Solvolysis cumyl chloride

Any Sn1-Sn2 mechanistic complication should be absent in the solvolysis of a-t-butylbenzyl tosylates [15], which have a neopentyl-type structure (Tsuji et al., 1990). Indeed, the substituent effect in the solvolysis is accurately described by (2) with an r value of 1.09 which differs from the value r = 1.0 for the a-cumyl chlorides solvolysis. Based on the linearity of the correlation of the substituent effects on the solvolyses of [14] and [15] in 80% aqueous acetone, an Sn1-Sn2 mechanistic duality is also unlikely to be the cause of the exalted r value observed in the solvolysis of [14]. 

Thus, if the value of D is larger than the value of L, as is the case in cumyl chloride solvolysis, this has been traditionally accounted for by a corresponding increase in 5. The same result can be achieved however, (in terms of defining a useful scale of constants) by simply setting p = dp = 1, and allowing the increased electronic demand of an electronically deficient active site to be accounted for by an increased value of D. This is the method which we have used. The resulting constants are set forth in Table 38. 

Rate constants for solvolysis in 90% aqueous acetone have been determined at 25 °C for 10 2-aryl-2-chloro-4,4-dimethylpentanes. Brown-Okamoto <7+ constants give p+ = -4.51, with r = 0.999, suggesting that such <7+ values based on cumyl chloride solvolysis are widely applicable for substrates solvolysing without appreciable nucleophilic solvent participation. There are further interesting details in the paper. 

Reactions that occur with the development of an electron deficiency, such as aromatic electrophilic substitutions, are best correlated by substituent constants based on a more appropriate defining reaction than the ionization of benzoic acids. Brown and Okamoto adopted the rates of solvolysis of substituted phenyldimeth-ylcarbinyl chlorides (r-cumyl chlorides) in 90% aqueous acetone at 25°C to define electrophilic substituent constants symbolized o-. Their procedure was to establish a conventional Hammett plot of log (.k/k°) against (t for 16 /wcra-substituted r-cumyl chlorides, because meta substituents cannot undergo significant direct resonance interaction with the reaction site. The resulting p value of —4.54 was then used in a modified Hammett equation. 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

An example of a reaction series in which large deviations are shown by — R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides, ArCMe2Cl54. This reaction follows an SN1 mechanism, with intermediate formation of the cation ArCMe2 +. A —R para-substituent such as OMe may stabilize the activated complex, which resembles the carbocation-chloride ion pair, through delocalization involving structure 21. Such delocalization will clearly be more pronounced than in the species involved in the ionization of p-methoxybenzoic acid, which has a reaction center of feeble + R type (22). The effective a value for p-OMe in the solvolysis of t-cumyl chloride is thus — 0.78, compared with the value of — 0.27 based on the ionization of benzoic acids. 

The pn- and p -values for electrophilic bromine additions to arylolefins are in the same range as those for other reactions via analogous benzylic carbocations. However, generally the comparisons are only qualitative because of significant differences in the experimental conditions and in the mechanisms. For example, as has already been mentioned, the reaction constant of t-cumyl chloride methanolysis is —4.82 (Okamoto et al., 1958), i.e. slightly higher than that for a-methylstyrene bromination in methanol, where the intermediate resembles that in the solvolysis of cumyl derivatives (Scheme 13). 

An example of a reaction series in which large deviations are shown by —R para-substituents is provided by the rate constants for the solvolysis of substituted t-cumyl chlorides. ArCMe2Cl82. This reaction follows an S l mechanism, with intermediate formation of the cation ArCMe2+. A —R para-substituent such as OMe may stabilize the... 

The amino group and its simple relatives, e.g. Me2N and PhNH, strongly accelerate the solvolysis of substrates such as benzhydryl chloride or tertiary cumyl chloride in solvents... 

The ct" " substituent constant scale was developed based upon the solvolysis of substituted cumyl chlorides, where cumyl cations are formed in the rate-determining ionization. For the carbocation-forming reaction of Eq. 20, there is a good correlation of log versus the sum of the constants for the substituents directly on the carbon. Such a correlation clearly does not exist for the reaction where the... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

Continuum solvation models have also been used to rationalize the Hammett p+ parameters determined [200] from SN1 solvolysis rate constants [201] of cumyl chlorides. In particular the SM5.42R/AM1 [112] model reproduces the experimental p+ within 24%. The use of continuum models for placing the empirical correlations of physical organic chemistry on a firmer basis is in its infancy. 

For the reactivity parameters Y, n, a+ (but not a) andN+ the lack of curvature is not unexpected. This is because these parameters are defined with respect to the rate of some standard reaction (solvolysis of t-butyl chloride, substitution of methyl iodide, solvolysis of cumyl chlorides, combination reaction of nucleophiles with a standard electrophile). Therefore the resultant plot is of the type log k vs. log k, while the curvature shown in a typical Br nsted plot (Figure 5) results from a plot of log k vs. log K. This curvature is due to a gradual change from a reactant-like transition state, which is insensitive to a perturbation in the reactivity parameter, to a product-like transition state in which equilibrium perturbations are largely reflected in the transition state (and hence the rate). A log k — log k plot is not expected to show this effect and hence is not expected to show curvature. 

The electrophilic substituent constants, given in Table 1, were defined by a set of apparent substituent constants, i.e. (1/p) log(A /Aro), derived from the solvolysis rates of a,a-dimethylbenzyl(a-cumyl) chlorides [2] (Scheme 1) in 90% aqueous acetone at 25°C. For the definition, the reaction constant p = -4.54, based exclusively on meta and ir-electron withdrawing (tt-EW) para substituents, was applied. 

The use of the Brown equation as a probe of reaction mechanism is essentially based on the alternative use of substituent parameters tr and a- the better correlation with one of the reference scales, i.e. in this analysis, indicates closer similarity in the mechanism or in the structure of the transition state to that of the reference reaction, solvolysis of a-cumyl chlorides [2]. While the broad applicability of the Brown treatment is widely appreciated, this (T treatment has the inevitable limitations of a single reference parameter relationship. 

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