Resonance - Electron Delocalization

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In certain types of organic molecule electrons are able to spread out over multiple atoms in order to stabilize the overall system. This requires conjugation in which orbitals are aligned in patterns that allow for extended interaction, for example between non-bonding electrons and pi bonds or pi bonds and empty p orbital(s). The two examples below show a localized lone pair (left), where the non-bonding electrons are located only on the O atom, and a delocalized system (right) where the lone pair is shared between O and C. The presence of the pi system is essential for resonance delocalization. 

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To recognize resonance in charged molecules the system must contain an electron source and an electron sink; the former is either a non-bonded pair or a pi bond, the latter is usually an atom capable of accepting a lone pair, for example a second row element or an empty p orbital. Typical examples are shown below for allyl-type anions (red) with the electron source being a lone pair and the sink being the terminal atom of the pi bond. The double-headed arrows are used to describe the flow of electron density in each case:

allylanions.gif

For allyl cation systems we are looking at an electron deficiency and not an electron excess as with the above anionic molecules. Here (on the left) the sink atom, usually carbon, has lost a pair of electrons from its octet, however conjugation to a pi bond allows for stabilization through delocalization. Again we are looking at a 3-atom allyl-type system but the direction of electron flow is reversed compared to the anions. In the example on the right, the carbocation is directly attached to a heteroatom that has non-bonding electrons available to share. In such cases only two atoms are involved in resonance, however the principle of the electron donor and electron sink still applies:

allylcations.gif

The allyl radical is a related species that features a half-filled p orbital on carbon such that C has 7 electrons. Having a neighbouring pi bond will stabilize this species in a similar way to the allyl cation except only one electron is needed to help stabilize the radical; the second pi bond electron must therefore be accounted for in the second resonance structure by placing it on the third carbon of the system. The typical allyl radical resonance structures are shown below (left) along with an orbital representation showing how the conjugated p orbitals interact. 

allylradicals.gif

YouTube videos that discuss resonance in more detail: