Resonance: Drawing Resonance Structures
a. A structure with a positive charge on a carbon immediately
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
is a benzene ring. Shifting all three double bonds provides the
other reasonable resonance structure. This resonance has a
influence on the extra stability associated with the benzene ring
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
adjacent to a benzene ring is termed a benzylic carbocation. It
several resonance forms that arise from shifting the position of the
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
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
donator atom, and the carbon atom of the carbonyl group (C=O) is the
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
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
g. Phosphorous is a third-row element, so it can
its octet to contain ten valence electrons. (Phosphorus is the
atom commonly encountered in organic chemistry that routinely expands
octet to ten valence electrons.) The oxygen atoms can each loan a
lone pair to the phosphorus, so several resonance structures
The resonance of phosphate ion has important biochemical consequences.
h. Any ion with three bonds to an oxygen atom that
a positive charge is called an oxonium ion. This oxonium ion has
no other resonance structures because it does not have adjacent donor