What is resonance?

What is resonance (in organic chemistry)? In one sentence, resonance is the concept where electrons (bonds) are delocalized over three or more atoms which cannot be depicted with one simple Lewis structure.

Resonance is one of those issues that you will have to deal with for both semester I & II of organic chemistry.  It is much better to have a solid understanding of it now, rather than have to worry about it later.  The basic goal of resonance structures is to show that molecules can move electrons and charges onto different atoms on the molecule.  Resonance generally makes a molecule more stable because the charge (or bond) is now delocalized and not “forced” onto an atom that might not want it.

Below are some handy rules of resonance.  If you learn these and think about them when tackling different resonance problems, you will be able to handle whatever is thrown at you.

1) Know each atom’s “natural state”. We talk about this in a different post on atoms’ natural state. You need to recognize what each atom we deal with generally looks like, in an uncharged state.  This will help you to construct the Lewis Dot structure on which you will base your resonance structures.  Remember that halogens and hydrogens are always terminal, meaning that are at the end of the molecule and only have one bond, and therefore, they will not (generally) participate in resonance.

2) Atom positions will not change.  Once you have determined that an atom is bonded to another atom, that order will not change in a resonance structure.  If they do change, it is no longer a resonance structure, but is now a constitutional isomer or a tautomer.

3) Check the structure you have created to make sure that it follows the octet rule.  This will become much easier once you have a better handle on the “natural state” of atoms.  If you violate the octet rule, you need to go back and check to make sure you didn’t make a mistake.

4) When two or more resonance structures can be drawn, the one with the fewest total charges is the most stable.  In the example below, A is more stable than B.

resonance in organic chemistry

5) When two or more resonance structures can be drawn, the more stable has the negative charge on the more electronegative atom.  In the example below, A is more stable than B.

resonance in organic chemistry

6) In the end, each resonance structure should have the same overall charge and total number of electrons (bonds + lone pairs) as when you started.  If it does not, you most likely made a mistake somewhere.

7) Resonance affect the length of a bond between two atoms.  We have another post on this topic, which we called resonance in organic chemistry.

 

Take Home Message:  What is resonance? Resonance is like telemarketers.  They are never going to go away, so you need to learn them well.