Substitution reactions, inhibition first-order

Scheme 22. The rate equation for this mechanism is described in (1). The authors determined that the reaction is first-order in allylic carbonate, aniline and catalyst, and inverse first-order in allylamine product. These results are consistent with the proposed mechanism. Thus, iridium-catalyzed allylic substitution is inhibited by product. In addition, the formation of the allyliridium intermediate is disfavored.

Lehn and coworkers have profitably employed tartaric acid-containing crown ethers as enzyme models. The rate of proton transfer to an ammonium-substituted pyridinium substrate from a tetra-l,4-dihydropyridine-substituted crown ether was considerably enhanced compared to that for a simple 1,4-dihydropyridine. The reaction showed first order kinetic data and was inhibited by potassium ions. Intramolecular proton transfer from receptor to substrate was thus inferred via the hydrogen bonded receptor-substrate complex shown in Figure 16a (78CC143). 

If (A i[X ]/A 2[Y ]) is not much smaller than unity, then as the substitution reaction proceeds, the increase in [X ] will increase the denominator of Eq. (8-65), slowing the reaction and causing deviation from simple first-order kinetics. This mass-law or common-ion effect is characteristic of an S l process, although, as already seen, it is not a necessary condition. The common-ion effect (also called external return) occurs only with the common ion and must be distinguished from a general kinetic salt effect, which will operate with any ion. An example is provided by the hydrolysis of triphenylmethyl chloride (trityl chloride) the addition of 0.01 M NaCl decreased the rate by fourfold. The solvolysis rate of diphenylmethyl chloride in 80% aqueous acetone was decreased by LiCl but increased by LiBr. ° The 5 2 mechanism will also yield first-order kinetics in a solvolysis reaction, but it should not be susceptible to a common-ion rate inhibition. 

Radical decay kinetics have been shown to be 3/2 order, falling to first order, and also second order, falling with time deviations are apparently due to side reactions of 118. Radical half-lives are strongly influenced by the nature of the aryl substituents, being particularly short for ortho-substituted Ar because of inhibited delocalization. The corresponding compounds 39 have, accordingly, enhanced thermal stability, a factor useful in some commercial thermo- and photographic processes. 

In assessing whether a reactor is influenced by intraparticle mass transfer effects WeiSZ and Prater 24 developed a criterion for isothermal reactions based upon the observation that the effectiveness factor approaches unity when the generalised Thiele modulus is of the order of unity. It has been showneffectiveness factor for all catalyst geometries and reaction orders (except zero order) tends to unity when the generalised Thiele modulus falls below a value of one. Since tj is about unity when 0 < ll for zero-order reactions, a quite general criterion for diffusion control of simple isothermal reactions not affected by product inhibition is < 1. Since the Thiele modulus (see equation 3.19) contains the specific rate constant for chemical reaction, which is often unknown, a more useful criterion is obtained by substituting l v/CAm (for a first-order reaction) for k to give ... 

Operating within the framework of the Chauvin mechanism, the main consideration for the reaction mechanism is the order of events in terms of addition, loss and substitution of ligands around the ruthenium alkylidene centre. Additionally, there is a need for two pathways (see above), both being first order in diene, one with a first-order dependence on [Ru] and the other (which is inhibited by added Cy3P) with a half-order dependence on [Ru]. From the analysis of the reaction kinetics and the empirical rate equation thus derived, the sequence of elementary steps via two pathways was proposed, one non-dissociative (I) and the other dissociative (II), as shown in Scheme 12.20. The mechanism-derived rate equation is also shown in the scheme and it can thus be seen how the constants A and B relate to elementary forward rate constants and equilibria in the proposed mechanism. 

Theoretical studies of the gas-phase hydrolysis or methanolysis of methylsul-fonyl chloride indicated a concerted Sn2 process involving a four-membered cyclic transition state. The tertiary amine-catalysed hydrolysis of benzenesul-fonyl chloride was shown to be inhibited by chloride ion and a nucleophilic mechanism of catalysis was favoured. Kinetic studies" of the solvolysis of p-substituted benzenesulfonyl chlorides in aqueous binary mixtures with acetone, methanol, ethanol, acetonitrile and dioxime showed that the reactions were third order processes, with first order rate constants determined mainly by the molar concentrations of the protic solvent, so that the reaction rates appear to be dominated by solvent stoichiometry. The solvolyses in methanol and ethanol yield both an alcoholysis (ap) and a hydrolysis product (hp). Solvolyses of electron-rich arylsulfonyl chlorides, under neutral or acidic conditions, exhibited surprising maxima in solvent-dependent S values as defined by Equation 15. 

In order to design superior catalyst systems and expand the applications of these first generation catalysts, it was necessary to understand the fundamental mechanism of ruthenium-catalyzed olefin metathesis reactions. Initial investigations focused on the activity of 1 and its derivatives for the catalytic RCM of diethyl diallylmalonate (Eq. 4.14) [86]. These studies revealed that, in all cases, the overall catalytic activity was inhibited by the addition of free phosphine, and that the turnover rate was inversely proportional to the concentration of added phosphine. This indicated that phosphine dissociation was required for catalytic activity, and further suggested that olefin metathesis may be initiated by the substitution of a phosphine ligand with an olefin substrate. 

The first study on Mnj(CO)u, by Wawersik and Basolo indicated a D mechanism for PRj substitution because the rate was inhibited by free CO. However, later work by Pog and co-workers claimed to find no CO inhibition and the homolytic cleavage mechanism in Scheme 5.6 was proposed. It is important to note that these were primarily studies of the decomposition of Mn CCOj, either in the absence or presence of Oj and mainly in decalin, at temperatures of 115-180 C. Poe and co-workers found that the rate is half-order in [Mnj(CO) o] under an argon atmosphere and proposed that the reaction proceeds by homolysis of the Mn— Mn bond, as shown in the following reaction ... 

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