Woodward–Hoffmann rules

The Woodward -Hoffmann rules developed in 1965 by Robert B. Woodward and Roald Hoffmann allow statements about the course and products of pericyclic reactions. They include electrocyclic reactions, sigmatropic rearrangements and cycloadditions. It is considered in the rules, that the reaction can proceed either thermally or photochemically. Hoffmann was honored for this work along with Fukui in 1981 with the Nobel Prize for Chemistry.

• 3.1 kon-/disrotatorische rotation

Rule

The Woodward -Hoffmann rule can be quite simple to express:

The Woodward -Hoffmann rules for concerted reactions involving π - orbitals are involved. They are not valid for reactions that proceed via reactive intermediates or via radical mechanisms.

The importance of the Woodward -Hoffmann rules is that they not only considers the π - orbitals, but takes into account their signs during the course of reaction. Basically, binding interactions, and thus a subsequent chemical bonding, only between lobes of the same sign possible.

Concerted reactions

Concerted reactions are characterized by the fact that they take place in one step. The dissolution of the old, and building of the new binding takes place in a single step, but not necessarily in sync. Thus, there are at concerted reactions no intermediates, but only transition states. Along the reaction is an intermediate of a local Energiemimimum, the transitional state, however, an energy maximum. Intermediates can be trapped and isolated as a rule, transition states do not.

Cycloadditions

Two molecules each with a π - system react to form a new cycle ( ring closure ).

Electrocyclic reactions

At the end of a linear system of π - electrons, a σ - bond is formed, the π - system is a π - bond shortened ( electrocyclic ring closure ) or vice versa:

At the end of a cyclic system of π - electrons is interrupted by three σ - bonds, the medium σ - bond is broken, the π system is extended by a π bond ( electrocyclic ring opening).

Sigmatropic rearrangement

Sigmatropic rearrangements are a special case of a pericyclic reaction. Electron system of the compound is rearranged. One with a σ - bond to the substituent bound system travels along the π - system.

Special Rules

The rules for electrocyclic reactions are as follows:

• Even number of conjugated π - bonds: thermal reaction → conrotatory
• Photochemical reaction → disrotatory

• An odd number of conjugated π - bonds: thermal disrotatory reaction →
• Photochemical reaction → conrotatory

Kon-/disrotatorische rotation

These terms describe the rotation of participating in the emerging / opening π bonding molecular orbitals. In a conrotatory rotation of the rotation of the two orbitals is identical, oppositely at a disrotatory rotation.

Examples

• Sigmatropic rearrangement Cope rearrangement, Claisen rearrangement ( = oxa -Cope rearrangement )

• Electrocyclic reactions Valence isomerization of benzene ( prismane, Dewar benzene, benzvalene ) bullvalene

• Cycloaddition thermal Diels -Alder reaction
• Dipolar cycloaddition
• Cheletropic reaction
• 2 2 cycloaddition ( antarafacial )

• 2 2 cycloaddition ( suprafacially )