Resonance Structures. 

Many organic and some inorganic compounds and ions are stable because of the fact that their electrons are not in a distinct position. We say, the electrons are delocalized. They move around in orbitals that cover many atoms holding these atoms tight together. For example, the carbonate ion, CO32- has three resonance structures making it highly stable. The four atoms are tightly held together by a pi orbital that extends itself over the whole ion. You can see that by drawing the three possible Lewis structures for the carbonate ion. Try this. The convention is to write a double-pointed arrow between the different resonance structures.

Next, try to draw the two resonance structures for the acetate ion, CH3COO-. You notice that the electrons in the COO- part of this ion are not in a distinct position, they are delocalized. The two Lewis structures are representations of a pi orbital that is extended over the OCO part of this ion.

Write all resonance structures for ion 1 and compound 2 hereunder. Each carbon in ion 1 has one H atom attached, so the formula of this ion is C5H5-. We will make the catalyst ferrocene in lab from this ion and iron 2+. The second one is C10H8, naphthalene, a compound in gasoline. In naphthalene the central carbons cannot have a hydrogen atom anymore, they have already four bonds. Check this! Answers for the resonance structures will follow soon in lecture.

1.                                                2.

                                       

 

Draw resonance structures of ozone O3, and nitrogen monoxide NO. Recall, NO an important hormone in our body, has a single electron and therefore is called a radical. Radicals are highly reactive species. [Other radicals, such as oxygen radicals are known to be mutagenic, they change our DNA.]
 

        Rules for resonance structures.

1. Electrons may have different positions, but the relative positions of all the atoms remain the same.

2. All resonance structures of a compound or an ion must have the same total number of paired and unpaired electrons.

3. The algebraic sum of the formal charges on each atom will remain the same. This follows logically from the two rules above.

4. Important resonance structures will have comparable energy. Structures with high formal charge values are not very likely to exist or better: are not contributing to the resonance hybrid. The structures with the lowest formal charges are the main contributors to the resonance hybrid.

Rule 4 shows that a considerable amount of intuition is needed to judge what low energy resonance structures are. You will develop that with experience.