Resonance Concepts
Major concepts
 Resonance is not a real phenomenon; it is a tool to more closely describe the reality of
molecular bonding
 Using the arrow formalism, one can draw a series of resonance structures
 Resonance can be used to explain charge distribution over a molecule
 A resonance hybrid of a molecule may be drawn based on major and minor resonance
contributors
 An understanding of structural charge distribution is essential to understanding the stability of a
structure.
Vocabulary
 Resonance hybrid
 Charge distribution
 Arrow formalism
Students should be able to:
 Recognize the difference between resonance structures and constitutional isomers
 Explain why resonance is a tool, not a phenomenon
 Recognize major and minor resonance contributors
 Use resonance structures to draw a resonance hybrid
 Describe the charge distribution of a molecule using resonance
 Draw alternate resonance structures for a Lewis structure, given arrows
Daily Problems
1. Fill in all hydrogens and lone pairs explicitly in these resonance structures. Notice how, although
some bonds and lone pairs are in different places, the atoms are still bonded in the same connectivity.
δ-
δ-
δ+
δ+
δ-
δMajor due to neg. charge on the
more electronegative atom
δ+
+
δ
δMajor
because
every
Major
because
there
is atom
a complete octet
nohas
charge
δ+
2. In each of the sets of resonance structures in problem 1, draw an arrow formalism that helps get to
the next structure.
3. Label each of the structures in problem 1 as major or minor resonance contributor, and explain why.
(Don’t worry about parts A and B.)
4. Draw a resonance hybrid for each of the molecules in problem 1.
5. Given the arrows, draw a resonance structure.
6. Circle all structures which are resonance structures of Compound A. Explain why the other structures
are not resonance structures. (Hint: Drawing in implicit H’s and lone pairs is helpful.)
A
C
B
D
B and D are resonance structures of compound A. A and C are NOT resonance structures of A. Drawing
in the implicit hydrogens will show you that they do not have the same molecular formulaand there is a
charge missing when the double bond gets “moved”.
Cumulative Problems
7. A beginning R340 student attempted to draw these resonance structures based on these arrows.
They are wrong. Explain what is wrong with the resonance structure and/or the arrows.
You cannot give electrons to a carbon atom that
already has a full octet.
The way the arrow is drawn implies that electrons
are being taken away from oxygen, yet the
resonance structure has a negative charge on the
oxygen. The arrow pushing is backwards.
These are two
different compounds.
There is an extra
hydrogen on carbon 2.
8. Using resonance and/or induction principles, indicate which of the carbon atoms in these molecules
are Lewis acidic and which are Lewis basic. (At this point, you have been given arrows to help you draw
resonance structures. After next lecture, they won’t be given.)
LB
LA
LA
LB
LB
LB
LA
LA
LA
9. Often, the charge distribution in a molecule shown by resonance stabilizes the molecule, but
sometimes it destabilizes the molecule.
A. Consider these two ions. The carbon attached to chlorine has what partial charge in each
case? Fill in the charge.
δ+
δ+
B. Based on the resonance arrows given, draw a second resonance structure.
C. Draw the resonance hybrid for each ion based on the resonance structures from part B. Then
indicate the partial charge due to induction from part A on the structure.
Compound A
δ-
δ-
Compound B
δ+
δ+
D. Which compound is stabilized by resonance? Which is destabilized?
Compound A is stabilized by resonance. The partial negative and partial positive (on the carbon
beside the chlorine) would attract therefore stabilizing the compound. Compound B is destabilized
because the partial positive charges would repel each other.
Extension problems
10. Vitamin C is drawn below, along with one resonance structure. Draw one more resonance structure
that puts the negative charge on a different oxygen atom.