Drawing Resonance Structures
Major concepts
 Looking at a Lewis structure, you can see features that let you know that there are additional
resonance structures.
 There are patterns that you can use to draw acceptable resonance structures in a systematic
way.
 Although a resonance structure may be acceptable, that doesn’t mean we will always draw it. It
may be acceptable but insignificant.
Vocabulary
 Acceptable resonance structure
 Significant resonance structure
 Resonance stabilization
Students should be able to:
 Recognize molecules that display resonance based on their structure
 Draw all significant resonance structures given a starting structure
 Answer some questions that apply resonance concepts
Daily Problems
1. Organic as a Second Language chapter 2 problems 2.1-2.73
Cumulative Problems These are extremely important cumulative problems because they are starting
to get similar to what you will see throughout the rest of the course. In these problems, and from
now on, you should always draw resonance structures and consider their impact on stability before
answering the problem.
2. Which carbon atom in 1-aminoprop-1-ene is most electron rich? Explain.
The second carbon is more electron rich due to the resonance structure shown above.
3. This compound could, in principle, undergo a reaction in which it lost an H+ to form one or the other
of these anions. Which product will form preferentially? (Remember—first consider resonance, then
consider its effect on stability.)
Due to the resonance-stabilized product that will form, the boxed compound will be preferentially
formed.
4. Proteins are formed when amino acids are stuck together through “peptide bonds”, which chemists
call amides. Proteins have a particular structure and they function the way they do because the carbonnitrogen bond of an amide acts sort of like a single bond and sort of like a double bond. Look at the
structure below, which is part of a protein. How can this bond act sort of like a double bond when the
Lewis structure looks like a single bond?
Amides have a resonance structure that places the negative charge on the oxygen, causing the bond
between the nitrogen and carbon to act like both a single and a double bond.
5. Name all the functional groups in the molecule in problem 4. Keep in mind that if the functional
group has lost H+ (and is therefore now negative), we say it is a “deprotonated” functional group. If the
functional group has gained an H+ and is now positive, we say that it is a “protonated” functional group.
thiol
amide
Deprotonated
carboxylic acid
Protonated
amine
Extension problems
6. Redraw these incorrectly drawn Lewis structures. How are they incorrectly drawn? What do you
think the physical reason is that the molecules are not in the shapes drawn below?
Atoms want to be as far away
as they possibly can within a
compound to avoid “knocking
into” other atoms (i.e. steric
clashing).