Kinetic evidence

Respiratory control is a consequence of the thermodynamic back-pressure exerted by the proton electrochemical potential upon the respiratory chain, limiting the rate at which further protons can be extruded. Since the chemiosmotic theory states that the only connection between the respiratory chain and the other proton-utilizing components in the membrane is through the proton electrochemical gradient, the respiratory chain should be incapable of distinguishing between the different ways of producing a given depression in A/tn+. 

Testing for such an identity provides a severe test for the chemiosmotic theory. While some groups [e.g., 36,37,46] show that the transition to State 3 or addition of small concentrations of proton translocator cause similar respiratory stimulations, other groups have reported major discrepancies. Thus, Padan and Rottenberg [47] found that a much greater depression of was required to produce a given 

A similar argument applies to the rate of ATP synthesis. From the chemiosmotic standpoint, this should be regulated by the relation between Ajiih+ and AG jp, and should not depend on the way in which Aju,p,+ was varied. However, Baccarini-Melandri et al. [52] found that the rate of ATP synthesis by bacterial chromato-phores varies with Aju.h+ depending on whether electron flow is limited or proton translocator added. 

These discrepancies could be due to experimental problems in the measurement of the potentials, or to the existence of micro-circuits out of equilibrium with the bulk phases. These would possess a significant resistance between the localized proton circuit and the bulk phase. The respiratory chain would see the localized 

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The kinetics of this type of polymerization are the same as for simple condensation for this reason, the use of the term polycondensation is perhaps more appropriate. Unless kinetic evidence suggests otherwise, polymerizations involving the formation of chain polymers from cyclic compounds, following ring scission, are classed as condensation polymerizations. Some important con-... 

Left side of Fig. 4 shows a ribbon model of the catalytic (C-) subunit of the mammalian cAMP-dependent protein kinase. This was the first protein kinase whose structure was determined [35]. Figure 4 includes also a ribbon model of the peptide substrate, and ATP (stick representation) with two manganese ions (CPK representation). All kinetic evidence is consistent with a preferred ordered mechanism of catalysis with ATP binding proceeding substrate binding. 

Such arguments are based on the assumption that the nitronium ion is the nitrating agent in all of the media under consideration as regards nitration with acetyl nitrate, they certainly do not prove the efficacy of the nitronium ion unless the participation of the latter can be shown to be also consistent with the kinetic evidence. 

On the basis of both thermodynamic and kinetic evidence-both of which are interpretable in terms of the strain associated with rings of certain sizes or similar structural factors we see that only rings with five or six atoms have any significant stability. Accordingly, we conclude the following ... 

Little is known about the existence of alkyl hydrotetraoxides, R—OOOOH. There is some kinetic evidence supporting methyl hydrotetraoxide [23594-84-5] as a very labile intermediate in the reaction of methylperoxy radical, , and hydroperoxy radical, OOH (63). 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and the kinetic evidence of a solvent isotope effect are consistent with rate-limiting protonation with simple alkenes such as 2-metlQ lpropene and 2,3-dimethyl-2-butene. 

In 1977, Mandolini and Masci reported clear kinetic evidence that a template effect was important to the cyclization of o-hydroxyphenyl-3,6,9,12-tetraoxa-14-bromotetra-decyl ether. The starting material and its subsequent reactions are illustrated in Eq. (2.4). 

The overall reaction stoichiometry having been established by conventional methods, the first task of chemical kinetics is essentially the qualitative one of establishing the kinetic scheme in other words, the overall reaction is to be decomposed into its elementary reactions. This is not a trivial problem, nor is there a general solution to it. Much of Chapter 3 deals with this issue. At this point it is sufficient to note that evidence of the presence of an intermediate is often critical to an efficient solution. Modem analytical techniques have greatly assisted in the detection of reactive intermediates. A nice example is provided by a study of the pyridine-catalyzed hydrolysis of acetic anhydride. Other kinetic evidence supported the existence of an intermediate, presumably the acetylpyridinium ion, in this reaction, but it had not been detected directly. Fersht and Jencks observed (on a time scale of tenths of a second) the rise and then fall in absorbance of a solution of acetic anhydride upon treatment with pyridine. This requires that the overall reaction be composed of at least two steps, and the accepted kinetic scheme is as follows. 

The next level seeks a molecular description, and kinetics again makes a contribution. As will be seen in Chapter 5, the experimental kinetics provides information on both the energetics of the reaction (i.e., the height of the energy barrier on the reaction path) and the atomic composition of the transition state. Any proposed mechanism must therefore be consistent with the kinetic evidence. 

Kinetic evidence for a common intermediate. Suppose a series of related reactants each separately reacts with a common reagent, and the rates of product formation are the same for each reactant ... 

Kinetic evidence in the hydroxide-catalyzed halogenation of acetone provides definitive mechanistic information. The overall reaction is... 

From the overall kinetic evidence, a bimolecular mechanism is... 

A hydrogen-bonded cyclic transition state can be postulated for a nucleophile like ethanolamine or ethylene glycol anion whose hydrogen bonding to an azine-nitrogen in aprotic solvents can facilitate reaction via a cyclic transition state such as 78, cf. Section II, F. Ethanolamine is uniquely reactive with 2-chloronitrobenzene by virtue of a cyclic solvate (17) of the leaving group, a postulate in line with kinetic evidence. 

The preceding mechanism of the nucleophilic opening of the isoxazole ring is in accordance with the kinetic evidence of Pino... 

Nanosecond flash photolysis of 1,4-dinitro-naphthalene in aerated and deaerated solvents showed a transient species with absorption maximum at 545nm. The maximum of the transient absorption was independent of solvent polarity and its lifetime seemed to be a function of the hydrogen donor efficiency of the solvent. The transient absorption was attributed to the lowest excited triplet state of 1,4-dinitronaphthalene. Based on spectroscopic and kinetic evidence, the triplet state of 1,4-dinitronaphthalene behaved as an n - Tt state in nonpolar solvents,... 

We will conclude this section on theory with such a case. In Section 8.3 it was shown that the influence of substituents on the rate of dediazoniation of arenediazonium ions can be treated by dual substituent parameter (DSP) methods, and that kinetic evidence is consistent with a side-on addition of N2. We will now discuss these experimental conclusion with the help of schematic orbital correlation diagrams for the diazonium ion, the aryl cation, and the side-on ion-molecule pair (Fig. 8-5, from Zollinger, 1990). We use the same orbital classification as Vincent and Radom (1978) (C2v symmetry). 

The nature of the electrophile in this nitrating mixture is still not wholly agreed upon whereas kinetic evidence can be interpreted as consistent with nitration by nitronium ion, the fact that substituents with lone pairs of electrons or it-electrons give markedly different ortho para ratios from other nitrating mixtures is usually conceded to be consistent with the electrophile being something other than the nitronium ion. The balance of evidence at present is in favour of pro-tonated acetyl nitrate being the electrophile. 

Additional kinetic evidence supporting molecular iodine as an iodinating species is sparse. Li325 found that the iodination of tyrosine in acetate buffers at 25 °C showed the mixed inverse dependence on iodide ion concentration noted above, so that part of the reaction appeared to involve the molecular species. Subsequently, Doak and Corwin326 found that the kinetics of the iodination of (N-Me)-4-carboethoxy-2,5-dimethyl- and (N-Me)-5-carboethoxy-2,4-dimethyl-pyrroles in phosphate buffers in aqueous dioxane at 26.5 °C obeyed equation (162), viz. 

There is one further piece of kinetic evidence which throws light on an aspect of the benzidine rearrangement mechanism, and this is comparison of the rates of reaction of ring-deuterated substrates with the normal H compounds. If the final proton-loss from the benzene rings is in any way rate-determining then substitution of D for H would result in a primary isotope effect with kD < kH. This aspect has been examined in detail42 for two substrates, hydrazobenzene itself where second-order acid dependence is found and l,l -hydrazonaphthalene where the acid dependence is first-order. The results are given in Tables 2 and 3. 

Brown and McDonald (1966) provided another type of kinetic evidence for these size relationships by determining secondary kinetic isotope effects in reactions of pyridine-4-pyridines with alkyl iodides. For example, the isotopic rate ratio in the reaction between 4-(methyl-d3)-pyridine and methyl iodide at 25-0 C in nitrobenzene solution was determined to be kjyfk = l-OOl, while that in the corresponding reaction with 2,6-(dimethyl-d6)-pyridine was 1-095. (Brown and McDonald (1966) estimate an uncertainty of 1% in the k jk values.) Furthermore, the isotopic rate ratio in the case of the 2-(methyl-d3)-compound increased from 1 030 to 1-073 as the alkyl group in the alkyl iodide was changed from methyl to isopropyl, i.e. the isotope effect increased with increasing steric requirements of the alkyl iodide. 

A kinetic evidence for reversibility of bromonium ion formation has been obtained in the reaction of tetraisobutylethylene and its Dg labeled derivative with Br2 in acetic acid (ref. 9). Owing to steric effects, the first formed bromonium ion cannot undergo backside attack to give the dibromide, but looses a proton to yield... 

There is a large amount of evidence for the Sn2 mechanism. First, there is the kinetic evidence. Since both the nucleophile and the substrate are involved in the rate-determining step (the only step, in this case), the reaction should be first order in each component, second order overall, and satisfy the rate expression... 

The kinetic evidence is a necessary but not a sufficient condition we will meet other mechanisms that are also consistent with these data. Much more convincing evidence is obtained from the fact that the mechanism predicts inversion of configuration when substitution occurs at a chiral carbon and this has been observed... 

Like the kinetic evidence, the stereochemical evidence for the SnI mechanism is less clear-cut than it is for the Sn2 mechanism. If there is a free carbocation, it is planar (p. 224), and the nucleophile should attack with equal facility from either side of the plane, resulting in complete racemization. Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5-20% inversion, though in a few cases, a small amount of retention of configuration has been found. These and other results have led to the conclusion that in many SnI reactions at least some of the products are not formed from free carbocations but rather from ion pairs. According to this concept," SnI reactions proceed in this manner ... 

There is kinetic evidence that the initiation takes place primarily by steps like 1, which are called symproportionation steps (the opposite of disproportionation, p. 246). ... 

According to this mechanism, it is the free amine, not the salt that reacts, even in acid solution and the active-hydrogen compound (in the acid-catalyzed process) reacts as the enol when that is possible. This latter step is similar to what happens in 12-4. There is kinetic evidence for the intermediacy of the iminium ion (13). 

Evidence that the cation plays an essential role, at least in some cases, is that when the Li was effectively removed from L1A1H4 (by the addition of a crown ether), the reaction did not take place. The complex 19 must now be hydrolyzed to the alcohol. For NaBH4 the Na" " does not seem to participate in the transition state, but kinetic evidence shows that an OR group from the solvent does participate and remains attached to the boron... 

There is kinetic evidence that the solvent cage shown may not be necessary Walling, C. J. Am. Chem. Soc., 1988, 110, 6846. 

No j3-bromo- or iodoalkyl complexes have yet been isolated. The reaction of vicinal dibromides or diiodides with [Co (CN)j] , [Co(CN)5H], or a Co(I)-DMG complex merely gives the olefin 32, 75,105,109,161), though kinetic evidence was obtained for the intermediate formation of the j8-bromo complex in the reaction of [Co"(CN)5] with a,j8-dibromopropionate and a,/3-dibromosuccinate (75). It is interesting that the pentacyanide produced is the bromo or iodo, and not the aquo, complex 32, 75), which suggests that the decomposition may involve a cis rather than a trans elimination of Co—X. The /3-chloroethyl complex can be prepared by tbe reaction of [Co(CN)5H]3- with CICH2CH2I 105). 

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