1.3: Resonance

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Resonance involves the delocalization of electrons throughout an extended network of π bonds (double and triple bonds). This occurs because it is lower in energy to distribute charge over multiple atoms. Thus, the electrons on the negatively charge terminal oxygen lie in an atomic p orbital that can overlap with the adjacent π system so that there is optimal orbital mixing – this is lower in energy! As a rule, any lone pair that sits adjacent to a π system and can delocalize into this π system will do so from an atomic p orbital on an atom that is sp²-hybridized.

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The Lewis structures we draw using line-angle notation are defective because they localize electrons. However, electrons like to be arranged in ways that are most stable and the resonance arrow can account for this. Be careful, though, because a resonance arrow does not mean the structures are interconverting (like equilibrium) – they are not! There is only a single structure that accounts for all resonance contributors, which happens to lie intermediate between all of them.

We call each of the Lewis structures on each side of a resonance arrow a resonance contributor, or resonance form. If a molecule is symmetrical, then the true structure is exactly in between the two forms. However, this is not always the case. Electronegativity can control which resonance form will predominate. For example, take the molecule formamide. The resonance contributor which has charge separation is a major contributor because the negative charge is on oxygen, a more electronegative atom.

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Biomedical Spotlight

Captopril™ is a blood pressure-lowering drug discovered by Bristol-Myers-Squibb in 1977. It works by preventing the enzyme ACE (angiotensin-converting enzyme) from cleaving its peptide substrate angiotensin-1 into angiotensin-2, the latter which results in increased blood pressure. A key feature of this drug is the amide bond (shown in red), which mimics the peptide bond between histidine (H) and proline (P) in the natural peptide. All peptides have resonance contributors where the lone pair on the nitrogen is resonance delocalized into the carbonyl group, leaving negative charge on the oxygen atom. Captopril has this same resonance contributor, which allows it to mimic the natural peptide angiotensin-1, inhibiting ACE from performing its function, and resulting in lower blood pressure in diseased patients.

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Adapted from Hallelujah Moments, by Eugene Cordes, 2014