Solvolysis rate constant

On the basis of either interpretation, k should equal the solvolysis rate constant and if MS is an isolatable intermediate, it should be shown to react rapidly with Y. Both consequences have been realized in certain systems. Although there has been some skepticism shown towards a dissociative mode of substitution k, k in (4.98)), there is growing evidence for its existence in cases where the importance of the associatively activated pathways can be reduced. For the first step of the reaction... 

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. ... 

In a series of papers beginning in 1968, Gassman and co-workers examined the solvolysis of N-terf-butyl-N-chloroanilines (14, Scheme 8) in MeOH and In EtOH buffered with AcOH/NaOAc, the solvolysis rate constants for 14a-f correlated well with The for the reaction was —6.4,... 

For lack of a better system, the ratio of rate in an ethanol-water mixture of the same Y value as acetic acid to rate in the much less nucleophilic acetic acid, ( Etoii/ acoh) yj has served as a measure of sensitivity to solvent nucleophilicity. More recently, the problem has received renewed attention, and two groups have proposed possible approaches.114 Of the two proposals, that of Bentley, Schadt, and Schleyer is easier to apply. Their scheme defines the solvent nucleophilicity, N, by Equation 5.21, where k is the solvolysis rate constant of methyl tosylate in... 

The method should work better for adamantyl chlorides or norbomyl chloride for which Sn2, and, presumably, preassociation processes are precluded. Speculative values of pATr1 and pATR for the relevant carbocations 54-56 are listed below based on a solvolysis rate constant in water for the... 

The relative solvolysis rate constants of 1-bromoadamantane, 74, and 76 explain this behavior 266> ... 

Fig. 4. Negative log of the relative solvolysis rate constants of 3-substituted adamantyl bromides in 80 % ethanol at 75 C plotted against a. (Reproduced from Ref. 2").

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. 

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. 

Figure 19. Correlation of solvolysis rate constants for neophyl chloride (46, X - Cl) at 50° and Y. O, EtOH/HjO , MeOH/HjO O, dioxane/H20 , AoOH/H20 9, Ac0H/HCO2H. (From Fainberg and Winstein, 1957 and reproduced by permission of the American Chemical Society.)...

The effects of substituents on the symmetrically disubstituted diarylethyl tosylates, [27(X = Y)j, can be described accurately in terms of the Y-T relationship with p = —4.44 and r = 0.53. The Y-T plot against the Y-T ascale with an appropriate r of 0.53 gives an excellent linear correlation for the whole set of substituents, indicating a uniform mechanism for all of them. When Y X, the overall solvolysis rate constant ki corresponds to the sum of the rate constants, k + kj, and hence k, cannot be employed directly in the Y-T analysis. The acetolysis of monosubstituted diphenylethyl tosylates gave a non-linear Y-T correlation, which is ascribed to a competitive X-substituted aryl-assisted pathway k and the phenyl-assisted k pathway. By application of an iterative non-linear least-squares method to (9), where the terms k and ks are now replaced by k and k, respectively, the substituent effect on kt can be dissected into a correlation with = —3.53, = 0.60, and an... 

Grob and coworkers (Grob and Cseh, 1964 Grob and Pfaendler, 1971) reported that the solvolysis rate constants for 1-arylethenyl bromides [28]-Br in 80% aqueous ethanol at 120°C correlate with Brown s o- substituent constants for a limited set of substituents (NH2, OMe, MeCONH and H) with p = -5.1. Stang et al. (1977) reported substituent effects on the solvolysis of the trifluoromethanesulphonates [28]-OTf in 80% (v/v) aqueous ethanol at 75°C. Kobayashi et al. (unpublished) studied the solvolysis of several derivatives of [28]-OTs in 80% (v/v) and 30% (v/v) aqueous ethanol, and... 

The solvolysis rate constants of l-aryl-2-methylpropenyl sulphonates, extrapolated to the tosylate solvolysis in 80% aqueous ethanol at 25°C, give an excellent Y-T correlation with r = 1.06, as shown in Fig. 11. The solvolysis of ( )-l-aryl-3,3-dimethylbutenyl bromides also gives an excellent correlation with r = 1.02. 

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

Comparison of the solvolysis rate constants of 2-chloro-2-methylpropane obtained in water and in benzene solution reveals a rate acceleration of ca. 10 with increasing solvent polarity [47] f The solvolysis rate of 1-bromoadamantane in... 

Recently an indication of path A has been shown by investigating solvolysis of the 7-substituted norbornadiene complexes of Cr, Mo, and W 90). It was found that in an 80% acetone-water solution at 25°C,(t7-C7H7C1)Mo (CO)4 gives free 7-norbornadienol instead of the expected (t7-C7H70H)Mo (CO)4- Moreover, the solvolysis rate constant was of the same order of magnitude as that for free 7-chloronorbornadiene. All these data support fast dissociation of the type... 

Bulk solvent properties such as dielectric constant (e) may sometimes correlate with solvolysis rate constants, but microscopic dielectric constants in the vicinity of ions can be very different from bulk values. For water within 1.5 A of an ion, e has been estimated to fall to 5, rising to —80 for a water 4 A from the ion (252). One notable effect of the lowering of the dielectric constant of a solvent, however, is to encourage ion-pair formation (222, 285). Correlations that employ a... 

Naively, one would expect that solvolysis rates constants, and ig, in solvents Si and S2 would be related as in (16). This would not be the case however, if one of the solvents were specifically involved in the transition state, i.e. if the solvolysis were not a pure S l process, but depended on solvent nucleophilicity (Winstein et ah, 1965). 

In the solvolysis of the benzhydiyl ester the titrimetrically determined solvolysis rate constant, k, is given by kikzKk i + k ). If the rate constant for total ionization of the ester, ki, can be found, R may again be determined. By supposing that complete equilibration of the carboxyl oxygens of the anion takes place within the ion pair prior to return to re-form the ester, ki may be determined as the sum of the solvolysis rate constant and the rate constant k for equilibration of the oxygen... 

The extent to which a solvent stabilises ions could be measured by comparing the equilibrium constant for a standard dissociation with the value for that in a standard solvent. The Grunwald-Winstein equation (Equation 48) uses as its standard the solvolysis of /-butyl chloride (Equation 49) and assumes that the transition state has almost complete carbenium ion character and that the formation of the ion-pair is rate determining. The rate constant would therefore measure the energy of formation of the free carbenium ion. Equation (48) defines the solvent parameter, K, where is the solvolysis rate constant of /-butyl chloride in the solvent (S) and the standard solvent (ss) is 80% EtOH/H2O. Other solvolyses can be used as standards and those of 1-adamantyl species has largely supplanted the original one for reasons given later. 

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