Supercritical water, rearrangements

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

HCl-HOAc-AcjO, POCls, BiCla, neat with FeCls, and polyphosphoric acid. The reaction has been done in supercritical water and in ionic liquids. A polymer-bound Beckman rearrangement has been reported. Sim-ply heating the oxime of benzophenone neat leads to A-phenyl benzamide. The oximes of cyclic ketones give ring enlargement and form the lactam,as in the formation of caprolactam (83) from the oxime of cyclohexanone. Heating an... 

Yamaguchi, Y., Yasutake, N., Nagaoka, M. Ab initio study of noncatalytic Beckmann rearrangement and hydrolysis of cyclohexanone-oxime in subcritical and supercritical water using the polarizable continuum model. THEOCHEM 200Z, 639,137-150. 

As a novel approach, non-catalytic Beckmann rearrangement was recently reported to occur near the critical temperature (647 K) in subcritical or supercritical water [66]. 

The pinacol/pinacolone rearrangement, which is a typical rearrangement catalyzed by acids, occurs in sub- and supercritical water in a certain temperature range without any addition of acids (Scheme 14.4). The reaction was carried out in the temperature range of 20 to 450 °C [49]. Below 300 °C no reaction was observed. Under most reaction conditions investigated, pinacolone was the only reaction product until conversion was complete. Between 375 and 380 °C and 22.5-25 MPa, 1,2,4-trimethyl-4-isopropencydohexene instead of pinacolone is the main product. This product is formed by the elimination of two molecules of water from pinacol and the subsequent reaction of the elimination product via a Diels-Alder reaction. 

Boreo, M., Ikeshoji, T., Liew, C. C., Terakura, K., and Parrinello, M. 2004. Hydrogen bond driven chemical reactions Beckmarm rearrangement of cyclohexanone oxime into e-caprolactam in supercritical water. /. Am. Chem. Soc. 126 6280-6286. 

The physical and chemical properties of supercritical water were studied by molecular dynamics. It was confirmed that the Beckmann rearrangement reaction of cyclohexanone oxime to e-caprolactam in the supercritical water occurs via a hydronium... 

Early examples of application of this reactive CPMD approach were the chemical rearrangement of azulene into naphthalene [62], where new mechanisms and alternative possible pathways were evidenced for the first time, the synthetic organic reactions in supercritical water for the production of nylon synthetic fibers [63], and phase transitions in various materials from zeolites to graphene [64]. 

The critical temperature of water is 374°C. This is too hot for most organic compounds. However, exploratory work has been done with supercritical and near critical water.210 At 300°C the polarity and density of water approach those of acetone at room temperature. Cyclohex-anol (8.20) can be dehydrated to cyclohexene in 85% yield at 278°C in 18 h. Pinacol (8.21) can be rearranged quantitatively at 275°C in 60 min. Quantitative ring opening of 2,5-dimethylfuran (8.22) occurs at 250°C for 30 min. Acetals and esters (8.23) can be hydrolyzed under such conditions. 

In many organic reactions such as hydrolysis or certain rearrangements, water is the solvent and catalyst via self-dissociation, and sometimes also a reactant [11, 12]. The advantage of the use of water is that the addition of acids and bases may be avoided. This means that cleaning the effluent is easier and less expensive. The ionic product of water increases with pressure (under supercritical conditions) therefore reaction rates e.g. of acid- or base-catalyzed reactions also increase. On the other hand, the reaction of free radicals, which are undesirable during pyrolysis, decreases with pressure (see Introduction), thus high selectivities can be achieved. 

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