Reactions with a Simple Kinetic Form

Some reactions follow relatively simple rate expressions. Their time course usually depends on the concentration of only a single species, designated A. We shall learn to recognize these situations and to analyze the resulting concentration-time data. We shall also consider data obtained by instrumental methods and by the method of flooding. 

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Binks and Ridd164 have made a complete kinetic study of the reaction of indole with several diazotized amines (p-nitroaniline, p-chloroaniline, sulfanilic acid, and aniline). Only the reaction with p-nitrodiazonium salt exhibits a simple kinetic form (pseudo first-order reaction) in the other cases the kinetics appear to be due to the superposition of two reactions, a normal azo-coupling reaction and an autocatalytic side reaction that removes diazonium ions, but does not form azo compounds. 

Why does the stabilized ylid prefer to react with the double bond In order to understand this, let s consider first the reaction of a simple, unstabilized ylid with an unsaturated ketone. The enone 1 has two electrophilic sites, but from Chapters 10 and 23, in which we discussed the regioselectivity of j attack of nucleophiles on Michael acceptors like this, you would expect that direct 1,2-attack on the i ketone is the faster reaction. This step is irreversible, and subsequent displacement of the sulfide i leaving group by the alkoxide produces an epoxide. It s unimportant whether a cyclopropane prod- uct would have been more stable ihe epoxide forms faster and is therefore the kinetic product. 

Since the forward reaction in (29) is exothermic, the equilibrium is displaced to the left by increase in temperature this factor accounts in part for the anomalous temperature coefficient of reaction rate mentioned above. The apparent catalysis by propagating base is also explicable as acid catalysis since the carbamic acid is stoichiometrically derived from the base by reaction (29). That true base catalysis is not operative has been shown by the observation that addition of tertiary bases does not affect the reaction rate [17]. Further, the polymerization is catalysed by other weak acids such as hydrocinnamic [17] and a-picolinic acids [10, 17], which, if present in sufficient concentration under conditions of low CO2 pressure, reduce the order in initiating base to unity. Thus, under such conditions, with hydrocinnamic acid (HX) as catalyst the simple kinetic form (30) is achieved. 

A simple kinetic scheme has been developed to explain many of the observed aspects of SiH4/N20 growth [19], It was suggested that the reaction is initiated by decomposition of N2O (Eq. 5.4), generating an oxygen radical which can abstract hydrogen from silane forming a hydroxyl radical (Eq. 5.5), that can react further with silane (Eq. 5.6). 

The local selectivity decreases with increasing conversion, since ri decreases as A is consumed and r2 increases as C is formed. The overall selectivity is an integrated average of the local values. Internal concentration gradients lower the selectivity because r is decreased and f2 is increased. If the reactions have similar simple kinetics, the change in local selectivity can be easily estimated using partial pressure differences. If... 

The reaction sequence depicted in Scheme 4.4 fits both observations (second-order and saturation-type kinetics) as is evident from the following. From a kinetic viewpoint this sequence is a simple consecutive reaction with a reversible first step once the enzyme is either reduced or oxidized and the product is released the enzyme is in a different form. Due to the absence of the second substrate it cannot re-launch into the catalytic cycle. In other words this is not a catalytic reaction and the enzyme responds as a normal reactant ... 

The integrated forms of several other simple-order kinetic expressions, obtained under the assumption of constant density, are listed in Table 1.2.4.2-1. Fig. 1.2.4.2-1 graphically represents the various integrated kinetic equations of Table 1.2.4.2-1 [Caddell Hurt, 1951). Note that for a second order reaction with a large ratio of feed components, the reaction order degenerates into pseudo-first-order. 

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