Elimination reactions solvolytic

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

The faster rate of dehydration of 2-ea o-bornanol over that of 2-endo-bornanol can be attributed in part to the anchimeric assistance of the C(l)-C(6) bonding electrons which are trans to the electrons bonding the OH group to C(2) carbon, as with solvolytic reactions 74), The occurrence of tricyclene as the primary product of dehydration of 2-ea o-bornanol can best be explained by an elimination reaction which takes place within the submicroscopioal pores of the aluminas. 

This enables one to use aliphatic systems as precursors to the radicals X-Y whose solvolytic (= redox) behavior can then be studied. Equations 2a, c describe what may be called oxidative solvolysis . This reaction sequence, the first step of which is in many cases induced by the OH radical, is of great importance in radical (and radiation) chemistry. It extends from /8-elimination reactions of monomeric radicals [6, 7] to the mechanism of DNA strand breakage [8]. An example for Eq. 2 in which it is shown that the radical XY can be produced by either step a or b is given in section 3.3. 

A. Thibblin, Mechanisms of solvolytic alkene-forming elimination-reactions, Chem. Soc. Rev. 1993, 22, 427. 

Full details of a study of leaving group-promoted solvolytic elimination reactions of 1-(1-methyl-l-arylethyl)pyridinium cations in 25 vol.% acetonitrile (aqueous) have been reported. Reactions of (34) and (35) are found to proceed via a common carbocation intermediate of ion-molecule pair type to give the suhstitution product (36) and elimination product (37) (Scheme 4). The total rate of reaction of (35) exceeds that for (34) by 1100-fold, corresponding to a Bronsted parameter of )S g = —0.93, and the fraction of (37) obtained is governed by = 0.12 for the dehydronation (kg) of the ion-molecule pair by the leaving group the product ratio is hardly affected hy the presence of substituted pyridines. For (34) and (35), = 1.85 0.10 (60 °C) and... 

Nucleophilic Substitution Solvolytic and Elimination Reactions Ring-opening of Epoxides Esters, Ethers, and Related Derivatives of Alcohols Oxidation Reduction Miscellaneous... 

The crucial step in this mechanism is the second one in which the a-haloether moiety 115 solvolyzes to give an oxocarbocation. In order to obtain the ionic bicyclobutane this step has to compete effectively with both protonation of the carbanion and the 1,3-elimination reaction. By using oxygen nucleophiles, e.g. MeO", EtO" and CF3CH2O", in pro tic solvents, the rate of these three reactions was found to decrease in the order solvolysis > elimination > protonation. Although an apolar medium is expected to enhance the elimination reaction and to slow down the solvolytic step, it was shown that for MeO" in THF, the ionic bicyclobutane route still prevails. ... 

The problem was again taken up in general terms at the Chemical Society Symposium on the Transition State (Maccoll, 1962). Here, the behaviour of carbonium ions in the mass spectrometer and in solvolytic reactions was examined in relation to the behaviour of the virtual carbonium ions postulated in the gas-phase elimination reaction. Such properties as the ease of formation of carbonium ions and their rearrangements were examined, and, where data were available, a linear relationship was found between the activation energy for elimination and the heterolytic bond dissociation energy. This relationship is shown in Fig. 6. 

A similar explanation has been suggested to account for the variation in the product distribution from the decomposition of the 2-phenyl-2-butyl cation generated from a variety of substrates-. In solvolytic elimination reactions of l,l,4.4-tetramethylcyclodecyl-6- or -7-tosylates jy/i-elimination appears to predominate in both acidic and basic solvents, an ion pair mechanism being suggested to account for the observed results . It is obviously unsound to generalise when considering stereochemistry of El reactions. 

One of the most commoidy used methods for forming carbon-carbon double bonds is by -elimination reactions of the types shown in Scheme 2.1, where X = e.g. OH, OCOR, halogen, OSO2R, NRa, etc. Included among these reactions are acid-catalysed dehydrations of alcohols, solvolytic and base-induced eliminations from alkyl halides or sulfonates and the Hofmann elimination from quaternary ammonium salts. They proceed by both E2 (elimination bimolecular) and E1 (elimination... 

Brown s investigation of the addition compounds of trimethyl borane, diborane, and boron trifluoride with amines has provided a quantitative estimation for steric strain effects in chemical reactions. He also investigated the role of steric effects in solvolytic, displacement, and in elimination reactions. His results demonstrate that steric effects can assist, as well as hinder, the rate of a chemical reaction. 

While there is a paucity of information on neighboring sulfur involvement in elimination reactions, the results of McCabe and Livingston suggest participation in the solvolytic elimination of hydrogen chloride from the chloroalkene (91) and in the pyrolytic elimination of adipic acid from the adipate ester (92). 

The preparation of di-w-butyl ether is illustrative (Scheme 2.6). No reaction occurred with n-butanol alone for 2 h at 200 °C. However, in the presence of 10 mol % n-butyl bromide, 26% conversion of the alcohol to the ether was obtained after 1 h, without apparent depletion of the catalyst. It is known that addition of alkaline metal salts can accelerate solvolytic processes, including the rate of ionization of RX [41]. This was confirmed when the introduction of LiBr (10 mol %) along with n-butyl bromide, afforded a conversion of 54% after 1 h at 200 °C. Ethers incorporating a secondary butyl moiety were not detected, precluding mechanisms involving elimination followed by Markovnikov addition. 

It should be pointed out that, compared to a-bromoboronic esters, p-bromoboronic esters are much less stable. For instance, dibutyl (2-bromoethyl)boronate readily undergoes P-elimination, even under solvolytic conditions [87]. Therefore, the reaction of 18 with NBS also reaffirms the regioselectivity of the hydrozirconation step. The reaction is highly general and works equally well for the preparation of a-chloro- and a-iodoboronic... 

Primary and secondary kinetic isotope effects are of general importance in the study of neighboring group participation. Isotopic substitution a to the incipient carbo-cation produces a secondary isotope effect whereas 0 and y substituents may give rise to both primary and secondary effects. For example, if the rate determining step of a solvolytic reaction involves a hydrogen shift or elimination, primary deuterium isotope effects are clearly implicated. 

In the solvolysis of cyclopentyl brosylate the increase in the isotope effect /c(H)/7c(/S d4) with increasing solvent ionizing power has been interpreted as evidence for a rate-limiting elimination in the more polar solvent207. Investigation of a much larger number of substituents is needed to understand the effect of electron-withdrawing a-substituents on solvolytic reactions. 

Solvolytic deoxymercuration catalysed by iodide has been reported and the reaction shows both a first- and second-order component in iodide - It seems unlikely that other halides will be as useful as iodide in dehalogenation as they are oxidised less easily. Rearrangement rather than gamma elimination is observed when 1,3-dihalosubstrates are treated with iodide ion . ... 

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