Jeffrey A. Manion

Gas-phase hydrogenolysis of chloroethene: rates, products, and computer modelling

Hydrogenolysis of chloroethene (vinyl chloride, VC) has been investigated in a tubular-flow reactor at atmospheric pressure between 872–1085 K. Ethene, ethyne, and HCl are major initial products of reaction. Hydrogen-atom addition to the substituted end of VC yields C2H4 after loss of a chlorine atom from the radical intermediate; addition to the unsubstituted end gives the 1-chloroethyl radical (1) and is reversible, but (1) may also be hydrogenated to chloroethane, which rapidly loses HCl. HCl elimination from VC to give C2H2 mainly occurs via a molecular reaction for which the parameters log k/s–1= 14.0 – 290kJ mol–1/θ,θ= 2.3 RT have been determined. Both C2H2 and C2H4 are hydrogenated and lead to C2H6 and, through splitting of the C–C bond in this product, to methane. Addition of HCl had only a small rate-enhancing effect on VC conversion, with increased production of ethene. A reaction model which accounts well for VC conversion rates and product, distributions has been developed. Features of VC hydrogenolysis have been compared with its pyrolysis.

Relative gas-phase desubstitution rates of chlorobenzene derivatives by hydrogen atoms near 1 000 K

Thermolysis of arene/H2 mixtures at atmospheric pressure near 1 000 K leads to desubstitution via attack of hydrogen atoms: (Ar–X + H˙→ ArH). Rates of dechlorination (and, where appropriate, dehydroxylation and defluorination) have been examined for a variety of Chlorobenzene derivatives, including o,m,p-dichlorobenzenes, 1,2,4-trichlorobenzene, o-,m-chlorofluorobenzenes, o-,m-,p-chlorophenols, and 2,4-dichlorophenol. Employing competitive studies and computer analysis of product distributions, the rates of desubstitution have been determined relative to (1), (C6H5Cl + H˙→ C6H6). Observed per-site rate constants for dechlorination of the substrates were 0.66–1.44 times that of C6H5Cl. Electron-withdrawing substituents retard the reaction rate, and electron-donating groups lead to rate enhancement. Substituent effects on dehydroxylation were found to be similar; rates of defluorination were less affected by ring substituents. The results are discussed in terms of the reaction mechanisms.

Gas-phase thermal hydrogenolysis of organic chlorine compounds: An alternative to incineration

Abstract Thermal hydrogenation of organic chlorine compounds is advanced as an alternative to incineration and as an environmentally sound process for conversion/ detoxification of industrial organic wastes. The chemical reactions proceed at ca. 600–900° C and lead to HCl (and analogous derivatives of non-hydrocarbon functional groups) and a mixture of hydrocarbons, especially CH 4 , C 2 ′s and benzene. There is little or no tar or soot formation. Toxins such as chlorinated dioxins are also effectively converted; the resulting products can be considered as a fuel. The requisite hydrogen can be supplied or made (in situ) from a source like methanol.

Substituent Effects in Gas-Phase Thermal Hydroxylation and Hydrogenolysis

The effect of substituents, especially Cl, on four basic free-radical gas-phase reactions is outlined, viz. H-abstraction from (chloro)benzenes by OH (1), ipso substitution ArCl + OH. ➙ ArOH (2), hydro-dechlori- nation ArCl + H.➙ ArH + Cl. (3) and H-abstraction from vinylchloride Cl. + CH2=CHC1 ➙ CH2=C.C1 + #CH=CHC1 (4a, b).