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Dioxan proton transfer

Curioni et al.148 studied the protonation of 1,3-dioxane and 1,3,5-trioxane by means of CP molecular dynamics similations. The dynamics of both molecules was continued for few ps following protonation. The simulation provided a detailed picture the evolution of both the geometry and the electronic structure, which helped to rationalize some experimental observations. CP molecular dynamics simulations were applied by Tuckerman et al.149,150 to study the dynamics of hydronium (H30+) and hydroxyl (OH-) ions in liquid water. These ions are involved in charge transfer processes in liquid water H20 H+. .. OH2 - H20. .. H+-OH2, and HOH. . . OH- -> HO-. . . HOH. For the solvatetd H30+ ion, a picture consistent with experiment emerged from the simulation. The simulation showed that the HsO+ ion forms a complex with water molecules, the structure of which oscillates between the ones of H502 and I L/ij clusters as a result of frequent proton transfers. During a consid-... [Pg.107]

Some cyclic thioacetals have an A-SE2 hydrolysis mechanism,206 as do some 2-aryl-2-methyl-l,3-dithianes, except for the 4-NO2 derivative, which looks more A2-like.207 In 10 vol% dioxane/aqueous HC104 mixtures, reactive 2-aryl-2-phenyl-l,3-dithianes are believed to have an A-SE2 hydrolysis mechanism, whereas the least reactive ones have an A2 mechanism.130 Isothiocyanates are believed to hydrolyze by a mechanism that involves simultaneous proton transfer to nitrogen and attack of water at carbon in a cyclic transition... [Pg.36]

Proton exchange rates in aqueous solutions are enhanced by small amounts (0.5% V/V) of hydrophobic substances (e.g., methanol, dioxane) because of a consequent increase in H-bonded water structure in the hydration shells through which the proton transfer is mediated (9). [Pg.70]

The Brpnsted coefficient /3b = 0.52 for deprotonation of 3-phenylcoumaran-2-one (108) by a series of bases in 50% (v/v) water-dioxane, and q bh = 0.48 for reprotonation by the conjugate acid of the buffer, are indicative of a fairly symmetrical transition state for proton transfer, although the primary KE, ku/ku = 3.81, found for proton abstraction by HO is lower than expected. " The moderate intrinsic rate constant for deprotonation of (108) suggests that generation of the charge in the transition state is accompanied by only a small amount of molecular and solvent reorganization. In acidic solution, below pH 5, O-protonation of (110) occurs initially to form (109)... [Pg.374]

The nitrosation of A-alkylureas in dioxane-acetic acid mixtures is governed by the expression v = fc[HN02][urea], at fixed pH, and dependent on rate-determining proton transfer from the protonated iV-alkyl-iV-nitrosourea to acetate anion the order of reactivity, which reflects relative impediment by the alkyl groups, is as for nitrosation in aqueous media (methyl- ethyl- propyl- butyl- > allyl-urea). [Pg.382]

On the basis of kinetic data, it was suggested that appreciable charge separation in the activated complex (equation 13) could be avoided by means of such proton transfers, where HA is a general acid (H2O, ROH, RO—OH). Upon change from a polar protic solvent to the nonpolar solvent dioxane, the reaction was observed to be second-order in hydrogen peroxide and the second molecule of H2O2 obviously played the role of HA in the 1,4-proton shift. The rate of oxidation was shown to increase linearly with the pfsTa of solvent HA. In general, it was concluded that solvent interactions provide a... [Pg.73]

No kinetic data are available for the hydrolysis of phenyldiazomethane, but some measurements have been done with p-nitrophenyldiazomethane [210]. A solvent isotope effect of kH/kD = 2.5 in 60 % dioxane—water with 0.014 M HC104 at 20 °C supplies evidence for rate-determining proton transfer in the mechanism of hydrolysis. Since the electron-withdrawing p-N02 group must destabilize the carbonium ion and increase the acidity of the diazonium ion, it may be expected that ku must be higher and must be lower for the unsubstituted compound. It follows that fen/fe i must be higher in the hydrolysis of phenyldiazomethane in comparison to p-nitrophenyldiazomethane. Therefore, it is very likely that rate-determining proton transfer occurs also in the hydrolysis of phenyldiazomethane. [Pg.67]

A 3 2 mixture of cis-trans isomers is obtained from the addition of secondary amines to butadiyne in dioxane . The ratio remains constant during the course of the reaction signifying that the isomers are formed in this ratio. This, coupled with the second-order kinetics observed and large negative values for the activation entropy (AS — 50 e.u.), led to the postulation of a mechanism involving ratedetermining attack by the amine on the diyne, followed by stereochemical equilibration of the dipolar ion and proton transfer, as illustrated in Scheme 7. [Pg.71]

The dependence of intramolecular proton transfer on solvent friction has been established for 2-(2 -hydroxy-5-methylphenyl) benzotriazole in alcohol and other solvents. Excited-state proton transfer in 2-(2 -hydroxyphenyl) benzothiazole has also been studied 2 Photophysical properties and laser performance of w, w -bis ( oxazol-2-yl)- -oligophenylenes in dioxane have been measured at room temperature. +p increases with the number of phenyl rings between terminally positioned oxazoyl groups. [Pg.13]

The second observation concerns the increase in the hydrogen-bonding interaction of the O—H moiety of the hydroxyarene. Several observations of this effect were reported in the past, for phenol, naphthol and pyrenol derivatives. Perhaps the most direct observation concerns the red shift observed in the IR absorption frequency of the complexed O—H bond. A shift of about 250 cm was observed for O—H- O and O—H- N type bonds of 1 1 complexes of 1-naphthol with water and ammonia when 1-naphthol was electronically excited. This shift translates to an about 0.7 kcalmol" increase in the hydrogen-bonding interaction in the excited state of the photoacid. A similar effect was observed in solution by Weller for the system 1-hydroxypyrene complexed with pyridine in methylcyclohexane. Other observations include phenol and 1- and 2-naphthol complexed with dioxane in isooctane ", and HPTA complexed with dioxane and DMSO in dichloromethane and dichloroethane. In all cases the hydrogen-bonding interactions of the photoacid were found to increase upon electronic excitation by 0.5-3 kcalmol. No proton transfer was observed in these systems. [Pg.503]

An interesting case of solvent-mediated proton transfer is proposed for 4-aminophthalimide (4-AP). In aprotic solvents like dioxane or acetonitrile (ACN),... [Pg.297]

Lumichrome [7,8-dimethylalloxazine, (23)], a flavin tautomer, has two fluorescence emissions with maxima at 440 and 540 nm in pyridine-dioxan mixtures. Nanosecond time-resolved fluorescence shows fast growth of the latter due to proton transfer from N-1 of the excited lumichrome (23 ) to N-10 during the lifetime of lumichrome singlet, and emission occurs from the excited flavinic chromophore (24 ). [Pg.72]

Examples are hydrolyses of methyl and butyl acetates [19]. Another example is formation of eximers and exiplexes in polyesters and methacrylate polymers that always favor large molecules over small ones [33]. Proton transfer reaction of poly(vinyl quinoline) [34] can serve as a third example. The emission, excitation, and absorption spectra of this polymer in a mixture of dioxane and water can be compared to that of 2-methylquinoline. The emissions coming from the protonated heterocyclic rings in the polymer occur sooner than from the low molecular weight compound [34]. [Pg.572]


See other pages where Dioxan proton transfer is mentioned: [Pg.279]    [Pg.325]    [Pg.442]    [Pg.85]    [Pg.116]    [Pg.177]    [Pg.183]    [Pg.917]    [Pg.42]    [Pg.43]    [Pg.35]    [Pg.250]    [Pg.79]    [Pg.8]    [Pg.69]    [Pg.133]    [Pg.102]    [Pg.1157]    [Pg.80]    [Pg.81]    [Pg.177]    [Pg.183]    [Pg.34]    [Pg.318]    [Pg.588]    [Pg.297]    [Pg.271]    [Pg.272]    [Pg.167]    [Pg.167]    [Pg.407]    [Pg.9]   
See also in sourсe #XX -- [ Pg.133 ]




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1.3- dioxane, protonation

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