Solvolysis dissociative

Winstein suggested that two intermediates preceding the dissociated caibocation were required to reconcile data on kinetics, salt effects, and stereochemistry of solvolysis reactions. The process of ionization initially generates a caibocation and counterion in proximity to each other. This species is called an intimate ion pair (or contact ion pair). This species can proceed to a solvent-separated ion pair, in which one or more solvent molecules have inserted between the caibocation and the leaving group but in which the ions have not diffused apart. The free caibocation is formed by diffusion away from the anion, which is called dissociation. 

In addition to simple dissolution, ionic dissociation and solvolysis, two further classes of reaction are of pre-eminent importance in aqueous solution chemistry, namely acid-base reactions (p. 48) and oxidation-reduction reactions. In water, the oxygen atom is in its lowest oxidation state (—2). Standard reduction potentials (p. 435) of oxygen in acid and alkaline solution are listed in Table 14.10- and shown diagramatically in the scheme opposite. It is important to remember that if or OH appear in the electrode half-reaction, then the electrode potential will change markedly with the pH. Thus for the first reaction in Table 14.10 O2 -I-4H+ -I- 4e 2H2O, although E° = 1.229 V,... 

The k term is independent of Y and would, therefore, appear to be dissociative, but it is in fact found to be solvent-dependent and so it is thought to be associative. (It is also found to be sensitive to steric effects in the same manner as the k2 pathway.) A plausible pathway for the k route is slow solvolysis followed by fast substitution... 

In amphiprotic solvents protolysis of acids and bases takes place. This reaction is also termed solvolysis, classically dissociation. The degree of solvolysis depends on the nature of both the dissolved acid or base and on the solvent. 

Either there is a route to skeletal isomerization which can occur without ligand dissociation or else solvolysis of isomer (XX), Scheme IV, can give 30% of the 1-D2 isomer. 

The use of both 8(CH3) values and iftM- H) magnitudes does, however, allow determination of the coordination mode in many instances (e.g., 462). Problems due to ligand dissociation are frequently evident in H-NMR studies of sulfoxide complexes. Thus the dimethyl sulfoxide adduct of niobium oxychloride (409) undergoes solvolysis in acetoni-... 

Hemochromes are the symmetrical bis(ligand) adducts of the Fe11 porphyrins, e.g. the bis(imidazole) moiety [11] presumably occurring in reduced cytochrome a or bs (1, 47). The bis(pyridine) hemochromes Fe(P)Py2 are all rather labile in solution the same is true for the corresponding hemichrome derivatives, [Fe(P)Py2] , which are the cationic oxidation products of the hemochromes. Reliable spectral and other physicochemical data on these species can only be obtained in solution when an excess of L is present to suppress the dissociation equilibrium (5) (M = Fe) (20, 24), which is the origin of subsequent oxidation (n = 0) or solvolysis reactions (n = 0 or 1). 

As with solvolysis reactions of octahedral complexes, the rate-determining step may be solvolytic or dissociative in any case, it is independent of the concentration of Y ... 

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... 

Solvolysis studies of meta- and para-substituted phenyl phosphates (240) in anhydrous Bu OH and in Am OH have revealed that generally reactions of dianions are much faster in alcohols than in water. For example, the dianion of p-nitrophenyl phosphate (240 X = 4-NO2) reacts 7500- and 8750-fold faster in Bu OH and Am OH, respectively, than in water." The results of a theoretical study of the reactivity of phosphate monoester anions in aqueous solution do not support the generally accepted view that Brpnsted coefficients fhg = —1.23 and jSnuc = 0.13 determined more than 30 years ago for the uncatalysed reaction of water and a monophosphate dianion (241) represent conclusive evidence for the dissociative mechanism. It is suggested that, instead, the observed LFERs could correspond to a late transition state in the associative mechanism." ... 

The specific rates of solvolysis of benzyl p-toluenesulfonate and nine benzylic-ring-substituted derivatives (324) have been satisfactorily correlated using Aij and Tots scales within the extended Grunwald-Winstein equation. The reactions of Z-phenylethyl X-benzenesulfonates (325) with Y-pyridines (326) in acetonitrile at 60 °C have been studied at high pressures. The results indicated that the mechanism of the reaction moves from a dissociative 5)vr2 to an early-type concerted 5)vr2 with increasing pressure. 

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... 

A solvent kinetic isotope effect (SKIE) of 0.44 from solvolysis in CD3CN-H2O versus CD3CN-D2O mixtures was in line with predicted values for the protonation-dissociation mechanism for which the SKIE should be between 0.48 and 0.33 (Figure 18a... 

The relative reaction rates and the stability of the aquo complex make it possible to identify the aquo complex as an intermediate and study the individual acts separately. However, if the solvento complex were less stable and the anation rate much faster than the solvolysis, it would not be possible to observe this intermediate, and the process would be kinetically indistinguishable from a unimolecular dissociative process. Both processes would exhibit overall first-order kinetics and the usual mass-law retardation and other competitive phenomena characteristic of an extremely reactive intermediate. 

Detailed kinetic studies indicate that about 80% of the time isomerization goes via the solvento intermediate. But this is not a rate-determining solvolysis, rather a temporary diversion of the intermediate of a dissociative reaction. Watts 33) has recently prepared the dimethylformamide complex, [Co en2 DMF Cl]+2 and has shown that it cannot be an intermediate in the isomerization of cis- and trans-[Co en2 Cl2]+ in dimethylformamide. 

The concept of preassembly as a requirement for substitution may throw light upon the vexed question of the mechanism of the base hydrolysis reaction. It has long been known that complexes of the type, [Co en2 A X]+n can react rapidly with hydroxide in aqueous solution. The kinetic form is cleanly second-order even at high hydroxide concentrations, provided that the ionic strength is held constant. Hydroxide is unique in this respect for these complexes. Two mechanisms have been suggested. The first is a bimolecular process the second is a base-catalyzed dissociative solvolysis in which the base removes a proton from the nitrogen in preequilibrium to form a dissociatively labile amido species (5, 19, 30). 

Experimentally it has been found that primary and secondary amines react by solvolysis, while only the tertiary amines generally produce reduction, if reduction is observed. It thus seemed appropriate to study the reaction of niobium (V) halides with pyridine, where proton dissociation need not be considered and any reaction would necessarily lead to a simple adduct of pyridine or reduction of the metal halide. In this work, reduction of the niobium(V) halides was observed, and the reaction products were characterized. Elucidation of the pyridine oxidation products has permitted an interpretation of the reaction mechanism in terms of the two-electron reduction of niobium(V) by the pyridine molecule. 

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

The mechanism provides (in equation 1) a pathway for proton exchange in amine complexes. (In fact, it was the observation that proton exchange in [Co(NH3)6]3+313 and the base hydrolysis of (Co(NH3)5Br]2+298 had the same kinetic form that led Garrick to propose this mechanism in 1937.314) The pathway for base catalyzed solvolysis is made up of (1) + (2) + (3) and the pathway for base catalyzed substitution is (1) + (2) + (4). It was the observation of base catalyzed ligand substitution that provided the first strong evidence for the dissociative nature of the process.315... 

Although the evidence for the direct attack on the substrate by the nucleophile is overwhelming it was necessary to examine the ki pathway in depth to decide whether it represented a parallel dissociative process or associative solvolysis. Trapping,417 AF+ measurements446 and the general observation that kj and k2 respond in a similar fashion to electronic and steric variations in the other ligands in the substrate suggest that both pathways possess the same mode of activation. The possibility of dissociative activation in cfl planar complexes is discussed separately below (Section 7.1.7.6) and further consideration of this question is best reserved until then. 

The product elimination step proceeds with cleavage of the catalyst-substrate bonds. This may occur by dissociation, solvolysis, or a coupling of substrate moieties to form the product. The last of these involves covalent bond formation within the product, and corresponds to the microscopic reverse of oxidative addition. Upon reductive elimination both the coordination number and formal oxidation state of the metal complex decrease. In most homogeneous catalytic processes, the product elimination step, while essential, is usually not rate determining. The larger kinetic barriers are more frequently encountered in substrate activation and/or transformation. 

---