Water solvolysis rate constant

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. ... 

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... 

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... 

FIGURE 5.22. Plot of logio against for solvolysis of arylmethyl chlorides in formic acid/water/dioxane. Rate constants are from Refs. 1 and 2. 

Sulfates having alkyl groups from methyl to pentyl have been examined. With methyl as an example, the hydrolysis rate of dimethyl sulfate iacreases with the concentration of the sulfate. Typical rates ia neutral water are first order and are 1.66 x lO " at 25°C and 6.14 x lO " at 35°C (46,47). Rates with alkaH or acid depend on conditions (42,48). Rates for the monomethyl sulfate [512-42-5] are much slower, and are nearly second order ia base. Values of the rate constant ia dilute solution are 6.5 X 10 L/(mol-s) at 100°C and 4.64 X 10 L/(mol-s) at 138°C (44). At 138°C, first-order solvolysis is ca 2% of the total. Hydrolysis of the monoester is markedly promoted by increasing acid strength and it is first order. The rate at 80°C is 3.65 x lO " ... 

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... 

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. 

Rate equation (1) indicates that ku should be inversely proportional to the activity of water for solvolysis by the AAil mechanism and independent of it if the bimo-lecular processes (pathways (i) and (ii)) pertain. Fig. 12 illustrates that acid independent rate constants at different volume fractions of D20 in CD3CN, /cH, were linearly dependent upon the inverse of ud2o in CD3CN as determined from the corresponding activities of H20 in CH3CN.142 This is in accord with the AA]1 mechanism (pathway (iii), Scheme 6). 

Interest within the physical organic community on the mechanism for the formation and reaction of ion-pair and ion-dipole intermediates of solvolysis peaked sometime in the 1970s and has declined in recent years. The concepts developed during the heyday of this work have stood the test of time, but these reactions have not been fuUy characterized, even for relatively simple systems. Richard and coworkers have prepared a short chapter that summarizes their recent determinations of absolute rate constants for the reactions of these weak association complexes in water. This work provides a quantitative basis for the formerly largely qualitative discussions of competing carbocation-nucleophile addition and rearrangement reactions of ion and dipole pairs. 

Fig. 2 Effect of added NaBr on the pseudo-first-order-rate constants, A obsd. for solvolysis of 1-Br in water at 25 °C and 7 = 6.00 (NaC104). The inset shows the linear replot of the data according to equation (3 A) of the text. [Reprinted with permission of the American Chemical Society from Ref. 32].

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. 

The rate constant for solvolysis of the model tertiary substrate 5-Cl is independent of the concentration of added azide ion, and the reaction gives only a low yield of the azide ion adduct (e.g., 16% in the presence of 0.50 M NaNa in 50 50 (v/v) water/trifluoroethanol]." Therefore, this is a borderline reaction for which it is not possible to determine the kinetic order with respect to azide ion, because of uncertainties about specific salt effects on the reaction." ... 

A further possibility arises where the carbocation intermediate of the solvolysis is so unstable as to react with water at the limiting rate of solvent relaxation with a rate constant of 1011 s 1. It is then likely that the reaction with water occurs at the stage of a carbocation anion pair and that the back... 

The reaction of azide ions with carbocations is the basis of the azide clock method for estimating carbocation lifetimes in hydroxylic solvents (lifetime = 1 lkiy where lq, is the first-order rate constant for attack of water on the carbocation) this is analogous to the radical clock technique discussed in Chapter 10. In the present case, a rate-product correlation is assumed for the very rapid competing product-forming steps of SN1 reactions (Scheme 2.24). Because the slow step of an SN1 reaction is formation of a carbocation, typical kinetic data do not provide information about this step. Furthermore, the rate constant for the reaction of azide ion with a carbocation (kaz) is assumed to be diffusion controlled (ca. 5 x 109 M 1 s 1). The rate constant for attack by water can then be obtained from the mole ratio of azide product/solvolysis product, and the molar concentrations of azide (Equation 2.18, equivalent to Equation 2.14) [48]. The reliability of the estimated lifetimes was later... 

The MEMED technique has been used to study the hydrolysis of aliphatic acid chlorides in a water/l,2-dichloroethane (DCE) solvent system [3]. It was shown unambiguously that the reaction proceeds via an interfacial process and shows saturation kinetics as the concentration of acid chloride in the DCE increases the data were well fitted to a model based on a pre-equilibrium involving Langmuir adsorption at the interface. First-order rate constants for interfacial solvolysis of CH3(CH2) COCl were 300 150(n = 2), 200 100(n = 3) and 120 60 s-1( = 8). 

Table 1. Rate constants (104/cobsd/s l) for solvolysis of 22 in 60 40 ethanol-water...

Previous studies of the reactions of guaiacol (orthomethoxy-phenol) (3 ), dibenzyl ether (4 ), and benzyl phenyl amine (5) in dense water elucidated parallel hydrolysis and pyrolysis pathways, the selectivity to the latter increasing with water density. Reactant decomposition kinetics were interestingly nonlinear in water density and consistent with two mechanistic interpretations. The first involved "cage" effects, as described for reactions in liquid solutions (6 ). The second led to parallel pyrolysis and solvolysis reaction pathways wherein associated rate constants were dependent upon pressure. These two schemes are probed herein through the reactions of benzyl phenyl amine (BPA) in water and methanol. 

Quantitative indications of the relative magnitudes of solvation of the anions during solvolytic reactions can be obtained using eqn (5). By comparing the rate constants for solvolysis in acetic or formic acids with the rate constant for the ethanol/water mixture having the same Y value, a parameter [ Ew/kRC02H]y can be calculated. As... 

The rate constant for the hydrolysis of t-butyl chloride at 298 K decreases as x2 increases in DMSO + water mixtures (Heinonen and Tommila, 1965). A clear-cut contrast between TA and TNAN mixtures is shown by the volumes of activation and related parameters for the solvolysis of benzyl chloride in acetone + water (TA) and DMSO + water mixtures (Fig. 57). Thus, in the latter system, the curves show no marked extrema but there is a shallow minimum in AV near x2 = 0 4. Extrema in Sm AH and T. 5m AS for the hydrolysis of benzyl chloride are also smoothed out when the co-solvent is changed from acetone to DMSO (Tommila, 1966). A similar trend is observed in the kinetic parameters for the hydrolysis of chloromethyl and methyl trifluoroacetates (Cleve, 1972a). For example, in the case of the chloro derivative, 6mACp decreases gradually over the range 0 < x2 < 0-2 for DMSO + water mixtures, whereas a minimum is observed in this range for acetone + water mixtures. 

Ionic complexes in water are generally are weak. For example, the association constant for formation of complexes between stable monocations and monanions ions in water are typically 0.1 M-1.26 Values of Xas (M-1) for association of nucleophilic anions and neutral electrophilic substrates for solvolysis have been estimated from the limiting rate constant ratios determined for stepwise nucleophilic... 

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