Resonance Structures
Lewis structures attempt to show the location of the valence electrons in molecular and ionic compounds. These types of drawings work very well for most organic molecules. However, for some compounds, the electrons do not appear to be localized in the fixed positions suggested by a single Lewis structure. Instead, the electrons appear to be "delocalized", or spread out over more than one location. While a more complete description of electron probability distributions can be obtained by use of Molecular Orbital (M.O.) theory, the use of "resonance structures" leads to a much simpler explanation.
Many molecules that contain multiple bonds and/or lone pairs have structures that can best be understood in terms of a combination of two or more Lewis structures. For example, the electrons in the acetate ion (CH3CO2-1) can be thought of as being located in two different arrangements (as shown below). Both of these structures contain a C=O bond, which is expected to be shorter and stronger, and a C-O bond. Experimentally, both CO bonds appear to be the same length, and are approximately halfway between the length expected for a C-O single bond and C=O double bond. The resonance explanation is that this molecule spends ~50% of its time in each of two possible resonance forms. (The double-headed arrow indicates resonance).
Resonance stabilizes molecules and ions, which can be used to explain a variety of chemical reactivity trends. Understanding these trends requires two closely related arguments:
- If a reactant (starting material) is resonance stabilized, chemical reactions of this molecule will be less favored (slower, more positive DG, etc.) than they would be in the absence of resonance.
- If a product is resonance stabilized, chemical reactions that form this product will be more favored.
The following examples illustrate the application of these concepts.
- Benzene Reactivity
- Benzene contains three double bonds, and as such is expected to undergo addition reactions like other alkenes. The resonance stabilization of this molecule results in a significant decrease in this type of reactivity (point #1 above).
- Acidity of Carboxylic Acids
- Both alcohols and carboxylic acids contain -OH groups, and both can lose H+ from this group. However, carboxylic acids are much stronger acids than alcohols. While a complete explanation of this fact requires an examination of the electronic effect of the the carbonyl group, the enhanced acidity of carboxylic acids can be explained based on the fact that two equivalent resonance forms can be drawn for the carboxylate anion (see the acetate ion structures shown above), but the alcohol product (alkoxide anion) doesn't have any good resonance forms.
- Amide Basicity
- Both amines and amides contain nitrogen atoms with a lone pair of electrons, and both are expected to act as bases. While this is correct for amines, resonance stabilization of amides results in a marked decrease in basicity for these compounds.
- a-Hydrogen acidity
Hydrogen atoms attached to carbon atoms next to a carbonyl group display an enhanced acidity. While the acidity of these protons is quite low, the C-H bond is weak enough to be cleaved by strong base. This is the first step in the aldol condensation reaction.
