The Possible Use of Hexachlorocyclopentadiene to Purify Olefins

Hexachlorocyclopentadiene is so reactive that it adds twice to naphthalene molecules; once at each of the double bonds in one of its unsubstituted rings. In fact, this use is its best-known application where it acts as a protecting group for that part of naphthalene. In fact, hexachlorocyclopentadiene reacts readily with any double bond that is not conjugated with an electron-withdrawing group. 

Geometric and positional isomers of compounds that contain isolated double bonds are often very difficult to separate. Could hexachlorocyclopentadiene be applied to this problem?     

 

There is a literature report that ordinary alkenes react at different rates with hexachlorocyclopentadiene. This is because 1-octene has an activation energy of 20 kcal and 4-methyl-1-cyclohexene has an activation energy of 24 kcal. These are examples of a Diels-Alder reaction occurring with unactivated alkenes but these olefins are either only mono or di-substituted. 

Still, there are problems when considering hexachlorobutadiene as a separation reagent.  Hexachlorocyclopentadiene is expensive when obtained as a laboratory chemical. Also, its' disposal is a problem. It is an environmental toxin. Nevertheless, any adduct derivative can probably be reversed and the reagent trapped as an adduct with a more reactive olefin, such as ethylene, which could be bubbled through a heated mixture.

Additionally,  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, substrate-limited, 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 be needed.  An excess of the hexachlorocyclopentadiene could be used and when the reaction has proceeded to the required extent as judged by in-process analysis, the excess could be removed with maleic anhydride. 

The titration of a mixture of reactive alkene, hexachlorocyclopentadiene, and adduct with bromine gives the residual amount of the olefin because neither the reagent nor the adduct reacts under ice temperature conditions.A

US3868397US3868397Afollowed by mild base extraction of this adduct as a sodium salt. This would leave the hexachlorocyclopentadiene adduct of the more reactive alkene impurity and the desired less reactive product.