Patent GB-652618-A (GB)
Title: Improvements in or relating to the manufacture of 3.4-dichloro-ª‡-methyl-styrene
IPC: C07C17/00
Filed: 1947-10-21
Abstract: 3,4 - Dichloro - a  - methyl - styrene (for preparation see Group IV (b)), is said to copolymerize with relatively large proportions of conjugated dienes.ALSO:3,4-Dichloro-a -methyl-styrene is made by alkylating o-dichlorobenzene with an isopropylating agent selected from propylene, isopropyl alcohol and isopropyl chloride to form 3,4 - dichloroisopropylbenzene and dehydrogenating the latter by contacting with a dehydrogenating-catalyst at an elevated temperature.  Alkylation is effected by conventional methods, e.g. by the use of a Friedel-Crafts' catalyst and the dehydrogenation is preferably effected in the presence of a diluent preferably super-heated steam at 400-800 DEG  C.  The 3,4-dichloroisopropenylbenzene should be free from impurities particularly 2- or 6-chloroisopropenylbenzenes which act as diluents in the polymerization reactions and should generally not exceed 15 or 20 per cent.  The product is preferably made by reacting equimolar quantities of o-dichlorobenzene and propylene with 0.1-0.2 of a mol. of anhydrous aluminium chloride at 0-40 DEG  C.  The reaction is facilitated if propylene is bubbled through isopropyl chloride before passing to the reactor.  The reaction mixture is washed with strong alkali, e.g. 50 per cent sodium hydroxide solution and distilled, high boiling fractions being recycled for use as isopropylation agents. 3,4-Dichloro-a -methyl-styrene so obtained is dehydrogenated as above, preferably at 575 DEG  \sB 75 DEG  C., catalysts specified being of conventional type, e.g. those containing a base of zinc oxide optionally with aluminium and chromium oxides and potassium hydroxide.  In examples: (1) o-dichlorobenzene is reacted with propylene, previously bubbled through isopropyl chloride at room temperature, in the presence of anhydrous aluminium chloride at 20-25 DEG  C. and 3,4-dichlorocumene isolated as indicated above when it is mixed with super-heated steam at 560 DEG  C., passed through a tube of catalyst pellets prepared from zinc, aluminium and chromium oxides and potassium hydroxide, and the product fractionated to yield 3,4-diisopropenylbenzene and undehydrogenated compounds which are recycled, in (2) the dichloro-cumene prepared as in (1) is mixed with steam superheated to 600 DEG  C. and in (3) with superheated steam in a ration of 2.1 : p  1, the reaction being effected at 625 DEG  C. and in both cases completed as in (1).  The Specification as open to inspection under Sect. 91 refers to the preparation of 3,4-dichloroisopropenylbenzene by forming the methyl ester of 3,4-dichlorobenzoic acid which is converted by a Grignard synthesis into 3,4-dichlorophenydimethylcarbinol, the latter being dehydrated to yield the required compound and in an example (4) 3,4-dichlorobenzoic acid is refluxed with sulphonyl chloride, 3,4-dichlorobenzyl chloride isolated by distillation and refluxed with methyl alcohol to yield methyl 3,4-dichlorobenzoate which is converted into 3,4-dichlorophenyldimethylcarbinol by treatment with methyl magnesium chloride, the carbinol dehydrating on distillation to give the desired compound.  It is also stated that other alkenylbenzenes may be obtained by use of the appropriate alkene as alkylating agent, normal alkenes of 3-5 carbon atoms being preferred. This subject-matter does not appear in the Specification as accepted.