14.3 - Delocalisation of Electrons

14.3 - Delocalisation of Electrons 14.3.1 - Describe the delocalisation of π electrons and explain how this can account for the structures of some species Not all molecules can be accurately represented by a single Lewis structure diagram. Some compounds are resonance structures, and need to be represented as a number of possible structures to fit the experimentally determined bond lengths and atomic arrangement. Resonance is the distribution of electrons within a π bond. The electrons that move about in resonance structures are located in the π bonds. Although the molecules discussed later can be represented using two or three structures, in reality they exist in a state that is the average of these structures, with the delocalised electrons moving freely within the π bonds. For example, in the case of ozone, the two bonds have been shown to be the same length in experiments. However, our knowledge of the oxygen atom and its electron arrangement tells us that this is not possible, unless the oxygen was forming more or less bonds than we know to be possible. Therefore, resonance structures are used to give:

The experimental data gives the average bond lengths of these two arrangements. Here, the bonds are constantly interchanged. The average arrangement is called the resonance hybrid. NO2This molecule contains a non-bonding pair of electrons and has two possible structures

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CO32Note the 2- charge on the ion. The average bond length for all three bonds is the same.

Benzene - C6H6 This aromatic ring is made up of six carbon atoms, which have been found to have C-C bonds of the same length. Although the bonds actually alternate between single and double bonds, resonance theory allows this to be explained. Each carbon atoms of the benzene contains three sp2 orbitals and one p orbital. The p orbital of each atom contains one electron, and these form the π bonds right around the ring. The electrons are delocalised within this area. Benzene has two possible structures:

However, the actual structure of benzene is as follows:

This arrangement makes benzene resistant to addition reactions because the energy of each bond is much higher.

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NO3The nitrate ion can form three structures with the nitrogen atom double bonded to one of the oxygen atoms. In each case, the same arrangement of atoms remains, but the electrons are distributed differently. Once they are averaged, the length of all three bonds is the same.

RCOOThese are the ions of the carboxylic acids, which have a 1- charge.

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