Halides solvolysis rates

Table 5.11. Relative Solvolysis Rates of Ethyl Sulfonates and Halides" ...

Leffek, Llewellyn and Robertson (1960a, b) made careful conductometric determinations of deuterium kinetic isotope effects on the solvolysis rates (in water) of some ethyl, isopropyl and n-propyl sulphonates and halides. In the case of the n-propyl compoimds,... 

Stabilized vinyl cations also can be formed by allylic double-bond participation in vinyl halide solvolysis. Grob and co-workers (151) have investigated the solvolysis of a number of substituted 2-bromo-l,3-butadienes, 165. Bromodienes 165 solvolyzed via first-order rates in 80% aqueous ethanol... 

The major defect of the Grunwald-Winstein treatment is that it is limited in its scope. It has been applied to reactions other than halide solvolysis, but is in general restricted to those reactions for which the major contribution to the rate-limiting step is of the form ... 

Alkyl cations are thus not directly observed in sulphuric acid systems, because they are transient intermediates present in low concentrations and react with the olefins present in equilibrium. From observations of solvolysis rates for allylic halides (Vernon, 1954), the direct observation of allylic cation equilibria, and the equilibrium constant for the t-butyl alcohol/2-methylpropene system (Taft and Riesz, 1955), the ratio of t-butyl cation to 2-methylpropene in 96% H2SO4 has been calculated to be 10 . Thus, it is evident that sulphuric acid is not a suitable system for the observation of stable alkyl cations. In other acid systems, such as BFj-CHsCOOH in ethylene dichloride, olefins, such as butene, alkylate and undergo hydride transfer producing hydrocarbons and alkylated alkenyl cations as the end products (Roberts, 1965). This behaviour is expected to be quite general in conventional strong acids. 

Extensive studies have been carried out on the concentrated salt effects on the solvolysis reaction rates of aliphatic halides and related compounds in acetone-water mixed solvents. The main outcome of the complicated results presented appears to be that Tt is proposed that one could simply distinguish 5n1 from 5n2 reactions merely by observing a substantial increase in the solvolysis rate constant at 1.0 mol dm LiC104 in aqueous mixed solvents. ... 

Despite our failure to find any supporting spectral evidence, the suspected presence of a-bromo-o-xylene and the absence of o-methyl-benzyl acetate in the oxidation products from o-xylene suggest a solvolysis rate for this benzylic halide lower than for the isomeric methylbenzyl bromides. 

Crunden and Hudson80, by comparison of solvolysis rates in formic acid and aqueous acetone (although the same authors have stated that this may be coincidental), obtained a value of 40% SN1 participation in aqueous acetone (65%) they also estimated the contribution of the SN1 route to the solvolysis of substituted benzoyl halides (Table 6). It must, however, be emphasized that the nature of the intermediate trapped in the work of Gold et al81 could either be the acylium ion RCO+ or the hydrated acylium ion [RC0H20]+. 

A very interesting paper80 reported studies of the reactions of several substituted benzhydryl carbenium ions, generated by laser flash photolysis, with halide ions in several solvents. This work provided the nucleophilicity N of chloride and bromide ions in several solvents. These data, along with the ionization rate constants and the solvolysis rate constants for the reactions of substituted benzyhdryl halides, was used to construct quantitative energy surfaces for the. S N 1 reactions of substituted benzhydryl halides in several solvents. 

Olah and coworkers, ° and Mayr and Striepe discussed the scope and limitations of aliphatic Friedel-Crafts alkylations. In particular, they considered factors that would favor reactions of the type shown in equation (121), where an alkene is alkylated by an alkyl halide. ° They reasoned that formation of the 1 1 addition products (42) can be expected, if (41) reacts faster with the alkene than (42), otherwise higher addition products will be formed. Mayr ° suggested that the relative dissociation rates of (41) and (42) induced by the Lewis acid should reflect their relative rates of addition to a common alkene. Furthermore, it was assumed that the solvolysis rates in 80% ethanol were proportional to the Lewis acid induced dissociation constants. A few examples where good yields of alkylated (addition) products were obtained are shown in equations (122) and (123). 

A mechanistic model involving nucleophilic assistance, but not taking into account the variable electrophilic assistance in different solvents, has been proposed (54, 55) for the solvolysis of tert-butyl halides. The analysis was based on a comparison of solvent effects on the solvolysis rates of tert-butyl and adamantyl substrates. The solvent properties were analyzed in terms of parameters N and Y the electrophilic assistance was incorporated into Y (54, 56). Such an approximation had been acceptable in the original wor4c (14-16), which dealt mostly with aqueous alcohols as solvents. This approximation is no longer permissible when materials like TFA and fluori-nated alcohols are used as solvents. In fact, Fainberg and Winstein (56) pointed out that different solvent mixtures could not be placed on the same correlation line. 

In a series of papers, " Grunwald and Winstein showed that the solvolysis rate constants for organic halides which exhibit values differing by more than 6 orders of magnitude in fliis parameter can generally be accurately described by the following equation ... 

The amino group also enhances solvolysis rates of alkyl halides by intramolecular participation with the formation of cyclic products. Available data on the effect on the rate of increasing distances between the amino group and the halogen show accelerated solvolysis rates for n = 2, 4, 5, and 6. ... 

Relative solvolysis rates of 1-phenylethyl esters and halides... 

Table 2 lists selected carbenium ions in order of increasing stability, along with their hydride affinity and the solvolysis rate constants of the corresponding alkyl halides. 

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