GB805816A - Dehydrochlorination process - Google Patents

Abstract

1:2:3-Trichlorobutane is converted into hydrogen chloride and chloroprene precursors, i.e. 1:2-dichlorobutene-2, 2:3-dichlorobutene-1 and 1:3-dichlorobutene-2 (hereinafter called 1:2-DCB-2 &c.), by heating in the presence of the hydrochloride or a quaternary ammonium chloride salt derived from an amine, the amine having a pK value of 3.0 to 9.0 and a boilingpoint at atmospheric pressure of 110 DEG to 250 DEG C. (pK value being defined as log10 1/K , where K is the product of the concentration of the ammonium and hydroxyl ions divided by the total concentration of unionized material present as either the free base or hydrate). The catalyst is preferably derived from a tertiary amine, e.g. alpha- and gamma-picolines, 2:4:6-collidine, the amine hydrochlorides advantageously being used. These may be formed in situ, by addition of the free amine to the reaction mixture which combines with hydrogen chloride liberated. The reaction is preferably conducted at 150 DEG to 220 DEG C., under pressure when necessary, and an inert solvent such as diphenyl ether or chloronaphthalene may be employed. Preferably, the reaction mixture contains at least 25 per cent by weight of catalyst and the reaction is carried out continuously by withdrawing product and adding fresh 1:2:3-trichlorobutane to keep the volume of the reaction mixture approximately constant. The continuous process may be effected using a fractionating column, the dichlorobutene products being obtained as distillates. The process may be combined with the preparation of chloroprene by the chlorination of butane, butene-1 or butene-2 to 1:2:3- and 2:2:3-trichlorobutanes, the former, pure or admixed with the latter, being treated according to the present invention and the chloroprene precursors so formed being dehydrochlorinated to give chloroprene, and the 2:2:3-trichlorobutane being dehydrochlorinated directly to give chloroprene. The chloroprene precursor 1:2-DCB-2 is advantageously isomerized to 2:3-DCB-1, by heat alone or under the influence of copper or a copper chloride catalyst, before being converted into chloroprene by, for example, uncatalysed pyrolysis at 450 DEG to 650 DEG C. In examples: (1) 1:2:3-trichlorobutane (1:2:3-TCB) and quinoline (pK 8.8; b.p. 238 DEG C.) are refluxed in a fractionating column to give 2:3-DCB-1 and 1:2-DCB-2; (2) N-ethyl-piperidine (pK 3.0; b.p. 131 DEG C.) replaces the quinoline of (1), the product containing additionally some 1:3-DCB-2; (3) 1:2:3-TCB containing some 2:2:3:3-tetrachlorobutane is treated as in (1) with pyridine hydrochloride (pyridine has pK 8.6; b.p. 115 DEG C.) to give 1:2-DCB-2 and 2:3-DCB-1; (4) a mixture of 1:2:3-TCB and 2:2:3-TCB is treated as in (1) to give the required 2:3-DCB-1 and 1:2-DCB-2 and unwanted 2:3-DCB-2; (5) 1:2:3-TCB containing some 1:2:3:4-tetrachlorobutane is treated as in (1) with sym-collidine (pK 6.9; b.p. 172 DEG C.) to give 1:2-DCB-2 and 2:3-DCB-1; (6) a mixture as in (3) is treated as in (1) with cyclohexylamine (pK 3.4; b.p. 134 DEG C.) to give 1:2-DCB-2 and 2:3-DCB-1; (7) 1:2:3-TCB and tributylamine hydrochloride (tributylamine has pK 3.2; b.p. 214 DEG C.) give 1:2-DCB-2, 2:3-DCB-1, and 1:3-DCB-2 with some 3:4-DCB-1 and other dichlorobutenes, 1-chlorobutadiene and chloroprene; (8) 1:2:3-TCB and a -picoline hydrochloride (a -picoline has pK 8.1; b.p. 128 DEG C.) give the products of (7); (9) the mixture of (3) and a -picoline give 1:2-DCB-2, 2:3-DCB-1, 1:3-DCB-2 and some 1:3-DCB-1 and chloroprene; (10) the mixture of (3) with gamma-picoline (pK 7.8; b.p. 143 DEG C.) gives 1:2-DCB-2 and 2:3-DCB-1; (11) 1:2:3-TCB with 5-ethyl-2-methylpyridine (pK about 8; b.p. 174 DEG C.) gives 2:3-DCB-1, 1:2-DCB-2 and some chloroprene; and (12) 1:2:3-TCB and 1-methylpyridinium chloride give 1:2-DCB-2, 2:3-DCB-1, 1:3-DCB-2 and some 3:4-DCB-1, chloroprene and 1-chlorobutadiene. The Provisional Specification refers to the use of any amine catalyst, used as such or as an acid salt or quaternary ammonium compound; and provides an additional example wherein a mixture of 1:2:3-TCB and 2:2:3:3-tetrachlorobutane is refluxed with N-ethylpiperidine to give 1:2-DCB-2 and 2:3 - DCB - 1. Specification 805,817 is referred to.