Question

Electrocyclic reactions are reversible. The cyclic compound is favored for electrocyclic reactions that form six-membered rings, whereas the open-chain compound is favored for reactions that form four-membered rings. Explain thoroughly in terms of the enthalpy changes that are occurring.

Answer 1

The reason for this behavior is that when ring closure of 1,3,5-hexatriene happens than it results in the cyclohexa-1,3-dience which results in one lesser double bonds and one more sigma bond then the reactant which adds to more stability.

The same thing happens in the case of ring opening when cyclobutadiene gets converted into 1,3-butadiene that forms one more pi bond and one less sigma bond then the reactant

This is the reason for ring closure six membered rings are favored and for open chain compounds four membered rings are formed