ChemActivity 10
ChemActivity
Resonance Effect
1
10
Resonance Effects
(How does the number of resonance structures affect the P.E. of a molecule or ion?)
Model 1: Wavelength and Energy
Figure 1a: Guitar Strings
high frequency ( ν)
short wavelength (λ)
high energy (E)
low frequency ( ν)
long wavelength ( λ)
low energy (E)
Table 1b: Properties of Various Kinds of Light
Type of Light
Wavelength (nm)
Effect on Humans
X-ray light
λ= 0.1 to 200 nm
Ultraviolet
light
Visible light
λ= 200 to 400 nm
λ= 400 to 800 nm
Will give you cancer if you are
exposed for more than a few
minutes over a lifetime.
Gives you a sun burn and
eventually can give you cancer.
Does not hurt you, but supplies
energy to plants
no proven effect on humans
λ= 0.2 to 20 meters
There are 1 billion nanometers (nm) in 1 meter (1 m = 109 nm).
Radio Waves
Relative
Energy
very high
energy
high energy
moderate
energy
low energy
Critical Thinking Questions
1. According to Model 1:
a) Light with a short wavelength is low or high [circle one] in energy.
b) Light with a long wavelength is low or high [circle one] in energy.
ChemActivity 10
Resonance Effect
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Model 2: Electrons in a Box (a thought experiment)
• Imagine an electron confided to a box.
• The electron is constantly moving around in its box at nearly the speed of light. For
this reason we say each electron is evenly spread out or delocalized over the entire
box. Chemists imagine the electron spread out into a mist they call an electron cloud.
• Electrons are so small that they behave like light and have a wavelength.
• The longer the box, the longer the wavelength of an electron in that box.
Figure 2: Electrons in Small and Large Boxes
wavelength of the electron in smallest box (λ = short)
electron
Each electron is zipping around
in its box at the speed of light!
It is delocalized (spread out)
over the whole box.
wavelength of the
electron in largest box
(λ = long)
Critical Thinking Questions
2. Which electron in Figure 2 has the smallest wavelength?
The one with the smallest box.
3. Which electron in Figure 2 has the lowest energy? Explain your reasoning.
The one with the largest wavelength, so the one with the largest box.
ChemActivity 10
Resonance Effect
3
Review: Rules for Drawing Resonance Structures
The following are two of the three “legal” ways of moving electrons to generate a set
of resonance structures. We will use the third way later on. Please study these two arrows
carefully. They are the key to drawing resonance structures.
H
1
H
2
H
C
H
H
C
H
C
C
C
C
H
H
H
H
• You may move a lone pair of electrons on an atom so as to form a new double bond
to that atom (see Arrow 1).
• You may break part of a double bond and make those two electrons a lone pair on one
of the atoms originally involved in the double bond (see Arrow 2).
In addition:
• You may not break any single bonds.
• You may not add or remove electrons.
• You may not move any nuclei.
• The resulting resonance structure must be a legitimate Lewis structure (e.g. you can’t
make 5 bonds to carbon)
Critical Thinking Questions
4. Draw all important resonance structures (if any) that go with the three Lewis
structures (I, II & III) below. (Keep track of electrons using the curved arrow
notation.)
I
H2
C
H3 C
no other resonance structures possible!!
CH2
H
C
H
C
II
H
C
H
C
III
H2C
H2C
CH2
H2C
C
H
CH2
H
C
H
C
CH2
H2 C
C
H
H
C
CH2
H2C
H
C
C
H
CH2
ChemActivity 10
Resonance Effect
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Model 3: More Resonance Structures = Larger “Box”
The electron pair on ion I (previous page) is stuck (localized) on C3. However, the
resonance structures you drew on the previous page tell you the electron pair on ion II is
delocalized (spread out) over three carbons. It is therefore less confined, which is
equivalent to saying it is in a “larger box.” Recall from Model 2 that an electron in a
large box has a long wavelength, and therefore a low potential energy.
The wavelength argument is not as important as the following end result called the
resonance effect: The Resonance Effect states that “the larger the # of important
resonance structures you can draw for a structure, the lower the potential energy of
that ion or molecule.”
Critical Thinking Questions
5. Explain why we can say “the electron pair on III is in the largest “box!”
Because the electrons on III are delocalized over a larger area, they have a larger
"box."
6. Which ion (I, II or III) [circle one] do you expect to be lowest in potential energy?
Explain your reasoning.
According to the resonance effect, III will have the lowest potential energy.
7. Fill in Columns 2 & 3 in Table 3a.
Table 3a: pKa Data for Various Conjugate Acid-Base Pairs
Total # of
P.E. of the
Conjugate Base
Important lone pair in
Conjugate Acid
R.S.’s
pKa units
H2
I. Strongest Base
C
H2
1
50 pKa
C
H
C
CH3
3
units
H3C
pKa
~ 50
CH2
II.
H
C
2
CH2
H2C
H
C
43 pKa
units
43
CH3
H2C
+ other resonance structure
III. Weakest Base
H
C
H2C
H
C
C
H
CH2
3
33 pKa
units
H
C
H2C
H
C
C
H
CH3
+ other resonance structures
8. Based on the pKa data in Table 3a, write strongest base next to one of the species in
Table 3a and “weakest base” next to one of the species in Table 3a. Are your
choices consistent with your answers to CTQ 7? Explain.
Yes. The species with the most resonance structures has the lowest P.E., and this fits
with the fact that it's conjugate acid has the lowest pKa. This tells you that it takes the
least energy to remove a H+ from the conjugate acid of III.
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ChemActivity 10
Resonance Effect
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Review: Hierarchy of Electron Effects
Strongest = Formal charge effect = neutral happier than negative
Very Strong = Electronegativity effect = e– pair on F happier than e– pair on O>N>C
Strong = Resonance effect = the more “delocalized” an electron, the happier it is.
Weak = Inductive effect = long range electronegativity effect
Figure 3b: pKa Data for Two Ions Demonstrating Resonance Effects
Conjugate Acid
H
C
H
C
IV
CH2
H2C
H2C
CH2
Stronger Base
H
C
V
O
H
C
H
C
H
C
CH2
O
CH3
H2C
CH2
O
CH3
pKa
43
17
Weaker Base
Critical Thinking Questions
9. Based on the pKa data, write “weaker base” next to the weaker of the two bases in
Figure 3b (IV or V).
a) Which conjugate base IV or V [circle one] is lower in potential energy?
b) For Base IV, the two resonance structures are equivalent. For Base V the
two resonance structures are not equivalent. One of the two resonance
structures has an arrangement of electrons that is better/more happy/lower in
potential energy. This better resonance structure is called the more
important resonance structure. Circle this more important resonance
structure for Base V.
c) The two bases in Figure 3b have the same number of resonance structures.
According to the resonance effect, they should have similar potential
energies. But they don’t! Explain why one base is much lower in potential
energy than the other.
V is lower in P.E. than IV because V has a much better resonance structure. This is to
say that IV has two ok resonance structures, while V has one ok resonance structure
and one excellent resonance structure. Another way of saying this is that the negative
charge on IV is split between two C's while the negative charge on V is split between a
C and an O. Since O holds its electrons more tightly than C, V is better able to lower
the potential energy of its electrons.
d) Experiments show that Base V in Figure 3b has a carbon-oxygen bond order
of 1.2 and a carbon-carbon bond order of 1.8. Is this information consistent
with your answer to part c)? Explain.
Yes. According to the bond order data, there is more double bond character between
the two C's of V, and more single bond character between the O and the C of V. This
tells us that the right hand resonance structure is closer to the true picture of V. This
fits with our analysis that this resonance structure is more important due to energy
considerations.
ChemActivity 10
Resonance Effect
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Exercises
1. Consider the following acid-base reactions:
H
O
pKa = 12
NH2
CH
O
O
CH
CH
A
H2C
H2C
H2C
H
O
O
pKa = 16
CH2
NH2
CH2
NH3
H3C
H3C
a)
b)
c)
B
Use curved arrows to show each acid base reaction.
Based on the pKa data, which alcohol is a stronger acid?
Which conjugate base is lower in potential energy (a weaker base)?
Warning!! Common Sticking Point!! Many students think that conjugate base B
should be lower in potential energy because A has an unfavorable negative carbon
associated with it. An additional resonance structure (even if it is not very favorable)
can only “help” lower the potential energy of a species. It cannot hurt. The following
analogy helps illustrate this point.
Consider two teams of postal workers racing to postmark 1000 envelopes. Team A
consists of a fast worker and a slow worker. Team B consists of a single fast worker.
Assume the two fast workers work at the same pace, and the slow worker doesn’t bother
his fast teammate
Team A
fast
slow
worker worker
Team B
fast
worker
This means Team A will postmark 1000 envelopes before Team B since the slow
worker helps some, and doesn’t hurt at all.
Explain how this story relates to the example at the top of the page.
NH3
ChemActivity 10
Resonance Effect
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2. A student made up the following analogy for the resonance effect: “A kid lives on a
block with very few fences separating the back yards. The larger the number of
yards he and the other kids have access to, the happier they are.”
a) Explain what each of the underlined elements of the analogy represent in
terms of chemistry.
A second student thinks this analogy is incomplete. She adds: “To figure out the
happiness of a kid you must consider both the number of yards he has access to and
the quality of the grass in each yard.”
b) What does the quality of the grass in each yard represent?
3. Use curved arrows to show the most likely acid base reaction between the following
pairs of molecules. Draw the products of the reaction including any important
resonance structures.
H
N
H2O
O
+
?
H
H
O
H
O
H
H2O
+
?
H2O
+
?
O
H
O
O
H
4. Construct an explanation for why sulfuric acid is such a strong acid. (Note that
sulfur is in the third row of the periodic table and can have more than 8 electrons.)
ChemActivity 10
Resonance Effect
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H
O
O
H
S
O
O
5. Consider the data in Table 3a.
• The pKa in row 3 tells you that it takes 33 pKa units of energy to remove an H+ from
the conjugate acid of III so as to create the new lone pair on conjugate base (III).
• Assume that all three conjugate acids in Table 3a have the same potential energy.
a) Draw three energy diagrams showing the three acid base reactions depicted
in Table 3a. (Show Conjugate Base + H+ à Conjugate Acid)
b) Construct an explanation for why the conjugate acid in Row 3 of Table 3a is
the strongest acid listed in the table.
6. The pKa of a molecule tells you the amount of energy it takes to remove the most
acidic hydrogen. In most cases you are expected to know which H is most acidic. If
you are not sure, sketch several conjugate bases and determine which one is lowest
in potential energy.
a) For each molecule below circle the most acidic hydrogen.
b) For each pair, put a box around the molecule you expect to have a lower pKa
and explain your reasoning.
H
O
O
H
H
O
O
H
H
O
H
N
N
H
H
O
H
7. Read the assigned pages in your text and do the assigned problems.
ChemActivity 10
Resonance Effect
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Which H (proton) Is Most Acidic?
The following are equivalent ways of asking about the acidity of an H atom:
1.
2.
3.
4.
5.
6.
What is the “most acidic proton” on the molecule?
Which proton is associated with the published pKa value?
Which proton on the molecule is easiest to remove?
Which proton on the molecule takes the least energy to remove?
For which H atom is removal least up-hill in energy?
Which bond to an H atom, when broken, results in the lowest P.E. lone pair
of electrons?
The easiest way to answer question #1 is to ask yourself question #6.
To answer question #6, consider all the different H atoms in the molecule and draw
the unique products (a.k.a. “conjugate bases”) that result from each one’s removal. Then
consider the hierarchy of effects to sort out which conjugate base has the lowest P.E. new
lone pair of electrons.
Formal Charge Effect > Electronegativity (Core Charge) Effect > Resonance Effect
> Inductive Effect
Make a graphical sketch of the energies of all conjugate bases. For compound “S” below,
there are 4 different kinds of unique H’s, and thus 4 unique conjugate bases (I-IV).
H
H
O
O
H
H
N
O
H
N
H
H
II
I
N
H
H
O
S
H
II or IV
O
H
N
N
H
III
I
III
S
H
IV
ChemActivity 10
Resonance Effect
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1. For the example above, answer the 6 questions in reverse order 6 ---> 1 and explain
which is the most acidic hydrogen on the compound “S.”
2. For each of the molecules below, draw ALL possible conjugate bases and
a) Circle the conjugate base that would require the least amount of energy to
make from the original molecule (conjugate acid).
Note: There is a different pKa associated with removal of each different H. A table
would list the lowest pKa of these. This would be the pKa associated with the most
acidic H and the lowest P.E. conjugate base.
b)
Circle the most acidic hydrogen on the original molecule (conjugate acid).
ChemActivity 10
HO
Resonance Effect
NH3
H
N
H
H
O
O
O
CF3
F 3C
3. For each pair of compounds below, put a box around the molecule with the most
acidic hydrogen (this is the molecule that would have a lower pKa listed next to it
on a table) and explain your reasoning.
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ChemActivity 10
HO
NH3
H2O
NH2
O
O
H
H
F 3C
O
O
OH
OH
N
O
Resonance Effect
O
12
ChemActivity 10
Resonance Effect
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