The Possible Use of Hexachlorocyclopentadiene to Purify Olefins

Compounds that contain double bonds often suffer from the presence of isomers in which the double bond is in a different location. These contaminants are usually very difficult to remove. Hexachlorocyclopentadiene is a reagent that reacts readily with a double bond that is not conjugated to any electron withdrawing group.  This is precisely the situation that is most likely to be difficult to separate. Hexachlorocyclopentadiene is reactive enough that it adds twice to naphthalene molecules once at each of the two double bonds in an unsubstituted ring. This use in fact is its best known application where it acts as a protecting group for an unsubstituted ring in a naphthalene.

 There is also a literature report that ordinary alkenes react at different rates with hexachlorocyclopentadiene. 1-Octene has an activation energy of 20 kcal and 4-methyl-1-cyclohexene has an activation energy of 24 kcal. These exemplify Diels-Alder reaction occuingr with unactivated alkenes. Although hexachlorocyclopentadiene is expensive when obtained from Aldrich, it is still probably available somewhere in bulk inexpensively. The disposal of the chemical must be handled carefully since it is an environmental toxin.

The adducts of this reactive dienophile are reported to be split back into their precursors by heating with phosphorus pentoxide and distilling the non-chlorinated adduct. This is an inconvenient, inefficient and damaging procedure that cannot promote widespread use. Using the adducts to remove an unwanted contaminant by reaction however would not have this disadvantage since the reversal of the reaction with an impurity would not need to be reversed.  An excess of the hexachlorocyclopentadiene could be used andwhen the reaction has proceeded to required extent as judged by in process analysis, the excess could be removed with maleic anhydride followed by mild base extraction of this adduct as sodium salt. This would leave the hexachlorocyclopentadiene adduct of the more reactive alkene impurity and the desired less reactive product.

The titration of a mixture of reactive alkene, hexachlorocyclopentadiene and adduct with bromine gives the residual amount of the olefin because neither reagent or the adduct react under ice temperature conditions.