Solvolysis relative rates

Solvent Effects on the Rate of Substitution by the S l Mechanism Table 8 6 lists the relative rate of solvolysis of tert butyl chloride m several media m order of increasing dielectric constant (e) Dielectric constant is a measure of the ability of a material m this case the solvent to moderate the force of attraction between oppositely charged par tides compared with that of a standard The standard dielectric is a vacuum which is assigned a value e of exactly 1 The higher the dielectric constant e the better the medium is able to support separated positively and negatively charged species 8olvents... 

Relative Rate of SnI Solvolysis of te/t-Butyl Chloride as a Function of Solvent Polarity ... 

Hydrolysis to Glycols. Ethylene chlorohydrin and propylene chlorohydrin may be hydrolyzed ia the presence of such bases as alkaU metal bicarbonates sodium hydroxide, and sodium carbonate (31—33). In water at 97°C, l-chloro-2-propanol forms acid, acetone, and propylene glycol [57-55-6] simultaneously the kinetics of production are first order ia each case, and the specific rate constants are nearly equal. The relative rates of solvolysis of... 

The solvolysis of isomeric 5-fluoro and 5-trimethyIstaimyl 2-adamantyl tosylates has been examined. The relative rates depend on the substituents and the stereochemistry of the reactants as shown. 

Correlations with o in carboxylic acid derivative reactions have been most successful for variations in the acyl portion, R in RCOX. Variation in the alkyl portion of esters, R in RCOOR, has not led to many good correlations, although use of relative rates of alkaline and acidic reactions, as in the defining relation, can generate linear correlations. The failure to achieve satisfactory correlations with cr for such substrates may be a consequence of the different steric effects of substituents in the acyl and alkyl locations. It has been shown that solvolysis rates of some acetates are related to the pA", of the leaving group, that is, of the parent alcohol. The pK of alcohols has been correlated with but this relationship... 

The real world of Sn reactions is not quite as simple as the discussion has so far suggested. The preceding treatment in terms of two clearly distinct mechanisms, SnI and Sn2, implies that all substitution reactions will follow one or the other of these mechanisms. This is an oversimplification. The strength of the dual mechanism hypothesis and its limitations are revealed by these relative rates of solvolysis of alkyl bromides in 80% ethanol methyl bromide, 2.51 ethyl bromide, 1.00 isopropyl bromide, 1.70 /er/-butyl bromide, 8600. Addition of lyate ions increases the rate for the methyl, ethyl, and isopropyl bromides, whereas the tert-butyl bromide solvolysis rate is unchanged. The reaction with lyate ions is overall second-order for methyl and ethyl, first-order for tert-butyl, and first- or second-order for the isopropyl member, depending upon the concentrations. Similar results are found in other solvents. These data show that the methyl and ethyl bromides solvolyze by the Sn2 mechanism, and tert-butyl bromide by the SnI mech-... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Phosphonoformic acid (85) decarboxylated in acid solution, and it was proposed that the uncatalysed reaction involved a simple decarboxylation of the zwitterion. The acid-catalysed reaction showed some kinetic similarity to that of mesitoic acid and an elimination of carbon dioxide as trihydroxymethylcarbonium ion was preferred. Participation of the trans vicinal phosphonyl group in the solvolysis of the halides (86) and (87) has been deduced from rate measurements. In the norbornene derivatives, the relative rates of loss of chloride from (87a) and (87b) were 5 x 10 1. 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

Much is known about the lifetimes of carbocation intermediates of solvolysis, and these data have proven critical in the design of experiments to estimate absolute rate constants for reorganization of ion pairs. Consider reorganization of an ion-pair reaction intermediate that exchanges the positions of the nucleophilic atoms of the leaving group (, Scheme 9) and that occurs in competition with diffusional separation to free ions (k-d) which is much faster than addition of solvent to the ion pair. Ion-pair separation is irreversible and will result in formation of solvolysis reaction products s ). Reorganization of the ion pair will result in formation of isomerization reaction product and the yield of this reaction product will provide a measure of the relative rate constant... 

Another historically important reaction is the reorganization of chiral ion pair intermediates of solvolysis of a chiral substrate that leads to racemization of substrate during solvolysis. This reorganization competes with other reactions of the ion pair intermediate of solvolysis of a chiral substrate, so that the relative rate constant for ion-pair racemization can be obtained by determining the relative rates of formation of products from partitioning of the ion pair reaction intermediate, including the enantiomer of substrate (Scheme 14). 

The relative rates of solvolysis of camphene hydrochloride and t-butyl chloride indicate that the free energy of activation is 5 kcal mole higher with the latter compound. This might be attributed either to non-classical stabilization of the camphenyl transition state... 

The first are relative rates for [14] - [17] obtained by Schleyer and coworkers and given below on the effects of substituents at the 6-position in augmenting solvolysis of 2-exo-norbornyl tosylates (Schleyer, 1972 Schleyer et al., 1965 Strang and Schleyer, 1968). 

Still another barrier to employing the shifts to resolve the norbomyl problem stems from recent studies of H. C. Brown and Peters (1973). These authors have measured the relative rates of solvolysis of 2-methyl-, 2-t-butyl-, 2-phenyl- and 2-cyclopropyl-2-propyl p-nitrobenzoate in 80% aqueous acetone to gather information about the electron-releasing ability of these groups. Upon comparing their results with the C shifts of the carbonium carbon of the same cation, they found no correlation. Although it might be... 

Richards and Hill have recently obtained quantitative evidence of the stabilization of a -metallocenyl carbonium ions (38, 95). They have shown that sol-volyses of methylmetallocenylcarbinyl acetates proceed via a carbonium ion mechanism, and that these acetates solvolyze with rates greater than even tri-phenylmethyl (trityl) acetate. Further, the relative rates of solvolysis and therefore the order of carbonium ion stabilities increas.e, proceeding from the iron to the osmium acetate. A portion of these data is summarized in Table II. 

Table II. Relative Rates of Solvolysis of Methylmetallocenylcarbinyl Acetates...

The relative rate data closely parallel the results obtained in the solvolysis studies. Such a result might be expected from reactions proceeding through similar transition states. The observed order of relative rates may result from better overlap as the size of the central metal atom and the polarizability of its electron shell increase. This would result in increased stabilization and therefore ease of formation of the carbonium ions, proceeding from lighter to heavier metal complexes. 

Trifan and Bacskai as well as Hill and Richards have reported the preparation and solvolysis of exo-acetate (XLIV and endo-acetate (XLV) (38, 120). They showed that the relative rate of solvolysis for XLIV was several thousand times greater than for XLV, indicating it is much more favorable for the acetate group... 

Ifebls 1 Relative Rates of Solvolysis of 1-Phenylethyl Esters and Halides... 

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