Resonance (chemistry)

As resonance (also resonance or resonant structure) in chemistry refers to the phenomenon that the bonding in a molecule or polyatomic ion, can not be represented by a single structural formula, but only by several limiting formulas. None of these limiting formulas describes the bonding and thus the distribution of electrons in a sufficient manner. The actual electron distribution in the molecule or ion is located between the electron distributions given by the resonance formulas. This is symbolized by the Mesomeriepfeil (resonance arrow) ↔. The Mesomeriepfeil should not be confused with the double arrow equilibrium Arrow ⇌ featuring a chemical equilibrium. The concept of resonance was introduced in 1933 by Christopher Kelk Ingold.

The real state of a molecule, ie, the intermediate state between the limiting structures is called the mesomeric state.

The energy difference between the border structures and the actual mesomeric state that can be estimated in many instances is referred to as Mesomerieenergie or resonance energy. The more resonance structures comprises a molecule or ion, the more stable it is.

As benzene

An example of a mesomeric compound is benzene ( see figure). Other aromatics are mesomeric compounds.

According to the octet rule, those molecules are very stable, in which each atom is surrounded by eight valence electrons. For benzene, two structural formulas can be set up, where this is the case ( resonance structures ).

The fact that neither of the two resonance structures of benzene is correct can be derived from the bond lengths of the bonds between the carbon atoms. The interconnected by double bonds carbon atoms should have smaller distances than those which are connected only by a single atomic bond. However, this is not the case. The bond lengths between the carbon atoms are to uniform 139 pm.

In the benzene ring, each carbon atom has four valence electrons, of which the two connecting atom to the adjacent carbon atoms. An electron binds the associated hydrogen atom. The remaining six π - electrons give formal three π - bonds, as expressed in the structural formula with three double bonds. In the orbital model currently in force, the six π - electrons form a delocalized charge cloud ( delocalized 6- π -electron system ) both above and below the plane of the carbon ring (multi -center bond ).

This results in a 151 kJ / mol lowered energy state which the binding energy is increased by the same amount, resulting in a greater stability with respect to the hypothetical boundary formulas ( cyclohexa ) results with three isolated double bonds. This energy difference is called resonance or resonant energy and arises from the difference in the heats of hydrogenation of the hypothetical Cyclohexatriens and of benzene. The same value is obtained from the difference of the combustion energies of both compounds. However, it can also have a different reference substance for Mesomerieenergie use. In the case of the comparison of the heats of hydrogenation or combustion energies of benzene with the corresponding linear molecule ( hexatriene ) results in a somewhat lower value. This resonant energy is referred to as a " dewar Resonance Energy (DRE ) ".