Understanding Resonance

Resonance is the method that organic chemists use to depict organic molecules having two or more Lewis structures with multiple representations. Nitrate ion (NO₃^–) has three equivalent resonance forms. These are not constitutional isomers, as the atoms and the connectivity stay the same. The difference is in the placement of electrons. These three structures are known as resonance structures. A resonance structure is one of two or more Lewis structures with multiple equivalent representations. The only difference between these three equivalent structures is the placement of the lone pairs and bonds connecting to oxygen. The lone pairs in each structure are delocalized. Delocalization occurs when bonding electrons or lone pairs are not localized on one atom but are spread over more than one atom. NO₃^– does not exist as any of these individual structures but is a hybrid of all three structures. A resonance hybrid is the weighted average of all of the multiple representations of Lewis structures for an organic molecule. When a molecule has resonance structures, that molecule will generally have lower energy and be more stable than a similar structure without resonance structures. If a molecule has resonance structures that contribute equally to the resonance hybrid, the structure will be even more stable or lower in energy. The lowering of the overall energy of a molecule due to the delocalization of electrons as seen in its resonance structures is known as resonance stabilization.

Drawing resonance structures helps in identifying the stability of molecules and their reactivity. The basic rules for drawing resonance structures are:

1. Never break sigma bonds, such as single bonds, when drawing resonance structures.
2. Only move electrons, and never move atoms when drawing resonance structures.
3. Atoms in the second period (C, N, O, F) may never exceed an octet (eight valence electrons) of electrons.

When identifying if and where to draw resonance structures in organic molecules, certain patterns become clear. In general, there are five main patterns that have been identified:

1. Molecules with a lone pair next to a carbocation (carbon with a positive charge)
2. Any polar $\pi$ bond, such as ${\rm {C{=}O}}$, ${\rm {N{=}O}}$, ${\rm S{=}O}$, and so on. Polar $\pi$ bonds are double bonds that are polarized so that the electron density is more localized on one end of the double bond.
3. Molecules with a lone pair on a carbon next to a double bond (an allylic lone pair)
4. Molecules with a carbocation on a carbon next to a double bond (an allylic carbocation)
5. Molecules with alternating double and single bonds in a cyclic ring (conjugated $\pi$ bonds)

A double-headed arrow is used to indicate resonance structures of a molecule as opposed to a chemical reaction. When drawing resonance structures, none of the three rules for drawing resonance structures can be violated.

Curved arrows are used to show the movement of electrons between resonance structures. In reality, the electrons are not "moving" because there is no single resonance structure, just a hybrid of all possible resonance structures. However, curved arrows allow visualization of how to draw one resonance structure from another resonance structure. Note that outside of resonance, curved arrows show the movement of electrons in mechanisms, which shows how reactions proceed.

Curved arrows are drawn from the source of electrons (lone pair or bond, not an atom) to the final destination of the electron, which is a site of electron deficiency. A curved arrow may never originate at a single bond because that would violate the rule about breaking single bonds. A curved arrow may never terminate at a second-row element (C, N, O, F) unless the atom is electron deficient (such as a carbocation) or the element has electrons in a double or triple bond, which could move because of the incoming curved arrow. When determining which resonance structure is the best, an analysis of the resonance structure will rely on three main points:

• completely filled octets
• the fewest (or no) formal charges
• If there are formal charges, they should be on appropriate atoms (negative on an electronegative atom).

When comparing resonance structures, if one resonance structure has completely filled octets, meaning that every atom (except hydrogen, which can only have a maximum of two valence electrons) has eight valence electrons and another resonance structure has one or more unfilled octets, the resonance with completely filled octets is more important. Unfilled octets are very unstable. Any resonance structure with a carbocation, which is an unfilled octet, is going to be a bad resonance structure.

When comparing resonance structures, if all resonance structures have completely filled octets, the next thing to consider is the amount of formal charges. When a resonance structure has a formal charge, it is not as stable as a resonance structure with no formal charge. The resonance structure with more formal charges will be the least stable and less important resonance structure.

When comparing resonance structures, if all resonance structures have completely filled octets and all resonance structures have an equal number of formal charges, the last thing to consider is which atom has the formal charge. A more electronegative atom is better able to handle a negative charge, and a less electronegative atom is better able to handle a positive charge. For example, if one resonance structure has a negative charge on oxygen and another resonance structure has a negative charge on carbon, the resonance structure with the negative charge on the oxygen atom will be the more important resonance structure.