Multistep mechanisms

The net change represented by a balanced chemical equation often occurs by a multistep mechanism consisting of a sequence of elementary reactions. For example, below 225 °C, the reaction 

we say that the reaction occurs by a two-step mechanism. 

The chemical equations for the elementary reactions in a multistep mechanism must always add to give the chemical equation of the overall process. In the present example the sum of the two elementary reactions is 

Simplifying this equation by eliminating substances that appear on both sides gives Equation 14.23, the net equation for the process. 

Because NO3 is neither a reactant nor a product of the reaction—it is formed in one elementary reaction and consumed in the next—it is called an intermediate. Multistep mechanisms involve one or more intermediates. Intermediates are not the same as transition states, as shown in FIGURE 14.20. Intermediates can be stable and can therefore sometimes be identified and even isolated. Transition states, on the other hand, are always inherently unstable and as such can never be isolated. Nevertheless, the use of advanced ultrafast techniques sometimes allows us to characterize them. 

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Short segments of poly(dG—dC) incorporated within plasmids, or citcular DNA, adopt the Z-conformation under negative superhehcal stress. This left-handed DNA may be important in genetic control. On the other hand, the stmctural alteration of the helix requited in a B-to-Z transition within a plasmid is radical, and would involve either a multistep mechanism or the complete melting and reformation of helix. The improbability of such transitions has led to questions concerning the feasibility of a biological role for Z-DNA. 

The reaction proceeds by a multistep mechanism and normally is reversible. Therefore, we expect the reaction to be thermodynamically controlled. 

Rate constant The proportionality constant in the rate equation for a reaction, 288 Rate-determining step The slowest step in a multistep mechanism, 308 Rate expression A mathematical relationship describing the dependence of reaction rate upon the concentra-tion(s) of reactant(s), 288,308-309 Rayleigh, Lord, 190... 

Cellular defense mechanisms against toxins (A multistep mechanism for elimination of toxic metabolites and xenobiotics. It involves various transport, oxidation, and conjugation steps.) are usually divided into several steps as it is visualized on Fig. 3. Organic anion transporting proteins (OATPs) are responsible for the cellular uptake of endogenous compounds and... 

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

If the formation and breakdown steps of a mechanism involving a tetrahedral intermediate respond differently to changes in pH or catalyst concentration, then one can find evidence from plots of rate versus pH or rate versus catalyst concentration for a change in rate determining step and thus for a multistep mechanism. An example would be the maximum seen in the pH rate profile for the formation of an imine from a weakly basic amine (such as hydroxylamine). On the alkaline side of the maximum, the rate determining step is the acid-catalyzed dehydration of the preformed carbinolamine on the acid side of the maximum, the rate determining step is the uncatalyzed addition of the amine to form the carbinolamine. The rate decreases on the acid side of the maximum because more and more of the amine is protonated and unable to react. 

A multistep mechanism may occur in the following form First, an electron of low angular momentum (s or p) is captured into a rovibrational level of a Rydberg state of H3 ... 

Chain termination during the oxidation of hydrocarbons usually is a result of the interaction of two peroxyl radicals by a multistep mechanism. The mechanism of dispropotionation is... 

Cycloaddition of pyridine Ar-oxides (see Section 11.13.2) led to careful examination of thermodynamic aspects, though no experimental measurement was provided. Thermodynamic profile for the ring-chain isomerization of [l,2,3]triazolo[l,5- ]pyridines via a ring-opening pathway (Equation 2) was calculated. Based on this computational study, a multistep mechanism was proposed <20050BC3905>. 

V,/V-dimethylaniline, especially when those strong donors are paired with the relatively electron-poor MES derivative of the bis(arene)iron(ll) acceptor. As such, the dark reactions arise via essentially the same multistep mechanism as that for charge-transfer de-ligation, the difference arising from an adiabatic electron transfer (10) as the initial step that is thermally allowed when the driving force -AGET is sufficient to surmount... 

Multistep mechanisms are dealt with in the same way, as an example ... 

The palladium catalyzed reactions of substituted vinylallenes with unactivated 1,3-butadienes proceeded with high selectivity133. A multistep mechanism, involving several palladacycles, was proposed to explain the high selectivities observed. 

Furthermore, on the basis of the multistep mechanism reported in equation 20 and considering that the electrophilic and/or nucleophilic step may be rate determining depending on halogen and/or olefin, fc0bsd is always a composite constant (equation 22), even under the simplified conditions where only one path contributes to the product formation and... 

Sulfenyl chlorides and most of the other sulfenyl derivatives react with alkenes to give generally anti addition products with a high stereoselectivity. Although the mechanism of these reactions is still under study, it is usually accepted that sulfenyl transfer from the carrier to nucleophilic double bonds is consistent with the multistep mechanism reported in equation 907b. 

As we have noted in Sec. 2.3.2, there are few authentic examples of changing ATT values with temperature that are understandable on the basis of (and therefore evidence for) a proposed mechanism. A rds in a multistep mechanism which changes with temperature might account for such behavior. 

C-5 substituent either a pyrimidine (98) (when = NR2) or an isothiazole (99) (when R = Ar) are finally obtained. Sodium hydride as base resulted in ring fragmentation to give the open-chain adduct (100). Multistep mechanisms are proposed for these transformations (89BCJ1086>. The behavior of methylthio and dialkylamino salts (5 R = SMe and NRj, respectively) is quite similar and both give with carbon nucleophiles (e.g. Meldrum s acid) the same alkylidene derivative (97) (X , = Meldrum s acid residue) <(85CC696,88JCS(P1)899>. 

The influence of Pb + ions on the kinetics of zinc electrodeposition on Zn electrode in acidic sulfate electrolyte was discussed [217] in terms of a reaction model involving hydrogen adsorption and evolution, a multistep mechanism for zinc deposition and the overall reaction for zinc dissolution. The strongly adsorbed Pbads inhibited all the reactions taking place on the zinc electrode. 

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

In deriving this equation for Kc, we have assumed a single-step mechanism. For a multistep mechanism, each step has a characteristic rate constant ratio, kf/kr. When equilibrium is reached, each step in the mechanism must be at equilibrium, and Kc for the overall reaction is equal to the product of the rate constant ratios for the individual steps. 

Seibig S, van Eldik R (1997) Kinetics of F e (11) EDTA oxidation by molecular oxygen revisited. New evidence for a multistep mechanism. Inorg Chem 36 4115-4120 SimandanT, Sun J, DixTA(1998) Oxidation of DNA bases,deoxyribonudeosides and homopolymers by peroxyl radicals. Biochem J 335 233-240 Spinks JWT, Woods RJ (1990) Introduction to radiation chemistry. Wiley, New York Strohmeier Gort A, Imlay JA (1998) Balance between endogeneous superoxide stress and antioxidant defenses. J Bacteriol 180 1402-1410... 

The rate-limiting step of nitrogenase is the dissociation of the reduced MoFe protein from the oxidized Fe protein and has a rate constant k = 6.4 0.8 sec-1. An implicit assumption in the multistep mechanism shown in Figure 13 is that the three elementary reactions coupling each state of the MoFe protein are unperturbed by the reduction level of the large protein. This assumption has been verified for the species E0 and ) [73],... 

A decade later, Low and co-workers24 restudied this hydrolysis and proposed a multistep mechanism. 

Finally, it should be recognized that the double well potential model is simple and qualitative, although it can explain the results obtained for several gas-phase ion/molecule reactions. However, there are also many examples of ion/molecule reactions which proceed through a multistep mechanism as will be shown further on in this review. Of course, the corresponding potential energy surface is in that case much more complicated than the one in Fig. 4. At any rate, for a clear presentation of the many reactions of organic anions studied over the last decade, the discussion below will be focused both on several types of reactions and on various classes of anions. 

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