Resonance structures

Resonance is a way of defining delocalised structures, such as benzene, of which a single Lewis structure is not sufficient. Numerous resonance structures can be used to show the structure, but no single one is correct. What this means is that an approximate intermediate between the resonance structures is in fact the real structure, often referred to as the resonance hybrid. If we take benzene as the example, we know that the Kekulé structure, devised by Freidrich August Kekulé in 1972, is incorrect and benzene, with its delocalised ring is the correct structure. It does not have three double bonds but six bonds that are half way between single and double carbon-carbon bonds and a delocalised pi cloud above and below it, bearing in mind it is a planar molecule. The idea behind the Kekulé structures (see image below) was that it shifted between the two structures so fast that it is never one or the other in one moment. We now know that is doesn’t shift but is a midway state.

A common analogy to explain these structures is that a rhinoceros is a resonance hybrid between a unicorn and a dragon, neither is actually real but both share characteristics of a rhinoceros. A rhinoceros does not shift between a unicorn and a dragon; it is just simply a rhino.

You hopefully have expected that benzene is not the only compound that resonance structures are used for, there are others such as the nitrile group (-NO₂), sulphur dioxide (SO₂), the carbonate ion (CO₃^2-) and more.

Above shows the two resonance structures for the nitrile group

Above shows the two resonance structures for sulphur dioxide

Above shows the three resonance structures used to show the carbonate ion

Note that not one of these contain double bonds but all do contain a delocalised structure that is half way between the length of a single and double bond.

Author: Grace Ronnie.