Is there a ring (usually six-membered) with alternating double bonds? If so, move the double bonds around in a circle so that they are in the alternate positions.
Look for Ẍ−Y=Z, Ẍ−Y≡Z, or Ẍ=Y=Z. (X, Y, and Z can have any formal charge.) In these cases, push the lone pair on X to make an additional π bond between X and Y, and break one of the π bonds between Y and Z and make it a new lone pair on Z. The X atom increases its formal charge by 1; and the Z atom decreases its formal charge by 1.
Look for Ẋ−Y=Z, Ẋ−Y≡Z, or Ẋ=Y=Z. In these cases, use the single-headed or fishhook electron-flow arrow to show the movement of single electrons. Push the unshared electron on X to start an additional π bond between X and Y, take one electron from a π bond between Y and Z and use it to complete the additional π bond between X and Y, and take the other electron from the π bond between Y and Z and make it a new unshared electron on Z. There are no changes in formal charges.
Look for +C−Y=Z, +C−Y≡Z, or +C=Y=Z. In these cases, the C atom is electron-deficient, having only a sextet of electrons. (Other electron-deficient atoms such as neutral, trivalent B can replace C+, but C+ is by far the most common case.) Use the electrons from one of the π bonds between Y and Z to make an additional π bond between Y and X. The C+ gains an octet and becomes uncharged; the Z atom becomes electron-deficient and increases its formal charge by 1.
The electrons of a π bond (the second or third bond between two atoms) can be split up between the two atoms forming that bond, with two electrons going to one atom, or one electron going to each atom. This kind of resonance always produces a worse (higher energy) resonance structure, because there are more electron-deficient atoms in the resonance structure than there are in the original structure. If you give two electrons to one atom, that atom decreases its formal charge by 1, and the other atom increases its formal charge by 1.
If you have multiple adjacent π bonds, you can move electrons multiple positions down the chain.
Return to the UK Chemistry Home Page