride and chloroacetyl chloride hydrolyze via concerted, one‐step S_N2 mechanism, with the attack of water at the sp² hybridized carbon atom of the C?O group, and the transition state (TS) has distorted tetrahedral geometry. Solvent molecules act as general base and general acid catalysts. The TS of chloroacetyl chloride is tighter and less polar than the TS of acetyl chloride. The structure of the S_N2 TS for the hydrolysis of benzoyl chlorides changes with the substituents and the solvent. Tight and loose TSs are formed for substrates bearing electron withdrawing (e‐w) and electron donating (e‐d) groups, respectively. In acetone, only the e‐w effect of the substituents increase the reactivity of the substrates, and the change of the structure of the TSs with the substituents is small. In water, polar and very loose TSs are formed in the reactions of benzoyl chlorides bearing e‐d substituents, and the rate enhancing effect of both e‐d and e‐w groups can be computed at higher level of theory. Calculated reactivities and the changes of the structure of the TSs with substituents and solvent are in accordance with the results of kinetic studies. In S_N2 nucleophilic substitutions late/early TSs are formed if the attacking reagent is poorer/better nucleophile than the leaving group, and loose/tight TSs are formed for substrates bearing e‐d/e‐w substituents and in protic/aprotic solvents. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinetics study of a Diels‐Alder reaction in mixtures of an ionic liquid with molecular solvents

The second‐order rate constants for cycloaddition reaction of cyclopentadiene with naphthoquinone were determined spectrophotometrically in various compositions of 1‐(1‐butyl)‐3‐methylimidazolium terafluoroborate ([bmim]BF₄) with water and methanol at 25 °C. Rate constants of the reaction in pure solvents are in the order of water > [bmim]BF₄ > methanol. Rate constants of the reaction decrease sharply with mole fraction of the ionic liquid in aqueous solutions and increase slightly to a maximum in alcoholic mixtures. Multi‐parameter correlation of logk₂ versus solute–solvent interaction parameters demonstrated that solvophobicity parameter (Sp), hydrogen‐bond donor acidity (α) and hydrogen‐bond acceptor basicity (β) of media are the main factors influencing the reaction rate constant. The proposed three‐parameter model shows that the reaction rate constant increases with Sp, α and β parameters. Copyright © 2008 John Wiley & Sons, Ltd.     

Effect of ortho substituents on carbonyl carbon 13C NMR chemical shifts in substituted phenyl benzoates

¹³C 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 ¹³C NMR chemical shift, δ_CO, was found to be described by a linear multiple regression equation containing the inductive, σ_I, resonance, σ°_R, and steric, E, or υ 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 (ρ_I < 0), the normal resonance effect (ρ_R > 0), and the negative steric effect (δ_ortho < 0) with the E 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 ¹³C NMR chemical shift of the carbonyl carbon. For all the meta‐ and para‐substituted esters, the reverse substituent‐induced inductive and resonance effects (ρ_I < 0, ρ_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 Bu₄NBr and 2.25 M Bu₄NBr, and the IR frequencies, ν_CO, for the ortho‐, meta‐, and para‐substituted phenyl benzoates and alkyl benzoates were correlated nicely with the corresponding ¹³C NMR substituent chemical shifts, Δδ_CO. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural conformations and electronic interactions of the natural product,oroxylin: a vibrational spectroscopic study

The oroxylin, 5,7‐dihydroxy 6‐methoxy flavone is a potent natural product extracted from ‘Vitex peduncularis’. Density functional theory (DFT) at B3LYP/6‐311G(d,p) level has been used to compute energies of different conformers of oroxylin to find out their stability, the optimized geometry of the most stable conformer and its vibrational spectrum. The conformer ORLN‐1 with torsion angles 0, 180, 180 and 0 degrees, respectively, for H₁₃ O₁₂ C₆ C₅, H₁₄ O₁₀ C₄ C₅, H₁₃ O₁₂ C₆ C₅ and H₁₄ O₁₀ C₄ C₅ is found to be most stable. The optimized geometry reveals that the dihedral angle φ between phenyl ring B and the chrome part of the molecule in − 19.21° is due to the repulsive force due to steric interaction between the ortho‐hydrogen atom H₂₉ of the B ring and H₁₈ of the ring C (H₂₉·H₁₈ = 2.198 Å). A vibrational analysis based on the near‐infrared Fourier transform(NIR‐FT) Raman, Fourier transform‐infrared (FT‐IR) and the computed spectrum reveals that the methoxy group is influenced by the oxygen lone pair‐aryl p_z orbital by back donation. Hence the stretching and bending vibrational modes of the methoxy group possess the lowest wavenumber from the normal values of methyl group. The carbonyl stretching vibrations have been lowered due to conjugation and hydrogen bonding in the molecules. The intramolecular H‐bonding and nonbonded intramolecular interactions shift the band position of O₁₀ H₁₄ and O₁₂ H₁₃ stretching modes, which is justified by DFT results. Copyright © 2008 John Wiley & Sons, Ltd.