Three-step mechanism

These common features suggest that carbocations are key intermediates m alcohol dehydra tions just as they are m the reaction of alcohols with hydrogen halides Figure 5 6 portrays a three step mechanism for the acid catalyzed dehydration of tert butyl alcohol Steps 1 and 2 describe the generation of tert butyl cation by a process similar to that which led to its for matron as an intermediate m the reaction of tert butyl alcohol with hydrogen chloride... 

Instead of the three step mechanism of Figure 6 9 the following... 

IS reversible with respect to reactants and products so each tiny increment of progress along the reaction coordinate is reversible Once we know the mechanism for the for ward phase of a particular reaction we also know what the intermediates and transition states must be for the reverse In particular the three step mechanism for the acid catalyzed hydration of 2 methylpropene m Figure 6 9 is the reverse of that for the acid catalyzed dehydration of tert butyl alcohol m Figure 5 6... 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

Chain Polymerization The growth process of a polymer postulates a three-step mechanism ... 

The reaction is an F.1 process and occurs through the three-step mechanism shown in Figure 17.6). As usual for El reactions (Section 11.10), only tertiary alcohols are readily dehydrated with acid. Secondary alcohols can be made to react, but the conditions are severe (75% H2S04,100 °C) and sensitive molecules don t survive. Primary alcohols are even less reactive than secondary ones, and very harsh conditions are necessary to cause dehydration (95% H2S04,150 °C). Thus, the reactivity order for acid-catalyzed dehydrations is... 

Reaction plot for a reaction with a three-step mechanism. The second of the three steps is rate-determining. 

Kinetics of radical chain polymerisation. Kinetics calculations on radical chain polymerisation are based on the three steps mechanism with notations as shown in Figure 19. 

Thus, according to this three-step mechanism, a bimolecular recombination reaction is second-order at relatively high concentration (cM), and third-order at low concentration. There is a transition from second- to third-order kinetics as cM decreases, resulting in a fall-off regime for kbi. 

The mass spectrum of cyclohexanone has been examined by deuterium-labeling to reveal the mechanism effective for propyl loss, [M-43], m/z 55, from the molecular ion, = 98. [38,39] The corresponding signal represents the base peak of the spectrum (Fig. 6.11). Obviously, one deuterium atom is incorporated in the fragment ion that is shifted to m/z 56 in case of the [2,2,6,6-D4]isotopomer. These findings are consistent with a three-step mechanism for propyl loss, i.e., with a double a-cleavage and an intermediate 1,5-H shift. 

A serpin is a xerine proteinase inhibitor that forms cata-lytically inactive complexes which, after cleavage of the pi pif linkage, releases the inhibitor very slowly. O Malley et al. recently demonstrated that antichymo-trypsin (1) binds to chymotrypsin (E) to form an E I complex via a three-step mechanism ... 

Rhodium oxide,200 cobalt carbonyl,201 rhodium and ruthenium carbonyls,202 and rhodium compounds203,204 were later found to be effective catalysts. A three-step mechanism with hydroformylation of the alkene to yield an aldehyde in the first step can be written [Eq. (7.24)]. Condensation to form an imine [Eq. (7.25)] (or enamine) and hydrogenation of this intermediate leads to the product amine ... 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

Asymmetric 0-0 bond homolysis of the tetroxide as a first step to product formation has been invoked (Khursan et al. 1990), and the idea of the Russell mechanism replaced by a three-step mechanism [reactions (52)—(54)]. 

A kinetic study in 50% aqueous DMSO has shown that the first step in the three-step mechanism (Scheme 24) proposed for the 5NV reaction between para-substituted (methylthio)benzylidene Meldrum s acids (61) and four aliphatic primary amines is rate determining.101 The evidence supporting this mechanism is that the reactions are second order kinetically and show no base catalysis. A value of /Wc = 0.32 for the reaction with primary amines is smaller than the /9nuc = 0.41 found for the reaction with the less reactive secondary amines, indicating that N-Ca bond formation is more... 

A kinetic study of the aminolysis of substituted (methylthio)benzylidene Meldrum s acids with aliphatic primary amines in aqueous DMSO has been reported.53 With all amines the reactions are strictly second order and proceed via a three-step mechanism. 

A study of the aminolysis of substituted (methylthio)benzylidene Meldrum s acids (81 z = MeO, Me, H, Br, CF3) with a series of aliphatic primary amines in aqueous DMSO revealed second-order overall kinetics, i.e. first order in (81) and first order in the amine. A three-step mechanism has been proposed, the first step being the rate-limiting addition of amines to form the tetrahedral intermediate which is followed by fast acid-base equilibration and then formation of (82) by a fast expulsion of the leaving group, catalysed by RNH3+ or H20.127... 

Scheme 11.23 Three-step mechanism for proteases (A = RCO—), phosphatases (A = —PO32-) and gly-cosidases (A = glycosyl group).

Similarly if the related, but slightly more complex, three-step mechanism is considered ... 

For the three-step mechanisms (Fig. 1) the weights for direct spanning trees are expressed as... 

For numerical studies [223] of system (20) corresponding to the three-step mechanism (8) its parameters were taken to be kx = k2 = l, k3 = 10, k = 0.01, and k 2 — 0.1. Values of PAz and PB were varied over a wide range. A sequence of phase portraits for reaction (8) with one or three steady... 

Let us examine one more simple three-step mechanism whose steady-state characteristics are also of the hysteresis type. In what follows we will show that their type differs considerably from the previous one. It is the mechanism including steps of "consecutive adsorption one gas-phase substance is adsorbed on unoccupied sites and is then joined by a second gaseous substance, whereupon the two intermediates interact. In the general form this... 

The stability of steady states is analyzed [139] like the investigation performed for the three-step mechanism. In stable steady state, the inequality dg(0o)/d0o > df(0o)/ddo is fulfilled. In the unstable steady state, the sign of this inequality reverses. It can easily been shown that the unique steady state is always stable. If there are three steady states, the outer are stable and the middle is unstable. It can be suggested that the addition to the three-step adsorption mechanism of the impact step that is linear with respect to the intermediate does not produce any essential changes in the phase pattern of the system. The only difference is that at k. x = k 2 = 0 the dynamic model corresponding to the two-route mechanism can have only one boundary steady state (60 = 0, 9C0 = 1). 

While these free energy calculations do not directly address the intercalation dynamics, we could nonetheless discuss [2] this mechanistic picture in the context of the two most extensive experimental kinetic studies [6,7] for dauno-mycin intercalation (which as noted in the Introduction are not in complete accord with each other). We do not enter into the details of this comparison here, but refer the reader to our original article [2]. Suffice it to say here that reasonable accord with some of the kinetics results of Chaires et al. [6] was obtained, and we assigned the first two steps of the three step mechanism of these authors in terms of the two step mechanism we have described above. 

The three-step mechanism of serine enzyme action is usually described as in Equation (1) . Even though the enzyme is recovered in the end, the deacylation step remains slow enough to lead to inhibition of the bacterial cell wall biosynthesis, resulting ultimately in cell lysis by activation of endogenous autolytic mechanisms . 

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