Why resonance occurs in chemistry

Search for:. Learning Objective Describe how to draw resonance structures for compounds. Key Points Lewis dot diagrams are often employed to visualize the covalent bonding between atoms in a compound.

However, when multiple equally valid structures can be drawn, these structures are called resonance structures. Resonance structures have the same number of electrons and therefore have the same overall charge. Resonance structures differ only in the arrangement of electrons; the atoms keep the same connectivity and arrangement.

Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet. We must convert one lone pair on a terminal oxygen atom to a bonding pair of electrons—but which one?

Depending on which one we choose, we obtain either. Which is correct? In fact, neither is correct. As you will learn, if the bonds were of different types one single and one double, for example , they would have different lengths. It turns out, however, that both O—O bond distances are identical, Equivalent Lewis dot structures, such as those of ozone, are called resonance structures. The position of the atoms is the same in the various resonance structures of a compound, but the position of the electrons is different.

Double-headed arrows link the different resonance structures of a compound:. The double-headed arrow indicates that the actual electronic structure is an average of those shown, not that the molecule oscillates between the two structures. When it is possible to write more than one equivalent resonance structure for a molecule or ion, the actual structure is the average of the resonance structures.

The electrons appear to "shift" between different resonance structures and while not strictly correct as each resonance structure is just a limitation of using the Lewis structure perspective to describe these molecules. A more accurate description of the electron structure of the molecule requires considering multiple resonance structures simultaneously. Most arrows in chemistry cannot be used interchangeably and care must be given to selecting the correct arrow for the job.

At this point, the carbon atom has only 6 valence electrons, so we must take one lone pair from an oxygen and use it to form a carbon—oxygen double bond. In this case, however, there are three possible choices:. As with ozone, none of these structures describes the bonding exactly. Each predicts one carbon—oxygen double bond and two carbon—oxygen single bonds, but experimentally all C—O bond lengths are identical.

We can write resonance structures in this case, three of them for the carbonate ion:. Like ozone, the electronic structure of the carbonate ion cannot be described by a single Lewis electron structure. While each resonance structure contributes to the total electronic structure of the molecule, they may not contribute equally.

Assigning Formal charges to atoms in the molecules is one mechanism to identify the viability of a resonance structure and determine its relative magnitude among other structures. The resonance energy can be used to calculate electronegativities on the Pauling scale. The concept of resonance was introduced by Linus Pauling in The term "resonance" came from the analogy between the quantum mechanical treatment of the H 2 molecule and a classical system consisting of two coupled oscillators.

In the classical system, the coupling produces two modes, one of which is lower in frequency than either of the uncoupled vibrations; quantum-mechanically, this lower frequency is interpreted as a lower energy.

The alternative term mesomerism popular in German and French publications with the same meaning was introduced by Christopher Ingold in but did not catch on in the English literature. The current concept of mesomeric effect has taken on a related but different meaning.

The double headed arrow was introduced by the German chemist Arndt also responsible for the Arndt-Eistert synthesis who preferred the German phrase zwischenstufe or intermediate phase. Due to confusion with the physical meaning of the word resonance , as no elements actually appear to be resonating, it has been suggested that the term resonance be abandoned in favor of delocalization.

The double headed arrows would be replaced by commas. The ozone molecule is represented by two resonance structures in the top of scheme 2. In the hybrid structure on the right the circle replaces three double bonds. Often, reactive intermediates such as carbocations and free radicals have more delocalised structure than their parent reactants, giving rise to unexpected products.

The classical example is allylic rearrangement. When 1 mole of HCl adds to 1 mole of 1,3-butadiene, in addition to the ordinarily expected product 3-chlorobutene, we also find 1-chlorobutene.

Isotope labelling experiments have shown that what happens here is that the additional double bond shifts from 1,2 position to 2,3 position in some of the product. This and other evidence such as NMR in superacid solutions shows that the intermediate carbocation must have a highly delocalised structure, different from its mostly classical delocalisation exists but is small parent molecule. This cation an allylic cation can be represented using resonance, as shown above.

This observation of greater delocalisation in less stable molecules is quite general. The excited states of conjugated dienes are stabilised more by conjugation than their ground states, causing them to become organic dyes. An well-studied example of delocalisation that does not involve pi electrons hyperconjugation can be observed in the non-classical ion norbornyl cation.

The allyl carbocation is stabilized in a similar manner with two resonance structures of the same energy. The hybrid structure spreads out the negative charge to stabilize the positive charge. Why does resonance occur?

Chemistry Covalent Bonds Resonance. Truong-Son N. May 8,