Bunton mechanism

The point has recently been made that the kinetic data which indicate that the cyclic intermediates are involved are also compatible with mechanisms in which the true reaction is a bimolecular one and the complexes of reactants are inert. In support of this reinterpretation is the observation of an induction period in the reaction with propane-1,2-diol. Although the point is valid, there are two reasons why the Bunton mechanism of Figure 6.65 should be maintained. The first is the rapid-reaction work, which clearly showed the kinetic competence of an absorbing complex. The second is Occam s razor, which would reject the parasitic equilibria plus unspecified bimolecular process in favour of the simpler Bunton proposals. 

The following discussion of chemical reactivity and mechanism will be based on the premise that for most thermal reactions equilibrium is maintained between water and the micelles, which can be regarded as distinct reaction media, and most kinetic treatments are based on this so-called pseudophase model (Cordes and Gitler, 1973 Bunton, 1973b). Reaction... 

The mechanisms of reactions of anionic nucleophiles with carboxyl derivatives and phosphate esters are similar in that negative charge tends to be localized on oxygen in the transition states, as shown for deacylation in Scheme 5 (Bunton, 1977). The transition state here should be stabilized by... 

Micellar rate enhancements of bimolecular, non-solvolytic reactions are due largely to increased reactant concentrations at the micellar surface, and micelles should favor third- over second-order reactions. The benzidine rearrangement typically proceeds through a two-proton transition state (Shine, 1967 Banthorpe, 1979). The first step is a reversible pre-equilibrium and in the second step proton transfer may be concerted with N—N bond breaking (17) (Bunton and Rubin, 1976 Shine et al., 1982). Electron-donating substituents permit incursion of a one-proton mechanism, probably involving a pre-equilibrium step. 

Anionic micelles strongly favor the two-proton mechanism, because of the increased concentration of hydronium ions at the micellar surface (Bunton and Rubin, 1976 Bunton et al., 1978b). 

The mechanism of oxidation of vicinal glycols by HI04 has been studied by G.J. Buisst and C.A. Bunton (J. Chem. Soc., 1406, (1954). According to these workers, the reactive species is the para form of periodic acid (H5I06) which is formed by the action of water on HI04. 

Activation parameters have been measured for these reactions, and are also in agreement with the unimolecular mechanism, since the entropies of activation are all positive. For lsO- exchange from solvent water into mesitoic acid, under the conditions described above, Bunton et al.49 found a AS of... 

A re-investigation of the mechanism of the hydrolysis of acetic anhydrides in water was made by Bunton and Fendler4. They found that the hydrolysis of trimethylacetic anhydride follows an A-2 mechanism both in water and in aqueous dioxan and is slower than that of acetic anhydride. If acetic anhydride follows an A-l mechanism in water, both anhydrides should have the same mechanism and both should have similar reactivities in aqueous acids. If the A-2 mechanism is followed, steric effects should make trimethylacetic anhydride the less reactive compound. The entropy of activation of the hydrolysis catalysed by perchloric acid was re-estimated, taking into account the electrolyte effect of the perchlorate ions upon the rate of spontaneous hydrolysis, and a new value for A5 of—35 eu (for the spontaneous hydrolysis) and — 10 eu at 2 M HC104 for the acid-catalysed hydrolysis was obtained, compared with the value of + 2.2eu obtained previously. The new value is in the order of magnitude of A 5 for A-2 reactions but smaller than that observed for trimethylacetic anhydride under similar acidic conditions (— 26 eu). Plots... 

Although, so far in this section, we have discussed the kinetic evidence in terms of the A-l and A-2 mechanisms favoured by Bunton et al. (p. 224), the same points which give rise to differences in detailed interpretation of mechanism for hydrolysis of acyl halides apply with equal weight to the hydrolysis of anhydrides. Butler and Gold67,68,298,299 have studied the spontaneous hydrolysis of acetic anhydride and presented the following scheme. 

Mechanism. The earliest mechanistic interpretation of alka- ini hydrogen peroxide epoxidation waa given by Bunton and Min- who found, for the case of ethylideneaoetone and mesityl oxide, iii>t-nrder kinetics with respect to both the unsaturated ketone and the hydroperoxide anion. Accordingly, the reaction was presumed to occur l.v the path shown in Eq. (Si), step (6) being rate-determining- The... 

Does bimolecular substitution on tricoordinate sulfur involve the formation of an intermediate, or is it a one-step process The evidence is somewhat inconclusive. For example, when sulfite ester (14) is hydrolyzed with HO containing 180, 180 is found in the product but no significant amount is present in the recovered unreacted ester.70 Bunton and co-workers interpreted this to mean that the mechanism shown in Reaction 4.37 in which the intermediate 15 is formed in a rapid equilibrium prior to the transition state for the reaction, is ruled out. If 15 were so formed, they reasoned, it would rapidly equilibrate with isoenergetic 16. Then loss of HO- from 16 would result in lsO in recovered... 

Unimolecular substitutions are discussed in detail in the following sources (a) C. A. Bunton, Nucleophilic Substitution at a Saturated Carbon Atom, Elsevier, Amsterdam, 1963 (b) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969 (c) A. Streitwieser, Jr., Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962 (d) E. R. Thornton, Solvolysis Mechanisms, Ronald Press, New York, 1964. 

Recently, Bunton et al. synthesized the Ci6H33-attached triamine copper(II) complex 8b [26], which promoted the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) at alkaline pH. The catalytic activity was almost the same as that for Menger s previously reported comicellar system with 3b. The active species was proposed to be the hydroxide-bound copper(II) complex 8b. The pvalue of the copper(II)-bound water molecule was speculated to be about 8 from the fact that the nonalkylated and tetradecyl homologous copper(II) complexes have a pvalue of 8 (determined by DNP hydrolysis kinetics). Since the micellar metal complex 8b was not fully characterized either in the solid state or in a micellar solution, its hydrolysis mechanism remains to be elucidated. 

Support for mechanism a has been provided by Rhind-Tutt and by Bunton and coworkers, who have studied the closely analogous, acid-catalyzed methanolysis of phenyl tetra-O-methyl-a-D-glucopyranoside. They have shown that the ratio of a to 3 anomer in the product is similar to the ratio obtained in the methanolysis of tetra-O-methyl-a-D-gluco-... 

The most probable mechanism for the rate determining step involves a transition state, such as 3, in which the proton transfer either precedes or is concerted with covalent bond breaking. The mechanisms of orthoester hydrolysis have been summarized and discussed by DeWolfe and Jensen (1963), Wenthe and Cordes (1964), Bunton and DeWolfe (1966), Cordes (1967), DeWolfe (1969), and Jencks (1969). 

Cox and Ramsey, 1964 Bruice and Benkovic, 1966 Bunton, 1968 Ingold, 1969 Jencks, 1969) should be consulted for discussions of the mechanisms of hydrolysis in specific cases and the intricacies and criteria involved in their elucidation. 

The acid cleavage of a-phenylvinyl diethyl phosphate, which most probably proceeds by an ASj 2 mechanism, was found to be catalyzed by micellar sodium dodecyl sulfate (NaLS) and hexadecyl sulfonic acid (Table 8), whereas that of phosphoenol pyruvic acid was slightly inhibited by cationic and anionic surfactants (Bunton and Robinson, 1969c). The considerably greater rate enhancement by sodium dodecyl sulfate in 0-01m than in 0-10 m acid (Table 8) can reasonably be attributed... 

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