Resonance Structures of Nitrate Ion
Resonance Hybrid of Nitrate Ion
Rules for Resonance Structures
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 bond, such as , , , and so on. Polar 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 bonds)
Five Patterns of Resonance Structures
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.
Curved Arrows in Resonance Structures
Incorrect Ways of Drawing Curved Arrows
- 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.