Unimolecular reaction rates compounds

The system of coupled differential equations that result from a compound reaction mechanism consists of several different (reversible) elementary steps. The kinetics are described by a system of coupled differential equations rather than a single rate law. This system can sometimes be decoupled by assuming that the concentrations of the intennediate species are small and quasi-stationary. The Lindemann mechanism of thermal unimolecular reactions [18,19] affords an instructive example for the application of such approximations. This mechanism is based on the idea that a molecule A has to pick up sufficient energy... 

To obtain a molecular perspective of reaction rates, consider the unimolecular reaction shown in Figure 15-3. At elevated temperature, the compound ds-2-butene can rearrange to form its isomer, trans-2-butene. The reaction occurs after collisions transfer enough energy to a ds-2-butene molecule to break the C — C jrbond. Once the bond breaks, rotation around the C — C a bond takes place rapidly until a jrbond forms again. 

All these reactions are endothermic and, in addition, occur with a loss of entropy. Back unimolecular reactions are exothermic and occur with an increase in entropy. So, the role of such reactions should be negligible due to high activation energy and very fast back reaction. The values of the rate constants for addition reactions CH302 + carbonyl compound, calculated by the IPM method are presented in the following table ... 

One of the most important characteristics of micelles is their ability to enclose all kinds of substances. Capture of these compounds in micelles is generally driven by hydrophobic, electrostatic and hydrogen-bonding interactions. The dynamics of solubilization into micelles are similar to those observed for entrance and exit of individual surfactant molecules, but the micelle-bound substrate will experience a reaction environment different from bulk water, leading to kinetic medium effects308. Hence, micelles are able to catalyse or inhibit reactions. The catalytic effect on unimolecular reactions can be attributed exclusively to the local medium effect. For more complicated bimolecular or higher-order reactions, the rate of the reaction is affected by an additional parameter the local concentrations of the reacting species in or at the micelle. 

As mentioned in Section 4, the analysis of rate data resulting from unimolecular reactions is considerably easier than the analysis of such data for bimolecular reactions, and the same is true for pseudounimolecular reactions. Kinetic probes currently used to study the micellar pseudophase showing first-order reaction kinetics are almost exclusively compounds undergoing hydrolysis reactions showing in fact pseudofirst-order kinetics. In these cases, water is the second reactant and it is therefore anticipated that these kinetic probes report at least the reduced water concentration (or better water activity in the micellar pseudophase. As for solvatochromic probes, the sensitivity to different aspects of the micellar pseudophase can be different for different hydrolytic probes and as a result, different probes may report different characteristics. Hence, as for solvatochromic probes, the use of a series of hydrolytic probes may provide additional insight. 

Because of the relatively slow rates of unimolecular reactions of excited acetone in solution at room temperature, acetone makes a convenient solvent-sensitizer for photosensitizatioh studies, provided that the substrate does not undergo competing chemical reactions with triplet acetone. A recent study of the effects of high-energy radiation on dilute acetone solutions of polynuclear aromatic molecules revealed that the triplet states of these compounds were being formed at close to the diffusion-controlled rate by collision with some pre-... 

Solvent effects on the kinetics and mechanism of unimolecular heterolysis of commercial organohalogen compounds have been investigated.9-11 The reaction rate is satisfactorily correlated by parameters for polarity, electrophilicity, and cohesion of the solvent, whereas the solvent nucleophilicity and polarizability exert no effect. 

One of the most common reasons for lowyields is an incomplete reaction. Rates of organic reactions can vary enormously, some are complete in a few seconds whereas rates of others are measured on a geological timescale. Consequently, to ensure that the problem of low yields is not simply due to low reactivity, reaction conditions should be such that some or all of the starting material does actually react. If none of the desired product is obtained, but similar reactions of related compounds are successful, the mechanistic implications should be considered. This situation has been referred to as Limitation of Reaction, and several examples have been given [32 ] the Hofmann rearrangement, for example, does not proceed for secondary amides (RCONHR ) because the intermediate anion 28 cannot form (Scheme 2.11). Sometimes, a substrate for a mechanistic investigation may be chosen deliberately to exclude particular reaction pathways for example, unimolecular substitution reactions of 1-adamantyl derivatives have been studied in detail in the knowledge that rear-side nucleophilic attack and elimination are not possible and hence not complications (see Section 2.7.1). 

The development of a theory of unimolecular reactions proceeded rapidly in the mid-1920s, initiated by Hinshelwood with an A whose collision-free lifetime for reaction was approximated by an energy-independent one. The analysis was much elaborated by Rice and Ramsperger [60] and Kassel [61], known later as the RRK theory, where now the lifetime was, as it is in modern times, energy-dependent [62]. These theoretical works of the 1920s stimulated many measurements of the unimolecular rates of dissociation of organic compounds as a function of the gas pressure. Within a few years, however, this entire field collapsed or, more precisely, evolved into a new field It was shown experimentally that the unimolecular reactions , assumed originally to consist of only one chemical step, in-... 

The m-(trifluoromethyl)phenyl groups are lost in pairs, stepwise, in a unimolecular reaction for which the ratio of the rate constants, x/ 2> is 28 at 73.7°C. Cairncross and Sheppard believe that this behavior may be typical of other arylcopper compounds which are not highly substituted by fluorine (37). 

Literature concerning the unimolecular reactions of oxygen containing compounds is very extensive. To cover all the kinetic studies in this field would be virtually impossible in a review of this kind. By necessity we have limited our coverage, in the main, to reactions for which Arrhenius or transition state parameters have been reported. Some relative rate and kinetic isotope studies judged to be reliable, and to contribute significantly to the elucidation of the kinetics, have also been included. Photochemical and irradiation induced reactions do not generally produce unimolecular reactions which can be studied quantitatively therefore, the vast majority of the reactions reviewed here are those induced thermally. 

The Lindemann mechanism for thermally activated unimolecular reactions is a simple example of a particular class of compound reaction mechanisms They are mechanisms whose constituent reactions individually follow first-order rate laws [JT, 20, 36,48,40, 5f, 52, 53, 54, 55 and 56] ... 

A. Chemical vs. Isotopic Competitive Methods Two tj pes oi competitive methods can and have been used. They are the chemical competitive and the isotopic fractionation tedmiques. In the chemical cmnpetitive method, the isotopic compounds A or A compete with a chemically different species, B, for reaction with C. Tire method is, therefore, not applicable to unimolecular reactions and requires samples of A and A of appreciable isotopic enrichment. Furthermore, the species B must react with C at a rate of similar order of magnitude as A or A do. Consider for simpUcity reactions first order in each of the reactants... 

The answer is D. According to the passage, the reactions in Experiment 3 are based on unimolecular substitution mechanism. Based on this information, we can say that compound B has the fastest reaction rate. The second fastest is Compound C. These observations rule out Choices A, B, and C. Compound A is a primary alcohol and is not likely to undergo S l reaction. 

Pincock [Pi 64] characterized solvents on the basis of the solvent dependence of the ionic decomposition of t-butyl peroxyformate. However, not only the rate, but also the mechanism of decomposition of this compound is solvent-dependent. For example, in chlorobenzene it decomposes in a slow unimolecular reaction in which the peroxide bond is split in n-butyl ether the decomposition proceeds via radical attack on the peroxide oxygen atoms and in the presence of pyridine a bimolecular elimination reaction occurs, with the formation of t-butanol and carbon dioxide. Pincock used the solvent dependence of the rate of this latter reaction to characterize the solvent. 

It is known that the thermal decay of these compounds at not very high concentrations (<10 mol/1) is a unimolecular reaction with activation energy of -120 kj/mol and pre-exponential factor of lO -lO s [52]. The kinetics of the 320 nm band decrease in the temperature range of 323 K to 346 K are also described by a first-order equation with the rate constant kj = 10 exp(-114 4// r)s- [50]. 

The specificity of solvation effects during heterolyses of cyclopentyl and cyclohexyl substrates has been investigated. The same group has reviewed data on reaction rate depression of the unimolecular heterolyses of organic compounds in the presence of neutral salts, with an ion in common with that generated by the substrate or a... 

A semi-polar cyclic five-membered transition state has been proposed to account for the unimolecular pyrolysis kinetics of primary, secondary, and tertiary 2-phenoxycarboxylic acids in the gas phase reaction rates increase in the order 2-phenoxyacetic, 2-phenoxypropionic, 2-phenoxybutyric, and 2-phenoxyisobutyric acid. Phenol formation in the rate-determining step is followed by formation and subsequent decompositon of an intermediate lactone to give carbon monoxide and the corresponding carbonyl compound however, in the case of 2-phenoxyisobutyric acid, a parallel elimination reaction gives phenol and methacrylic acid." ... 

The principle underlying the use of organomercury compounds for carbene generation (entry 6, Scheme 9.1) is again the a-elimination mechanism. The carbon-mercury bond is much more covalent than the C-Li bond, however, so that the mercury reagents are generally stable at room temperature and easily isolated. They then decompose to the carbene when heated in solution with an appropriate alkene." The decomposition appears to be a reversible unimolecular reaction, and the rate is not greatly influenced by the alkene. This observation implies that a... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

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