First Kinetics

From these results it appears that the 5-position of thiazole is two to three more reactive than the 4-position, that methylation in the 2-position enhances the rate of nitration by a factor of 15 in the 5-position and of 8 in the 4-position, that this last factor is 10 and 14 for 2-Et and 2-t-Bu groups, respectively. Asato (374) and Dou (375) arrived at the same figure for the orientation of the nitration of 2-methyl and 2-propylthiazole Asato used nitronium fluoroborate and the dinitrogen tetroxide-boron trifluoride complex at room temperature, and Dou used sulfonitric acid at 70°C (Table T54). About the same proportion of 4-and 5-isomers was obtained in the nitration of 2-methoxythiazole by Friedmann (376). Recently, Katritzky et al. (377) presented the first kinetic studies of electrophilic substitution in thiazoles the nitration of thiazoles and thiazolones (Table 1-55). The reaction was followed spec-trophotometrically and performed at different acidities by varying the... 

The first kinetic results in the area were obtained by studying the quatemization of 4-alkyl-, 5-alkyT, and 2-alkylthiazoles with methyl iodide (253-255). A deeper and more exhaustive study of this reaction has been carried out recently with more elaborate substrates (152). 

The mechanism of anionic polymerization of cyclosiloxanes has been the subject of several studies (96,97). The first kinetic analysis in this area was carried out in the early 1950s (98). In the general scheme of this process, the propagation/depropagation step involves the nucleophilic attack of the silanolate anion on the sUicon, which results in the cleavage of the siloxane bond and formation of the new silanolate active center (eq. 17). 

Conant and Peterson123 made the first kinetic study of the coupling of diazonium ions with aromatics, and measured the rates of reaction of diazotised aniline and its 2-MeO, 4-Me, 4-Br and 4-S03H derivatives with the sodium salts of 4-hydroxybenzenesulphonic acid, 2-naphthol-3,6-disulphonic acid, 1-naphthol-3,8-disulphonic acid, and l-naphthol-4-sulphonic acid. The reaction was second-order, viz. 

The first kinetic study appears to have been that of Martinsen148, who found that the sulphonation of 4-nitrotoluene in 99.4-100.54 wt. % sulphuric acid was first-order in aromatic and apparently zeroth-order in sulphur trioxide, the rate being very susceptible to the water concentration. By contrast, Ioffe149 considered the reaction to be first-order in both aromatic and sulphur trioxide, but the experimental data of both workers was inconclusive. The first-order dependence upon aromatic concentration was confirmed by Pinnow150, who determined the equilibrium concentrations of quinol and quinolsulphonic acid after reacting mixtures of these with 40-70 wt. % sulphuric acid at temperatures between 50 and 100 °C the first-order rate coefficients for sulphonation and desulphonation are given in Tables 34 and 35. The logarithms of the rate coefficients for sulphonation... 

Derbyshire and Waters202 carried out the first kinetic study, and showed that the chlorination of sodium toluene-m-sulphonate by hypochlorous acid at 21.5 °C was catalysed more strongly by sulphuric acid than by perchloric acid and that the rate was increased by addition of chloride ion. A more extensive examination by de la Mare et al.203 of the rate of chlorination of the more reactive compounds, anisole, phenol, and />-dimethoxybenzene by hypochlorous acid catalysed by perchloric acid, and with added silver perchlorate to suppress the formation of Cl2 and C120 (which would occur in the presence of Cl" and CIO-, respectively),... 

The first kinetic study used chloromethyl methyl ether as chlorinating reagent and acetic acid as solvent, viz. reaction (188)381... 

The final aspect of the mechanism, namely the effect of different electron supply in the alkyl groups of ArSiR3 has now been settled. From the ease of cleavage of MR3 groups (M = metal) noted above, one would expect that increased electron supply from R would increase the reaction rate. The first kinetic studies664 in fact indicated the opposite, as shown by the data in Table 229, and although the... 

The first kinetic study of the acid-catalysed cleavage of the aryl-germanium bond, reaction (268)... 

When the drug is nonionizable in water, three hydrolytic pathways are available [Eq. (33)] it can degrade by specific acid catalysis represented by the first kinetic term in Eq. (33), water hydrolysis (second term), and specific base catalysis (third term) ... 

The catalyst can be regenerated by evacuation to remove cyclohexene oxide and addition of fresh BuOOH at the end of the first kinetic run. When a second dose of cyclohexene vapor was introduced to 3, very similar kinetic behavior was observed, Figure lb. However, the smaller absorbance change imphes that less cyclohexene was epoxidized, likely because of incomplete removal of the epoxide which blocks the active sites. 

The additivity of substituent effects, demonstrated by (16), was the first kinetic evidence for a symmetrical charge distribution in the rate-limiting transition states of alkene bromination. 

The first kinetically stable dibenzosilafulvene (7), whose structure and properties should more correctly be described by the resonance hybrid 7a 7b with a great contribution of the ylide form 7a, reacts with phosphorus ylide to form betaine (8), which is rearranged, under thermodynamically controlled conditions, into the salt (9) (Scheme 4).24,25... 

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The first kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

It is unfortunate that many workers have not appreciated how essential a clue to the kinetics can be provided by the kinetic order of the whole reaction curve. The use of initial rates was carried over from the practice of radical polymerisation, and it can be very misleading. This was in fact shown by Gwyn Williams in the first kinetic study of a cationic polymerization, in which he found the reaction orders deduced from initial rates and from analysis of the whole reaction curves to be signfficantly different [111]. Since then several other instances have been recorded. The reason for such discrepancies may be that the initiation is neither much faster, nor much slower than the propagation, but of such a rate that it is virtually complete by the time that a small, but appreciable fraction of the monomer, say 5 to 20%, has been consumed. Under such conditions the overall order of the reaction will fall from the initial value determined by the consumption of monomer by simultaneous initiation and propagation, and of catalyst by initiation, to a lower value characteristic of the reaction when the initiation reaction has ceased. 

The basic study was performed on copper complexes with N,N,N, N1-tetramethylethane-1,2-diamine (TMED), which were known to be very effective oxidative coupling catalysts (7,12). From our first kinetic studies it appeared that binuclear copper complexes are the active species as in some copper-containing enzymes. By applying the very strongly chelating TMED we were able to isolate crystals of the catalyst and to determine its structure by X-ray diffraction (13). Figure 1 shows this structure for the TMED complex of basic copper chloride Cu(0H)Cl prepared from CuCl by oxidation in moist pyridine. 

The first kinetic study of acceleration of some Diels-Alder reactions in water by Breslow et al. has set the stage for worldwide interest in organic... 

In the first kinetic experiments, the substrate concentration was varied by changing the propene partial pressure in the feed gas (at constant total pressure) between 0.9-3.2bar at temperatures in the range 65-140 °C. The rate... 

The apparently first kinetic study of a metal-assisted electrophilic substitution in a Co(III) complex is recent. The bromination of Co(NH3)5imidH is complicated by the presence of different bromine species in solution (Brj, HOBr and Brj"). In addition, successive brominations of the coordinated imidazole occur. Rate data can be interpreted in terms of reaction of the conjugate base of the Co(III) complex with Brj, and a suggested mechanism for the first steps is (Rq = Co(NH3)5 ")... 

Until the 1950s, the rare periodic phenomena known in chemistry, such as the reaction of Bray [1], represented laboratory curiosities. Some oscillatory reactions were also known in electrochemistry. The link was made between the cardiac rhythm and electrical oscillators [2]. New examples of oscillatory chemical reactions were later discovered [3, 4]. From a theoretical point of view, the first kinetic model for oscillatory reactions was analyzed by Lotka [5], while similar equations were proposed soon after by Volterra [6] to account for oscillations in predator-prey systems in ecology. The next important advance on biological oscillations came from the experimental and theoretical studies of Hodgkin and Huxley [7], which clarified the physicochemical bases of the action potential in electrically excitable cells. The theory that they developed was later applied [8] to account for sustained oscillations of the membrane potential in these cells. Remarkably, the classic study by Hodgkin and Huxley appeared in the same year as Turing s pioneering analysis of spatial patterns in chemical systems [9]. 

The first kinetics measurements about coalescence were reported by Kabalnov and Weers in water-in-oil emulsions [40]. These authors measured the characteristic time at which the layer of free water formed at the bottom of the emulsions corresponded approximately to half of the volume of the dispersed phase. This time was assumed to be equal to t. By measuring r at different temperatures, the activation energy was deduced from an Arrhenius plot. Kabalnov and Weers were able to obtain the activation energy for a water-in-octane emulsion at 50%, stabilized by the nonionic surfactant C12E5 (pentaethylene glycol mono n-dodecyl ether), above the phase inversion temperature (PIT), and found a value of 47 kgTr, Tr being the room temperature. 

The first kinetic study of the aquation of [MoCl ] to [MoClgfHjO)] = 87 min at 0 °C) has been reported. No conclusive proof that the final product is [Mo(H20)g] could be obtained, and the spectral profile reported earlier for [Mo(H20)e] is not considered to preclude the existence of dimeric molybdenum(iii) species. The reaction of [WCl (MeCN)2] with... 

Measurement of rate constants provides information not only on the rate process but on the reactant state as well. The first kinetic study on the addition of RLi to ketones goes back to 1950, when Swain and Kent reported the results of kinetic experiments for the addition reaction of RLi and RMgX with ketone . In these studies, a fiow method was used to measure the reactivity of these fast reactions. The reaction was carried... 

A very interesting and complex protonation mechanism has been snggested for the hydride cluster [W3S4H3(dmpe)3]PF6 in CH2CI2 solutions. In the presence of an excess of HCl, a careful kinetic study of the process in eq. (10.4) by the stopped-flow technique [9] has revealed three kinetically distinguishable steps very fast, fast, and slow, with rate constants A 1, ki, and k3. The kinetic order in the initial hydride cluster in the slow step has been measured as 1. At the same time, rate constants k and A 2 have corresponded to a second-order dependence on acid concentration, while the third step has shown a zero kinetic order on HCl. The rate constants have been determined as A i =2.41 x 10 M-2/s, k2 = 1.03 X 10 M /s, A 3 = 4 X 10 s . Note that the protonation process becomes simple at lower concentrations of HCl. Under these conditions it shows a single step with a first kinetic order on the acid. 

Soon after the isolation of 136, Tokitoh et described the synthesis of the first kinetically stabilized diarylstannylene stable in solution, that is, Tbt(Tip)Sn (169), by treatment of TbtLi with stannous chloride followed by addition of TipLi (Scheme 14.74). Under an inert atmosphere, stannylene 169 was found to be quite stable even at 60 °C in solution, and it showed a deep purple color (A,max =561 nm) in hexane. The Sn NMR spectrum of 169 showed only one signal at 2208 ppm, the chemical shift of which is characteristic of a divalent organotin compound as in the case of a monomeric dialkylstannylene (136). The bandwidth and the chemical shift of 169 were almost unchanged between —30 and 60 °C, indicating the absence of a monomer-dimer equilibrium. 

One of the first kinetic studies of the fragmentation of a C—O bond in an ether radical anion was reported by Maslak and Guthrie. " In this study substituted benzyl phenyl ethers (as well as some other benzyl-type phenyl ethers) were treated with 2,4,6-tri-tert-butylnitrobenzene radical anion to produce ArCH20Ph or PhCH20Ar and the unimolecular decay of the anion radical was monitored using EPR. Despite some discrepancies between the values of the reported rate constants, ... 

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