Solvolysis reactivity mechanisms

The generally observed identity of the r value for solvolysis reactivity and gas-phase stability AAG(c+> of the corresponding carbocation leads to an important prediction concerning the solvolysis transition state. In a typical (limiting) two-step SnI mechanism with a single dominant transition state, the r values of transition states for the various nucleophile-cation reactions should be essentially controlled by the intrinsic resonance demand of the intermediate cation the substituent effect should be described by a single scale of substituent constants (a) with an r value characteristic of this cation. In a recent laser flash-photolysis study (Das, 1993) on the recombination of stable trityl and benzhydryl cations with nucleophiles and solvents, McClelland et al. (1986, 1989) have treated the substituent effects on solvent-recombination processes by (2). 

A detailed study of the solvolysis of L has suggested the following mechanism, with the reactivity of the intermediate M being comparable to that of L. Evidence for the existence of steps ki and k 2 was obtained fiom isotopic scrambling in the sulfonate M when it was separately solvolyzed and by detailed kinetic analysis. Derive a rate expression which correctly describes the non-first-order kinetics for the solvolysis of L. 

A recent investigation by Macomber (100) of the system where Rj = CH3, 126 (R2 and all other R = H), sheds additional light on the mechanism of homoallenic participation in solvolysis. In this case, the value of the integrated rate constant increased significantly throughout acetolysis, implying rearrangement to a system with comparable reactivity. The results were interpreted by means of an ion-pair mechanism shown in Scheme VII. 

It is evident from the foregoing sections that simple alkylvinyl halides do not react via an Sn 1 mechanism, if at all, even under extreme solvolytic conditions (146,149). More reactive leaving groups, such as arylsulfonates, were clearly needed to investigate the possible solvolytic behavior of simple alkylvinyl systems, but the preparation of vinyl sulfonates until recently was unknown. Peterson and Indelicato (154) were the first to report the preparation of vinyl arylsulfonates via reaction of the appropriate disulfonate with potassium t-butoxide in refluxing f-butanol. They prepared and investigated the solvolysis of 1-cyclohexenyl tosylate 169 and c/s-2-buten-2-yl tosylate 170 and the corresponding p-bromobenzenesulfonates (brosylates). Reaction... 

In this review, the chemical properties of these electrophilic metabolites (i-XIIl) are discussed in terms of their metabolic formation and reactivity with nucleophiles, solvolysis and redox characteristics, reaction mechanisms, and their role as ultimate carcinogenic metabolites. 

From the above one might be tempted to attribute ultrasonically enhanced chemical reactivity mainly to the mechanical effects of sonication. However this cannot be the whole reason for the effect of ultrasound on reactivity because there are a variety of homogeneous reactions which are also affected by ultrasonic irradiation. How, for example, can we explain the way in which power ultrasound can cause the emission of light from sonicated water (sonoluminescence), the fragmentation of liquid alkanes, the liberation of iodine from aqueous potassium iodide or the acceleration of homogeneous solvolysis reactions ... 

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." ... 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

Electrophilic replacement constants crXr have been obtained for all the positions of benzo[6]thiophene from the solvolysis of isomeric l-(benzo[ >]thienyl)ethyl chlorides in 80% ethanol-water. These constants signify replacement of the entire benzene ring by another aromatic system (74JOC2828). The positional order of reactivity was determined to be 3>2>6>5>4>7, all positions being more reactive than benzene. The same order was also derived from the kinetic data for pyrolysis of the isomeric l-(benzo[6]thienyl)ethyl acetates (78JCS(P2)1053). A modified extended selectivity treatment has been developed to correlate electrophilic substitution data in benzo[Z> ]thiophene, which assumes a dual activation mechanism (79JOC724). 

VII. Mechanism of Solvolysis of Reactive Silanes A. The Silicon-Hydrogen Bond... 

West (91), amplifying on these results, argued that since the solvolysis is bimolecular it must proceed either through a normal Sn2 bimolecular displacement or involve a rather stable pentacovalent intermediate. Both mechanisms. West believes, must involve a 5-coordinate transition state, and therefore may really be thought of as equivalent. West found that silacyclopentane was 13 times as reactive as diethylmethylsilane and 200 times as reactive as silacydohexane (which could be construed as evidence for I-strain in silacyclopentane). Since this order of reactivity is the same as that found in carbocyclic compounds, it was concluded that similar considerations of energy and entropy of reaction are encountered, a possibility that had also been advanced by Price. 

Solvolysis rates of isopropenyl chloroformate, CH2=C(Me)0C(0)Cl, were shown to be very similar to those for 4-nitrobenzoyl chloride in mechanism and reactivity.29 Stepwise mechanisms were observed in the aminolysis of aryl chloroformates by a series of substituted quinuclidines - which were more reactive than isobasic secondary alicyclic amines.30 The pH-independent hydrolysis of 4-nitrophenyl chloroformate in MeCN-H20 mixtures proceeded via a cyclic transition state containing the ester and two water molecules.15 Solvolyses of 2-furancarbonyl chloride in most... 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

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