Solvolysis rates with alkyl halides

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. ... 

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. 

An example from Ref. [7] will be treated here with the simpler Arrhenius equation and will be used again later to compare this method to more sophisticated treatments. The data consists of measurement of the solvolysis of an alkyl halide (1-chloro, 1-methyl cycloheptane) in a solution of 80% ethanol for what is nominally an SNl replacement of the chloride ion. In a later chapter, we will examine this reaction in more detail as a model of a steric hindrance in a carbocation substitution reaction but here we are just interested in the measured reaction rates at various temperatures. 

Rate constants and products have been reported for solvolysis of benzhydryl chloride and /7-methoxybenzyl chloride in 2,2,2-trifluoroethanol (TFE)-water and-ethanol, along with additional kinetic data for solvolysis of r-butyl and other alkyl halides in 97% TFE and 97% hexafluoropropan-2-ol. The results are discussed in terms of solvent ionizing power Y and nucleophilicity N, and contributions from other solvation effects are considered. Comparisons with other 3 nI reactions show that the solvolyses of benzhydryl chloride in TFE mixtures are unexpectedly fast an additional solvation effect influences solvolysis leading to delocalized cations. 

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). 

In most SnI reactions, the solvent is the nucleophile. For example, the relative rates given in Table 10.4 are for the reactions of alkyl halides with water in water. Water serves as both the nucleophile and the solvent. Reaction with a solvent is called solvolysis. Thus, each rate in Table 10.4 is for the solvolysis of the indicated alkyl bromide in water. 

The rate of an SnI reaction depends only on the concentration of the alkyl halide. The halogen departs in the first step, forming a carbocation that is attacked by a nucleophile in the second step. Therefore, carbocation rearrangements can occur. The rate of an SnI reaction depends on the ease of carbocation formation. Tertiary alkyl halides, therefore, are more reactive than secondary alkyl halides since tertiary carbocations are more stable than secondary carbocations. Primary carbocations are so unstable that primary alkyl halides cannot undergo SnI reactions. An SnI reaction takes place with racemization. Most SnI reactions are solvolysis reactions The solvent is the nucleophile. 

The multiplicity of the reaction products limits deamination as a useful synthetic method for the preparation of olefins but it is of great interest to compare the mode of reaction of the intermediate carbonium ion with those derived from solvolysis of alkyl halides and esters or acid-catalysed dehydration of alcohols. As nitrosation is the rate-determining step, evidence for carbonium-ion intermediates has to come from the nature and stereochemistry of the reaction products rather than from kinetic studies. 

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. 

Previous investigations (Brady, 1949 Grayson, 1952 Bonner, 1952) in the Purdue laboratories were concerned with the influence of alkyl groups on the rate of ionization of phenyldimethylcarbinyl chloride. These studies indicated that first-order rate constants could be determined with high accuracy for the solvolysis reaction. Moreover, the entropies of activation were invariant in this series of halides. These considerations led to further study of other substituted phenyldimethylcarbinyl chlorides in an attempt to gain a further understanding of the influence of substituent groups on relative reactivity and as a possible model reaction for the assessment of parameters for electron-deficient reactions. 

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