Final answer:
Activating groups with lone pairs of electrons, like the -OH group, donate electrons to a benzene ring, increasing its nucleophilicity and resulting in a more reactive and stable resonance hybrid. Delocalization of electrons contributes to resonance stabilization, thereby lowering the molecule's potential energy.
Step-by-step explanation:
The fourth resonance structure formed by activating groups that have an unshared pair of electrons is crucial because it affects the reactivity and stability of the molecule. When an atom like oxygen in the hydroxyl group (-OH), with a lone pair of electrons, is bonded to an sp²-hybridized carbon atom on a benzene ring, it can delocalize those electrons into the ring through resonance. This electron donation makes the benzene ring more electron-rich, increasing its nucleophilic character and resulting in enhanced reactivity at the positions ortho and para to the substituent. This is because the electron density in these positions is increased due to the resonance effects, which can facilitate electrophilic aromatic substitution reactions. Activating groups such as -OH are beneficial for chemical synthesis as they make the benzene ring more reactive towards electrophiles.
Additionally, the delocalization of π- or non-bonding electrons contributes to lowered potential energy through resonance stabilization. The resultant resonance hybrid is typically more stable than any individual resonance form because the electron density is spread over a larger structure, thus stabilizing the molecule.