20227-79-6Relevant articles and documents
Mechanistic change in the reactivity of substituted phenyl acetates over phenyl thiolacetates toward imidazole in aqueous phase
Rajarathnam,Jeyakumar,Nadar, P. Ananthakrishna
, p. 211 - 221 (2007/10/03)
The kinetics of aminolysis of several substituted phenyl acetates by imidazole is studied in aqueous medium at 20°C and an ionic strength of 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 272-401 nm), under excess free imidazole, pseudo-first-order rate constants (κobs) are obtained. For the esters with good nucleofuges, the reaction follows clean second-order kinetics and the plots of (κobs - κH) against free imidazole concentration are linear at constant pH. The macroscopic nucleophilic substitution rate constants (κN) are obtained as the slopes of these plots and found to be pH independent. For the esters with poor nucleofuges, a rate dependence on more than first power of the free imidazole and a linear dependence of κ′2 on free imidazole is observed. Accordingly, the microscopic rate constants for the assisted paths viz. κga and κgb have been disseminated besides for simple bimolecular attack. The Broensted-type plots and Hammett plots were constructed whose slope values are consistent with a stepwise mechanism through a bipolar tetrahedral addition intermediate whose formation or breakdown being rate determining for various paths. Comparison of this reaction of oxyesters with the earlier reported works on similar reaction of analogue thiolesters under identical reaction conditions showed remarkable mechanistic differences which are discussed in detail. The discussion is extended to include the details on previously studied ammonolysis of these two types of esters wherein thiolesters showed differed reactivity than that reported in the present study.
Acid dissociation constants of phenols and reaction mechanism for the reactions of substituted phenyl benzoates with phenoxide anions in absolute ethanol
Um, Ik-Hwan,Hong, Yeon-Ju,Kwon, Dong-Sook
, p. 5073 - 5082 (2007/10/03)
Acid dissociation constants of 10 substituted phenols have been measured by a kinetic method together with second-order rate constants for the reactions of aryl benzoates (X-C6H4CO-OC6H4-Y) with Z-substituted phenoxide (Z-C6H4O-) and EtO- in absolute ethanol at 25.0 ± 0.1°C. The kinetic results support a stepwise mechanism for the present acyl-transfer reaction.
The Effect of Hydrogen Bonding between Methyl-Substituted Phenols and Dipolar Aprotic Solvents on the Rate Constants for Protonation of Anthracene Anion Radical
Nielsen, Merete Folmer,Hammerich, Ole
, p. 883 - 896 (2007/10/02)
The kinetics for the protonation of anthracene anion radical (A-anion radical) by phenol and 13 methyl-substituted phenols (ArOH) have been studied by derivative cyclic voltammery in dimethyl sulfoxide, N,N-dimethylformamide, propylene carbonate and acetonitrile.The observed rate constants were corrected for the stoichiometric effects of formation of the homoconjugation complexes, ArOH/ArO- and Ar/OH/ArOH/ArO- and the kinetic contributions from protonation of A-anion radical by ArOH dimers.Available data for equilibrium (1) show that ArOH exists in solution predominantly as the ArOH/solvent hydrogen bond complex with only minor amounts of the 'free' ArOH present.It is suggested that the protonation of A-anion radical involves only the free ArOH in the proton transfer step (II).The unique rate constants corrected for the effect of equilibrium (I) were between two and three orders of magnitude higher than those usually referred to as the second-order rate constants for protonation of A-anion radical.ArOH/solvent ArOH + solvent (I) A-anion radical + ArOH AH-radical + ArO- (II).The kinetic effect of methyl substituents in the 2,6-positions of the phenol was observed to depend on the solvent.It was found that these effects could be satisfactority accounted for by the influence of the 2,6-methyl substituents on the magnitude of K1.The 'best' values of the substituent constants, ?b2 and ?b2.6, defined by the protonation of A-anion radical were found to be between those defined by equilibrium (1) and those defined by the unique equilibrium acidity of the phenols.This is taken as an indication that the phenol O-H bond is stretched considerably in the transition state for protonation of A-anion radical.From the relation between kII and ΔG deg it was found that the Broensted α value for the process is close to 0.5.
