Unimolecular and Solvolytic Reactions

The rate constants for unimolecular and solvolytic reactions generally show a monotonic decrease (i.e., micellar inhibition) - or a monotonic increase (i.e., micellar catalysis) - or insensitivity (i.e., micellar-independent rate) - to an increase in micellar concentration. There seems to be no exception to this generalization and, if there is one, it is owing to some specific chemical or physical reasons. For example, the nnimolecular decarboxylation of 6-nitrobenzisox-azole-3-carboxylate ion (1) in CTABr micelles is enhanced by the salts of hydrophilic anions and slowed by the salts of hydrophobic anions, whereas salts such as sodium tosylate increased reaction rate when in low concentration, and retarded it when in high concentration. The first theoretical model, known as the 

Pseudo-first-order rate constants, k bs, for hydrolysis of ionized phenyl salicylate in the presence of different concentrations of CTABr were used to calculate k Kg and Kg (considered to be unknown parameters) from Equation 3.2 and such calculated values of k Kg and Kg are 0.61 + 0.24 Af- sec and (6.3 1.1) X 10 M respectively. These values of k Kg and Kg yielded k as 9.6 X 10 sec , which is exactly the same as the one calculated from Equation 3.2 considering k and Kg as unknown parameters. The quality of the data fit of a set of observed data to Equation 3.2 remained unchanged with the change in the choice of unknown parameters from k, and Kg to k Kg and Kg. This analysis thus rules out the inherent perception of a possible compensatory effect between the calculated values of k and Kg when they, rather than k, Kg and Kg, are considered unknown parameters in using Equation 3.2 for data analysis. 

Although the values of and Ks should be calculated from nonlinear Equation 3.2, very often the linearized form of Equation 3.2 (i.e., Equation 4.1) is used, probably because it does not require an approximation method, such as nonlinear least-squares method, and computer for data analysis and, furthermore. Equation 4.1 gives the exact solution. 

But the main problem in using Equation 4.1 is the fact that the statistical reliability of k - k ts of obs - W decreases as [DJ 0 and l/(k - ko,J or l/(kobs - kw) °o as [D ] — 0. Thus, the use of Equation 4.1 for calculation of k and Ks suffers from the disadvantages of placing a very high emphasis on the values of ko s as [DJ - 0 and being very sensitive to even small errors when 

Another practically usable linearized form of Equation 3.2 is Equation 4.2, which has been used to calculate and CMC with known values of k and k (the value of k, can be easily obtained from the plateau region of the k(,s vs. [Surf] plot). ° The disadvantage of using Equation 4.2 is similar to that described for using Equation 4.1. 

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The unimolecular reaction of the ion aggregate follows a similar course and the intermediate faces the same three possibilities for reaction. The rate of bond fission will not necessarily be the same as that of the free ion because the solvation environment has changed. We see this effect in the ion pair-catalyzed solvolytic reactions (7). In addition, since the reagent Y is in position before the five-coordinate intermediate is formed, the path by which X re-enters the coordination shell becomes less probable as a result of more effective competition by Y, and the rate is increased. 

Probably both reactant stabilization and the already evaluated relative instability of the cationic transition state contribute to the slowness of the solvolysis of vinyl components, but other factors are certainly involved. The most obvious experimental problem is whether the compounds compared react by a unimolecular mechanism or nucleophilic attack by the solvent is involved to a certain extent. In the case of vinylic systems, for instance, nucleophilic solvation from the rear is in general much more hindered than in the case of saturated compounds and the transition state is likely to be stabilized only by electrophilic solvation of the leaving group (Rappoport and Atidia, 1970). The low m values observed in the case of vinyl halides or sulphon-ates may be taken as a strong indication of poor solvation of the transition state in solvolytic reactions of vinyl derivatives. These and other complications, such as differences in hyperconjugation, differences in electronegativity of the -—C= and —- bonds (Jones and Maness,... 

Most of the work concerned with micellar catalysis of nucleophilic substitution refers to reactions of the Aac2 and SN2 types and will not be reviewed here. To date only a few systems have been examined in which a micellar medium affects the partitioning of solvolytic reactions between unimolecular and bimolecular mechanisms. The effects of cationic (hexadecyltrimethylammonium bromide = CTAB) and anionic (sodium lauryl sulfate = NaLS) micelles on competitive SN1 and SN2 reactions of a-phenylallyl butanoate 193) have been investigated189. The rate of formation of the phenylallyl cation 194) is retarded by both surfactants probably as a consequence of the decreased polarity of the micellar pseudo phase. The bimolec-... 

There are a number of reaction mechanisms by which nitrates can undergo hydrolytic decomposition, as summarized in simplified form in Fig. 9.1 [23], Pathway a is the unimolecular solvolytic route, which forms nitrate and a carbonium ion (Pathway a, Reaction a). The latter can add a nucleophile,... 

Gas-phase intracomplex substitution in (R)-(- -)-l-arylethanol/CHs OH2 adducts. It is well established that bimolecular Sn2 reactions generally involve predominant inversion of configuration of the reaction center. Unimolecular SnI displacements instead proceed through the intermediacy of free carbocations and, therefore, usually lead to racemates. However, many alleged SnI solvolyses do not give fully racemized products. The enantiomer in excess often, but not always, corresponds to inversion. Furthermore, the stereochemical distribution of products may be highly sensitive to the solvolytic conditions.These observations have led to the concept of competing ° or mixed SNl-SN2 mechanisms. More recently, the existence itself of SnI reactions has been put into question. ... 

Unimolecular substitutions are discussed in detail in the following sources (a) C. A. Bunton, Nucleophilic Substitution at a Saturated Carbon Atom, Elsevier, Amsterdam, 1963 (b) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969 (c) A. Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962 (d) E. R. Thornton, Solvolysis Mechanisms, Ronald Press, New York, 1964. 

It is known that for SnI ewaction (substitution-nucleophilic-unimolecular) [140-141] a key intermediate is a carbocation, therefore the more reactive substrate will be the one that can produce the most stable carbocation. The reactivity of methyl, ethyl, 2-propyl, and 2-methyl-2-propyl tosylates under SnI reaction conditions is inversely proportional to the calculated hydride affinity of the corresponding carbocations. The calculated values were in agreement with the experimental findings which were obtained through solvolysis rate measurement of these tosylates under SnI conditions [142, 143]. Correlation of the cation stability-hydride and affinity-solvolytic rate of the reaction under Sn 1 reaction conditions was observed for the allyl cation (allyl, 3-penten-2-yl, and 2-methyl-3-butene-2-yl cations)[144] and the benzyl cation (benzyl, 1-phenylethyl, and 2-phenyl-2-propyl cations) [145] series. The most reactive substrates were the ones that formed the carbocations with the lowest hydride affinity. 

SOLVOLYTIC RATE COEFFICIENTS AND PRODUCT DISTRIBUTION IN UNIMOLECULAR REACTIONS OF SOME ALKYL BROMIDES " AND CHLORIDES "" AT... 

Analyzes how micelles affect reaction rates and rate constants in unimolecular, solvolytic, and bimolecular organic reactions... 

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