Kinetic studies exchange

This genera] scheme could be used to explain hydrogen exchange in the 5-position, providing a new alternative for the reaction (466). This leads us also to ask whether some reactions described as typically electrophilic cannot also be rationalized by a preliminary hydration of the C2=N bond. The nitration reaction of 2-dialkylaminothiazoles could occur, for example, on the enamine-like intermediate (229) (Scheme 141). This scheme would explain why alkyl groups on the exocyclic nitrogen may drastically change the reaction pathway (see Section rV.l.A). Kinetic studies and careful analysis of by-products would enable a check of this hypothesis. 

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

Each act of proton abstraction from the a carbon converts a chiral molecule to an achi ral enol or enolate ion The sp hybridized carbon that is the chirality center m the start mg ketone becomes sp hybridized m the enol or enolate Careful kinetic studies have established that the rate of loss of optical activity of sec butyl phenyl ketone is equal to Its rate of hydrogen-deuterium exchange its rate of brommation and its rate of lodma tion In each case the rate determining step is conversion of the starting ketone to the enol or enolate anion... 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

Purely parallel reactions are e.g. competitive reactions which are frequently carried out purposefully, with the aim of estimating relative reactivities of reactants these will be discussed elsewhere (Section IV.E). Several kinetic studies have been made of noncompetitive parallel reactions. The examples may be parallel formation of benzene and methylcyclo-pentane by simultaneous dehydrogenation and isomerization of cyclohexane on rhenium-paladium or on platinum catalysts on suitable supports (88, 89), parallel formation of mesityl oxide, acetone, and phorone from diacetone alcohol on an acidic ion exchanger (41), disproportionation of amines on alumina, accompanied by olefin-forming elimination (20), dehydrogenation of butane coupled with hydrogenation of ethylene or propylene on a chromia-alumina catalyst (24), or parallel formation of ethyl-, methylethyl-, and vinylethylbenzene from diethylbenzene on faujasite (89a). 

Electrophilic aromatic hydrogen exchange reactions fall into two classes, namely those reactions catalysed by acid and those reactions catalysed by base. Of these the former are by far the most common and have been subjected to the most extensive and intensive kinetic studies. 

Kinetic studies of the mechanism of acid-catalysed hydrogen exchange... 

Kinetic studies of substituent effects in electrophilic aromatic hydrogen exchange... 

Kinetic studies of base-catalysed hydrogen exchange have been carried out by Roberts, by Shatenshtein, and by Streitweiser and their coworkers. In earlier work, potassium amide was used as base in liquid ammonia as solvent, whereas later workers used lithium and caesium cyclohexylamides in cyclohexylamine. The reaction can be represented by equilibria (239) and (240)... 

Kinetic studies of base-catalysed hydrogen exchange of heterocyclic compounds have been carried out. Paudler and Helmick515 measured second-order rate coefficients for deuteration of derivatives of imidazo[l,2-a]pyridine(XXXIII), imidazo[l,2-a]pyrimidine(XXXIV), and 1,2,4-triazolo[1,5-a]pyrimidine(XXXV)... 

The first extensive kinetic study of these reactions was carried out by Ingold et a/.7I3b, who demonstrated that for the two-alkyl exchange reaction, i.e. the reverse direction of equilibrium (283), the kinetics in acetone and ethanol were cleanly second-order, viz. 

Anderson and Bonner made the first detailed kinetic study on the exchange using the isotopic method ( Cr) and a separation method based on the conversion of Cr(II) into Cr(IIl) oxalate and an ion-exchange treatment. To prevent oxidation of Cr(II) during exchange a hydrogen atmosphere was maintained over the reaction mixture. The rate law found to be obeyed for the concentration ratio range Cr(III)/Cr(II) of between 3.3 x 10 and 2.0 in perchlorate media was... 

Horne has studied the kinetics of exchange in aqueous perchlorate media at temperatures down to —78 °C by the isotopic method ( Fe) and dipyridyl separation. The same rate law in these ice media as in aqueous solution was observed, although the acid dependence was small. Horne concluded that the same exchange mechanism occurs in solid and liquid solvent. Evidence for a Grotthus-type mechanism has been summarised. ... 

Four recent kinetic studies have been made of the exchange in the presence of the sulphate ion in perchlorate media, some measure of agreement being observed between the results of the various workers. 

Kinetic studies indicated that external H interacted with the renal brush border Na /H exchanger (resistant-type) at a single site with apparent pKa 7.3-7.5. Because the imidazolium ring of histidine is the principal group that is titratable in this... 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

Direct kinetic studies of water exchange on [Pd(H20)4]2+ and [Pt(H20)4]2+, the parameters for which appear in Table XIV (267-274), were first reported well after the chemistry outlined in the preceding paragraph had become established. Solvent and ligand exchange on... 

Kinetic studies on racemization, oxygen exchange reaction using H2lsO, and theoretical studies clarified the mechanism for racemization of optically active chalcogenic acids.34,37 The mechanism of racemization of tellurinic acids is different from that of the corresponding seleninic acids. In fact, the mechanism for racemization of optically active tellurinic acids was found to involve an... 

---