Elimination Reactions in Pyrolysis

The pyrolytic elimination is a model reaction, which probably dominates many pyrolytic processes. The (3 elimination with two groups lost from adjacent atoms is common in pyrolysis. A model pyrolytic elimination takes place with no other reagent present and often requires gas phase. For this reason, the typical E2 mechanism where a proton and another group from a molecule depart simultaneously, the proton being pulled by a base, is not common in pyrolysis in gas phase. The same is true for the Ei mechanism. More common for the gas phase pyrolysis is an E, mechanism. However, for polymers where the pyrolysis takes place in condensed phase, E2 and Ei mechanisms are not excluded. There are also several other mechanisms that have been found to operate in pyrolytic eliminations. 

A first type of mechanism involves a cyclic transition state, which may be four-, five- or six-membered [1]. No discrete intermediate is known in this mechanism 

The two groups (one being the H in the above examples) leave at about the same time and bond to each other. The designation of this mechanism is E, (in Ingold terminology). There are typical characteristics for the E mechanism  

In some cases, an Ei mechanism appears to be followed and the more stable olefin is formed. Instead of Hofmann s rule, Zaitsev s rule is followed (the double bond goes mainly toward the most highly substituted carbon). Also, in some reactions the direction of elimination is determined by the need to minimize steric interactions, sometimes even when the steric hindrance appears only during the transition state. 

Cases of Ei eliminations are common in pyrolysis. Most of these reactions occur with double or triple bond formation. Several examples are given below. 

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