Solvolysis, unimolecular

It is evident that the results of Grob on the a-bromostyrene system, 145, are most consistent with path D i.e., a unimolecular ionization and formation of an intermediate vinyl cation. Further evidence is provided by the very large effect of substituents upon the solvolysis rate, with the p-amino compound, for example, reacting some 10 times faster than the parent bromostyrene. The log... 

Essentially similar results and conclusions were obtained by Peterson and Indelicato (158) in the solvolysis of the corresponding tosylates and brosylates, 171 b (R = p-CHj Cfi H4 or p-BrCg H4 ) and 171 c (R = p-CHj 5 H4, p-BrCg H4 ), in 50% aqueous methanol at 130°. In this case, the trans isomer was found to react at a rate 10 times that of the cis isomer. Furthermore, the trans isomer gave 95% 2-butyne and 5% 2-butanone, whereas the cis isomer gave 72% 2-butyne and 28% 2-butanone as products. Also, as expected (vide supra) for a unimolecular solvolysis reaction, the cis brosylate reacts at a rate four times that of the corresponding tosylate. 

Solvolyses of these cyclic vinyl triflates at 100 in 50% aqueous ethanol, buffered with triethylamine, lead exclusively to the corresponding cyclo-alkanones. Treatment of 176 with buffered CH3COOD gave a mixture of cyclohexanone (85%) and 1-cyclohexenyl acetate (15%). Mass spectral analysis of this cyclohexanone product showed that the amount of deuterium incorporation was identical to that amount observed when cyclohexanone was treated with CH3COOD under the same conditions. This result rules out an addition-elimination mechanism, at least in the case of 174, and since concerted elimination is highly unlikely in small ring systems, it suggests a unimolecular ionization and formation of a vinyl cation intermediate in the solvolysis of cyclic triflates (170). The observed solvent m values, 174 m =. 64 175 m =. 66 and 16 m =. 16, are in accord with a unimolecular solvolysis. 

A unimolecular ionization was shown to be the mechanism of solvolysis by means of rate studies, solvent effects, salt effects, and structural effects (179,180). The products of reaction consist of benzo [bjthiophen derivatives 209 or nucleophilic substitution products 210, depending upon the solvent system employed. By means of a series of elegant studies, Modena and co-workers have shown that the intermediate ion 208 can have either the open vinyl cation structure 208a or the cyclic thiirenium ion 208b, depending... 

Vinyl cations have been postulated as intermediates in the aqueous acetone solvolysis of 257 (214). Unimolecular kinetics were observed, and the sole products of solvolysis were the corresponding amides, 258. A rho value of about p = —1.2 was observed for the effect of substituents X and p = —.7 for substituents Y in 257. The relatively small value of rho for the aryl substituents... 

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. 

Pseudo unimolecular rate constants k for sulfuric acid-catalysed solvolysis of 25c in CD3CN/D20 (adjusted to a constant ratio of 3.8 1) were found to be linearly dependent upon the acid concentration (Fig. 11) and the gradient afforded a composite rate constant of (2.41+0.10) x 10 2lmol 1 s-1 at 308K. From the intercept, ka, the rate constant for uncatalysed solvolysis, was at least three orders smaller and zero within experimental error. A similar linear dependence and near-zero uncatalysed rate constant was demonstrated for other /V-acetoxy-TV-alkoxybenzamides given in Table 3. 

Perhaps the first suggestion of INCs came from Rylander and Meyerson. [172,173] The concept that the decomposition of oxonium and immonium ions involve INCs (Chap. 6.11.2) was successfully put forth by Bowen and Williams, [143,157,158,167,174] and the analogies to solvolysis were described by Morton. [168] Nonetheless, mass spectrometrist were too much used to strictly unimolecular reactions to assimilate such a concept without stringent proof. 

Unimolecular solvolysis of this conjugate base in steps (4.45) and (4.46) produces an aqua amide complex that rapidly converts to the final product (4.47) ... 

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. 

Converging lines of evidence have led to a general acceptance of the monomeric metaphosphate mechanism for the hydrolysis of phosphate monoester monoanions. The pH rate profile for aryl and alkyl phosphate monoester hydrolyses commonly exhibits a rate maximum near pH 4. where the concentration of the monoanion is at a maximum. The proposed mechanism is based on these principal points of evidence (a) a general observation of P-O bond cleavage (b) the entropies of activation for a series of monoester monoanions are all close to zero, which is consistent with a unimolecular rather than a bi-molecular solvolysis where entropies of activation are usually more negative by 20 eu7 (c) the molar product composition (methyl phosphate inorganic phosphate) arising from the solvolysis of the monoester monoanion in a mixed methanol-water solvent usually approximates the molar ratio of methanol ... 

The detailed mechanism, or mechanisms, of the solvolysis of acid chlorides is still a matter of dispute. There are at least four possible mechanisms, (a)-(d) below, all of which have been proposed either to act separately or in various combinations, and there is a unified mechanism, that of Minato93 which will be discussed later. The bimolecular mechanisms (a) and (b) differ in that (a) includes a tetrahedral intermediate whereas (b) does not. The former is commonly accepted as the most likely for the bimolecular mechanism and the arguments against (b) have been stated in the introduction. There is, however, good evidence for (A), at least in the case of the hydrolysis of chloracetyl chloride94. The acylium ion mechanism (c) and the hydrated carbonium ion mechanism (d) are both unimolecular mechanisms. Whereas the acylium ion XXVII has never been directly observed in hydrolysis or alcoholysis reactions, it is favoured as an intermediate by many workers, although it is kinetically indistinguishable from XXVIII. 

It will be seen in the next section that benzoyl chloride in 50% aqueous acetone reacts by both unimolecular and bimolecular mechanisms. The solvolysis of benzoyl fluoride, in addition to being acid-catalysed is also base-catalysed and, towards hydroxide ion, benzoyl fluoride actually reacts faster than ben-... 

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. 

A concerted unimolecular mechanism has been proposed to account for the formation of elimination products on solvolysis of the tertiary l-(4-methoxyphenyl)-3-methy 1-3-butyl derivatives (38-X) in aqueous solvents.19 Thus, in 50 50 (v/v) CF3CH20H-H20, (38a) and (38b) give 39% and 56%, respectively, of the alkene... 

Most important mode of reactions of hypervalent A3-iodanes is their reductive transformation to univalent iodide. This process is very facile and energetically favorable, and often proceeds without the assistance of the added reagent. The rate of this unimolecular process was measured by solvolysis of alkenyl(aryl)-A3-iodanes. 

A through-bond interaction between the nitrogen lone pair and the developing carbocation orbital is believed to be responsible for the hugely different reactivities of the bicyclic chlorides 2 and 3 towards unimolecular solvolysis.4... 

In a series of experiments utilizing geometrically constrained five- and six-membered rings, Lambert et al. examined the dependence of the unimolecular solvolysis rates of (3-trimethylsilyl-substituted esters upon the Si-C-C-OR dihedral angle. The substrates 31,52 32,l2 40 33,53 34,52 and 4,12- 40 which had... 

The unimolecular solvolysis of a-(pentamethyldisilanyl)benzyl halides provides evidence for stabilization of positive charge by hyperconjugation with a... 

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