How does electron delocalisation contribute to resonance?

Electron delocalisation contributes to resonance by allowing electrons to move freely across multiple atoms, stabilising the molecule.

In more detail, electron delocalisation is a phenomenon where electrons are not confined to a single bond or atom but can move across different parts of the molecule. This is particularly common in molecules with conjugated systems, where alternating single and double bonds allow for the free movement of pi electrons.

Resonance is a concept in chemistry that describes the delocalisation of electrons in molecules with conjugated systems. It is a way of representing the real structure of the molecule, which is often a hybrid of multiple possible structures, or resonance forms. These resonance forms are not actual structures of the molecule, but rather hypothetical extremes. The real structure is a more stable, intermediate form.

Electron delocalisation contributes to resonance by allowing the molecule to exist in this more stable, intermediate form. The electrons can move freely across the molecule, spreading out the charge and reducing areas of high electron density. This stabilises the molecule, as areas of high electron density can be energetically unfavourable.

For example, in the case of benzene, a molecule with a conjugated system of six carbon atoms, the electrons are delocalised across the entire ring. This is represented by a resonance structure with a circle in the middle of the ring, indicating the delocalised electrons. This delocalisation makes benzene more stable than if the electrons were localised in alternating single and double bonds.

In summary, electron delocalisation is a key factor in resonance. It allows electrons to move freely across the molecule, spreading out the charge and stabilising the molecule. This is particularly important in molecules with conjugated systems, where the alternating single and double bonds allow for this free movement of electrons.