Orbital Symmetry in Cycloaddition Reactions:
Cycloaddition reactions are extremely vital in organic syntheis work. These types of reactions are relatively simple. Some, such as the Diels-Alder reaction, do not require any reagents or catalysts which may be expensive or difficult with which to work. The Diels-Alder is sometimes called a [4+2] cycloaddition, indicating a hydrocarbon of four carbons combines with one of two carbons. The four-carbon molecule is a 1,3-butadiene derivative, the two-carbon is an ethylene derivative. 1,3-butadiene contains two elements of unsaturation, meaning there are present four p electrons. Ethylene has two p electrons. The reaction can be summarized as:
Note the lack of reaction arrows showing movement of electrons. The reaction is concerted, meaning all changes in the electron distribution occur simultaneously or in one step. Below is a MO energy diagram for this reaction.
The interaction between the second p orbitals on both 1,3-butadiene and ethylene is favorable for two reasons. First, the phase symmetry of the orbitals is correct for allowing complete bonding interaction between the two. Second, the two orbitals are relatively close to each other in terms of energy. According to perturbation theory, the greatest amount of thermodynamic stabilization is derived from orbital mixing when the two reactant orbitals are similar in energy. Below is a diagram showing the HOMO of 1,3-butadiene and the LUMO of ethylene approach each other, forming the Diels-Alder transition state.
