Chem 242
Spring 2008
Answers to Problem Set 4
Question 1. Provide molecular orbital diagrams for each of the following compounds. In
addition, label each compound as aromatic, antiaromatic or nonaromatic (nonaromatic
compounds are neither aromatic or antiaromatic).
B
A
aromatic
antiaromatic
D
C
antiaromatic
E
aromatic
Ψ2
nonaromatic
Ψ3
A
Ψ1
Ψ2
Ψ3
B
Ψ1
Chem 242
Spring 2008
Ψ4
Ψ5
Ψ2
Ψ3
Ψ1
Ψ4
Ψ5
Ψ2
Ψ3
Ψ1
Chem 242
Spring 2008
Ψ4
Ψ3
Ψ2
Ψ1
Chem 242
Spring 2008
Question 2. Classify the following molecules as aromatic, antiaromatic or nonaromatic.
Give an explanation for your answer. The last one is very tricky…
N
sp3 carbon
H
10e-, aromatic
12e-, antiaromatic
nonaromatic
N
12e-, antiaromatic
H
10e-, aromatic (the nitrogen
contributes 2 e-)
H
N
O
6e-, aromatic (the oxygen
contribues 2 e-)
H
nonaromatic (see below)
For this molecule to be planar, the two hydrogens indicated above would have to occupy the same
space. Therefore, even though the π-system would contain 10e-, geometric restraints force the
molecule to twist and disrupt the continuous π-overlap. A tricky problem!!!
Chem 242
Spring 2008
Question 3. Which of the following compounds is more basic (i.e., which lone pair of
electrons shown below is more likely to become protonated)? Clearly explain your
choice.
O
O
Me
Me
O
4-pyrone
acetone
H
H
O
O
more basic
+ other resonance structures
O
O
6e-, aromatic
multiple resonance structures stabilize the protonated form; the aromatic resonance structure is
particularly stable
H
H
O
O
less basic
Me
Me
Me
Me
only one resonance structure stabilizes the protonated form
Chem 242
Spring 2008
Question 4. Why does the following compound have a large molecular dipole?
azulene
6e-, aromatic
6e-,
aromatic
Chem 242
Spring 2008
Question 5. Rank the following compounds from most to least basic. Clearly explain
your reasoning.
N
pyridine
most basic
N
N
N
H
H
pyrole
pyrolidine
pyrolidine has a very basic sp3-hydrbridized lone pair. Aromaticity is not
a consideration.
H
pyridine is less basic than pyrolidine becuase the lone pair is in an sp2hydrbidized orbital**. However, protonation of the nitrogen does not
disurpt the aromatic π-system.
N
least basic
N
pyrole is not very basic becuase the lone pair on nitrogen is involved in πbonding with the aromatic system. Protonation disrupts the aromaticity.
H
** Why does hybridization matter? That's an excellent question. Can you think of any other topics
that we have discussed where this is true?