Solvolysis of alkyl chlorides

The solvolysis of alkyl chlorides and aralkyl p-nitrobenzoates in phenol, an ambident nucleophile, gave C- and O-alkylated phenols. The logarithm of the ratio of product concentrations was found to be a linear function of the logarithm of the solvolytic rate constant for the particular substrate. 

Table 17.8 Rates of solvolysis of alkyl chlorides in 50% aqueous ethanol at 44.6 °C ...

The kgjko isotope effect for the solvolysis of chloromethyl methyl ether in isopropyl alcohol at 0° is 1.24 + 0.08 per deuterium atom " . This is in the range of other unimolecular reactions. The values of and AS for the chloroether in isopropyl alcohol are 10.7 0.6 kcal.mole" and —28.8 + 1.6 eu, respectively. In contrast, the corresponding values for solvolysis of alkyl chlorides are in the range of about 20-25 kcal.mole and —4 to —12 eu. The lower values of A/f and AS for the solvolysis of chloromethyl methyl ether are tentatively attributed to the double bond character between oxygen and carbon at the transition state (see above) . 

A substantial body of evidence indicates that allylic carbocations are more stable than simple alkyl cations. For example, the rate of solvolysis of a chloride that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride. 

Additional evidence for the SnI mechanism, in particular, for the intermediacy of carbocations, is that solvolysis rates of alkyl chlorides in ethanol parallel carbocation stabilities as determined by heats of ionization mea.sured in superacid solutions (p. 219). It is important to note that some solvolysis reactions proceed by an Sn2 mechanism." ... 

Kinetics of the solvolysis of acyl chlorides and alkyl chlorides in hydroxylic solvent mixtures have been measured conductimetrically at various temperatures and pressures. The activation parameters A V, AH, and AS were calculated from the rate constants. The authors appear to have been interested mainly in acyl chlorides, but conclude that, whereas p-methylbenzoyl chloride reacts via a dissociative... 

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. 

To assess the accuracy and precision of these other factors influencing relative reactivity. Recently, solvent effects were shown to produce important variations in the relative reactivity of very similar molecules. Thus the relative influence of m-methyl and m-t-butyl substituents on the rate of solvolysis of benzhydryl chloride depends on the solvent (Shiner and and Verbanic, 1957). Less remarkable but equally important variations in reactivity were detected among the p-alkylated benzhydryl chlorides (Shiner and Verbanic, 1957 Berliner and Chen, 1958). A full analysis of solvent influences (Clement et al., 1960) requires much detailed... 

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... 

A quite different heterolytic mechanism has been put forward for the reaction of diphenylmethylene with alcohols to form diphenylmethyl alkyl ethers (Kirmse, 1963). The ability of alcohols to suppress the reaction of the photolytically generated carbene with oxygen increased with increasing acidity of the alcohol. When sodium azide was present, the ylids of diphenylmethyl azide and alkyl ether were close to those obtained by solvolysis of diphenylmethyl chloride under the same conditions. Equation (22) is a plausible formulation of the reaction. 

Traditionally, relative stabilities of carbocations have been derived from the comparison of the rates of solvolysis reactions following the SN1 mechanism, for which the designation Dm + An has recently been proposed [36], The comparison of solvolytic rate constants for substrates of a large structural variety is hampered by the fact that the published solvolysis rates refer to different solvents, different temperatures, and precursors with different leaving groups. Dau-Schmidt has, therefore, converted solvolysis rates of a manifold of alkyl chlorides and bromides to standard conditions, i.e., soiv of RC1 in 100% EtOH at 25° C (Scheme 6) [37]. Although from a theoretical point of view, ethanol is not an ideal solvent for observing unassisted SN 1-type reactions (nucleophilic solvent participation), it has been selected as the reference solvent because most available experimental data have been collected in solvents of comparable nucleophilicity, a fact which made conversions to 100% ethanol relatively unproblematic [38],... 

Scheme 6 Solvolysis rate constants of alkyl chlorides converted to standard conditions (RCI, 100% EtOH, 25° C). (Data from Ref. 37.)...

On the basis of these considerations it has been concluded that under given reaction conditions (Lewis acid/solvent) the reactivity maximum is found for an alkylating system (RA7R+) that is approximately half-ionized [60,61]. Scheme 11 suggests that the electrophilic reactivity of RA" increases with increasing stabilization of R+ if only small equilibrium concentrations of carbocations are involved. In accord with this analysis, the relative alkylating abilities of alkyl chlorides have been found to be proportional to their ethanolysis rates (Fig. 2) [62]. The only compound that deviates from this correlation is trityl chloride which alkylates considerably more slowly than expected from its solvolysis rate. 

The effect of the substituents attached to the nitrogen (R ) on the rate of solvolysis of imidoyl chlorides in acetone/water is similar, i.e., R = alkyl > R = aryl, and electron-withdrawing substituents attached... 

Figure 5.5 Correlation between stability, measured by solvolysis rate in 80 percent aqueous acetone, and selectivity, determined by relative rate of reaction with azide ion (kN) and water (kw), for carbocations derived from alkyl chlorides. Reprinted with permission from D. J. Raber, J. M. Harris, R. E. Hall, and P. v. R. Schleyer, J. Amer. Chem. Soc., 93, 4821 (1971). Copyright by the American Chemical Society.

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

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