Unimolecular rate-determining step

Activation parameters that have been measured for Aal1 reactions are generally consistent with a unimolecular rate-determining step. The volume of activation for the hydrolysis of f-butyl acetate in 0.01 M HCI at 60°C is zero, within experimental error70. No significant change in rate is observed from atmospheric pressure up to 2 kbar, although this increase in pressure almost doubles the rate of hydrolysis of ethyl acetate in 0.1 M HCI at 35°C. 

The termolecular kinetics result from two equilibria starting with the amide and involving two hydroxide ions followed by a unimolecular rate-determining step, and the termolecular rate constant 3 is actually a product of the two equilibrium constants and a unimolecular rate constant k = kx Ki x K2. 

They found that the rate of hydrolysis depends only on the concentration of tert-butyl bromide. Adding the stronger nucleophile hydroxide ion, moreover, causes no change in the rate of substitution, nor does this rate depend on the concentration of hydroxide. Just as second-order kinetics was interpreted as indicating a bimolecular rate-determining step, first-order kinetics was interpreted as evidence for a unimolecular rate-determining step—a step that involves only the alkyl halide. 

Carbocation formation is unimolecular rate-determining step... 

Assuming that the rate determining step in the reaction of cyclohexanol with hydrogen bro mide to give cyclohexyl bromide is unimolecular write an equation for this step Use curved arrows to show the flow of electrons... 

Butanol Rate determining step is unimolecular therefore Sfjl... 

As depicted, the E2 mechanism involves a bimolecular transition state in which removal of a proton to the leaving group is concerted with departure of the leaving group. In contrast, the rate-determining step in the El mechanism is the unimolecular ionization of... 

Like the reaction of terr-butyl alcohol with hydrogen chloride step 2, in which tert-butyloxonium ion dissociates to (CH3)3C and water, is rate-deter-mining. Because the rate-determining step is unimolecular-, the overall dehydration process is refened to as a unimolecular elimination and given the symbol El. 

The first step, which is rate determining, is an ionization to a carbocation (carbonium ion in earlier terminology) intermediate, which reacts with the nucleophile in the second step. Because the transition state for the rate-determining step includes R-X but not Y , the reaction is unimolecular and is labeled S l. First-order kinetics are involved, with the rate being independent of the nucleophile identity and concentration. 

The rate law of a reaction is an experimentally determined fact. From this fact we attempt to learn the molecularity, which may be defined as the number of molecules that come together to form the activated complex. It is obvious that if we know how many (and which) molecules take part in the activated complex, we know a good deal about the mechanism. The experimentally determined rate order is not necessarily the same as the molecularity. Any reaction, no matter how many steps are involved, has only one rate law, but each step of the mechanism has its own molecularity. For reactions that take place in one step (reactions without an intermediate) the order is the same as the molecularity. A first-order, one-step reaction is always unimolecular a one-step reaction that is second order in A always involves two molecules of A if it is first order in A and in B, then a molecule of A reacts with one of B, and so on. For reactions that take place in more than one step, the order/or each step is the same as the molecularity for that step. This fact enables us to predict the rate law for any proposed mechanism, though the calculations may get lengthy at times." If any one step of a mechanism is considerably slower than all the others (this is usually the case), the rate of the overall reaction is essentially the same as that of the slow step, which is consequently called the rate-determining step. ... 

The first step (loss of the leaving group) is the rate-determining step, much Uke we saw for SnI processes. The base does not participate in this step, and therefore, the concentration of the base does not affect the rate. Because this step involves only one chemical entity, it is said to be uiumolecular. Unimolecular elimination reactions are called El reactions, where the 1 stands for unimolecular. ... 

Figure 8.17 Reaction of an alkyl halide with hydroxide ion. (a) A primary halide reacts by an SN2 mechanism, causing Walden inversion about the central, chiral carbon, (b) A tertiary halide reacts by an SN1 mechanism (the rate-determining step of which is unimolecular dissociation, minimizing the extent of Walden inversion and maximizing the extent of racemization). Secondary alcohols often react with both Sn 1 and SN2 mechanistic pathways proceeding concurrently...

So the tertiary halide reacts by a different mechanism, which we call SnI- It s still a nucleophilic substitution reaction (hence the S and the N ) but this time it is a unimolecular reaction, hence the 1 . The rate-determining step during reaction is the slow unimolecular dissociation of the alkyl halide to form a bromide ion and a carbocation that is planar around the reacting carbon. 

An Arrhenius expression, k = 1012 exp(—24,300/Hr) sec-1, was quoted for the coefficient of the rate-determining step. It is doubtful whether this step is a true unimolecular reaction. The effect of pressures has not been studied. The range of temperatures used appears to be too narrow to discuss the temperature dependence... 

As far as propagation is concerned, comparison of rates is hazardous because under some conditions the rate-determining step for isobutene [85], like propene [86], may be a unimolecular process, i.e., of zero order with respect to monomer (see sub-section 5.2). Moreover, comparison is complicated further by the consideration that in every system free cations and cations forming part of an ion-pair or higher aggregate may participate in the polymerization, and that therefore the extent of such participation must be ascertained before meaningful rate constants can be evaluated. This matter will be discussed in Section 6. 

According to Fontana s theory, the rate-determining step in a polymerization involving a monomer-complexed cation is a unimolecular isomerization of the complex, which is kinetically of first order ... 

The formation of a complex between the propagating end and one or more molecules of monomer can have two extreme consequences. If the incorporation of a monomer molecule from the solvation shell of the cation is the growth-rate determining step, the propagation becomes a unimolecular reaction and the rate of polymerisation becomes of zero order with respect to monomer concentration. Such a model was developed by... 

A distinction between "molecularity" and "kinetic order" was deliberately made, "Mechanism" of reaction was said to be a matter at the molecular level. In contrast, kinetic order is calculated from macroscopic quantities "which depend in part on mechanism and in part on circumstances other than mechanism."81 The kinetic rate of a first-order reaction is proportional to the concentration of just one reactant the rate of a second-order reaction is proportional to the product of two concentrations. In a substitution of RY by X, if the reagent X is in constant excess, the reaction is (pseudo) unimolecular with respect to its kinetic order but bimolecular with respect to mechanism, since two distinct chemical entities form new bonds or break old bonds during the rate-determining step. 

Figure 11.1 illustrates the behavior of Equation 11.6. By the assumption of rapid equilibrium the rate determining step is the unimolecular decomposition. At high substrate composition [S] KM and the rate becomes zero-order in substrate, v = Vmax = k3 [E0], the rate depends only on the initial enzyme concentration, and is at its maximum. We are dealing with saturation kinetics. The most convenient way to test mechanism is to invert Equation 11.6... 

Despite occasional apparent anomalies such as this, the rate expression gives us valuable information about the likely reaction mechanism. If the reaction is unimolecular, the rate-determining step involves just one species, whereas the rate-determining step involves two species if it is bimolecular. As indicated in Table 5.1, we can then deduce the probable reaction, and our proposed mechanism must reflect this information. The kinetic rate expressions will be considered further as we meet specific types of reaction. 

The abbreviation SnI conveys the information sub-stitution-nucleophilic-unimolecular . The reaction achieves much the same result as the Sn2 reaction, i.e. the replacement of a leaving group by a nucleophile, but is mechanistically different. It is unimolecular, since kinetic data indicate that only one species is involved in the rate-determining step ... 

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