We're going to make a lot of use of resonance to show how delocalization of electrons influences chemistry. This page presents a few rules to keep in mind when drawing the structures.
- Molecules that require description with resonance are those for which we can write two or more Lewis structures that differ only in the position of electrons
- If the structures differ in the position of atoms, they are isomers, not resonance structures
- Because they differ only in the positions of electrons, we can show how resonance structures are related by showing the electron motions that would change one resonance structure into the other.
- It is customary to use a double-headed arrow to connect resonance structures; do not mistake this for the double arrow indicating equilibrium.
- Only unshared electrons and electrons that are part of multiple bonds (p-bonds) can be moved in drawing resonance structures; that is, resonance can occur only in those parts of molecules that are composed of sp or sp2 atoms; p orbitals are an absolute requirement for resonance!
To start, we need a valid Lewis structure for the species we are examining.
Without changing the number of unpaired electrons (and switching between ions and radicals always does this, so we avoid doing it), use curly arrows to move one or two pairs of electrons to create another valid Lewis structure.
Here are some things to keep in mind when deciding which electron pairs to move.
- Move fewer electron pairs rather than more;
- In general, move electrons from regions of high electron density (unshared pairs, C=C double bonds) toward regions of low electron density (carbocation centers, C=O double bonds, nitro groups);
Here is an example, in which we write the resonance structures for the species in the middle:
- Follow the red arrow first. See that it converts an unshared pair into a bonding pair.
- Work out the new formal charges; since we moved electrons away from the nitrogen, it should become positive, and the carbon receiving them should become more negative (going from +1 to 0 is becoming more negative).
- Now follow the blue arrows. The left arrow converts an unshared pair into a bonding pair; the other blue arrow does the opposite conversion.
- We moved electrons away from a negative carbon; it becomes more positive. A positive carbon received electrons and became neutral.
Some general rules:
- The head of a curly arrow always makes a bond or an unshared pair.
- The tail either breaks a bond or makes an unshared pair into a shared pair.
- If several possibilities for moving electrons exist within a given structure, try each of them, but separately.
- Both of the resulting structures in the example are valid resonance contributors. However, some resonance structures are more equal than others (George Orwell):
- Structures in which every atom has an octet, like the one on the right, are better than those in which an atom is missing electrons, like the one on the left.
- Structures that result from creating charges on formerly neutral structures, on the contrary, are unfavorable; avoid them if possible.
Finally, although formal charges on individual atoms may change from structure to structure, the net charge on the structure MUST remain the same. Otherwise, you are creating or destroying electrons, and Mother Nature doesn't like that.
Notice here that the starting structure has formal charges that add up to zero; so do both resonance structures that we drew.
Here's another example:
Here, the original structure is on the left, and we generated the third structure from the second. One cannot know ahead of time whether all structures can be generated from the original or from each other, or both. One simply keeps trying, until no new structures seem to appear.
Species with unpaired electrons can have resonance structures also.
- In these cases, move electrons one at a time rather than in pairs
- Use curly arrows with only one barb (curly fishhooks?) to move single electrons
Here are some exercises to use to practice: