Resonance: Drawing Resonance Structures

Exercise Solutions

a. A structure with a positive charge on a carbon immediately adjacent to an alkene is termed an allylic carbocation.  Shifting the p electrons gives us the only other reasonable resonance structure.

b. This molecule is toluene.  The functional group is a benzene ring.  Shifting all three double bonds provides the only other reasonable resonance structure.  This resonance has a profound influence on the extra stability associated with the benzene ring (Chapter 9 of the text).  The direction of the electron flow (clockwise or counterclockwise) does not matter, as it gets us to the same place.

c. A structure with a positive charge on a carbon immediately adjacent to a benzene ring is termed a benzylic carbocation.  It has several resonance forms that arise from shifting the position of the positive charge as well as shifting the position of the double bonds within the benzene ring as in the previous problem.

d. Just because a molecule is neutral does not mean it does not have resonance structures.  We see this with CO (carbon monoxide) as well as with toluene (question b).

e. When drawing resonance structures, a lone pair can be moved in the same way as a p bond.  In this case, the carbon bearing the lone pair and negative charge is the donator atom, and the carbon atom of the carbonyl group (C=O) is the acceptor atom.  An molecule that bears a negative charge next to a carbonyl group is termed an enolate.  Resonance involving a lone pair of an electrons or a positive charge next to a p bond occurs in many structures.

f. This is another case of lone pair-p bond resonance interaction.  The lone pair does not have to be on an atom that bears a formal negative charge as in the previous question.

g. Phosphorous is a third-row element, so it can expand its octet to contain ten valence electrons.  (Phosphorus is the only atom commonly encountered in organic chemistry that routinely expands its octet to ten valence electrons.)  The oxygen atoms can each loan a lone pair to the phosphorus, so several resonance structures result.  The resonance of phosphate ion has important biochemical consequences.

h. Any ion with three bonds to an oxygen atom that bears a positive charge is called an oxonium ion.  This oxonium ion has no other resonance structures because it does not have adjacent donor and acceptor atoms.
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