Organic Chemistry Help: Resonance

Hi Everybody–Resonance is one of those issues that you will have to deal with for both semester I & II 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.  This makes the molecule generally more stable because the charge is now delocalized and not “forced” on an atom that does 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”.  You need to recognize what each atom 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.  In most uncharged cases:

       – C has four bonds and no lone pairs

       – N has three bonds and one lone pair

       – Halogens (F, Cl, Br, I) have one bond and three lone pairs. 

       – O has two bonds and two lone pairs

       – H has one bond and no lone pairs

       – With the exception of H, everyone in group I & group II are only counterions (+1 or +2 and not involved in resonance).

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 participate in resonance.

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

 

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.

 

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.

 

 

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.

 

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.