A B C D E - Colby College to PS4.pdf · aromatic antiaromatic antiaromatic aromatic nonaromatic Ψ1...
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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). Ψ 1 Ψ 3 Ψ 2 aromatic antiaromatic nonaromatic antiaromatic aromatic Ψ 1 Ψ 3 Ψ 2 A B D E C A B
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Transcript of A B C D E - Colby College to PS4.pdf · aromatic antiaromatic antiaromatic aromatic nonaromatic Ψ1...
Chem 242 Spring 2008
Answers to Problem Set 4
Question 1. Provide molecular orbital diagrams for each of the following compounds. Inaddition, label each compound as aromatic, antiaromatic or nonaromatic (nonaromaticcompounds are neither aromatic or antiaromatic).
Ψ1
Ψ3Ψ2
aromatic antiaromatic nonaromaticantiaromatic aromatic
Ψ1
Ψ3Ψ2
A B D EC
A
B
Chem 242 Spring 2008
Ψ3
Ψ1
Ψ2
Ψ4 Ψ5
Ψ3
Ψ1
Ψ2
Ψ4 Ψ5
Chem 242 Spring 2008
Ψ1
Ψ2
Ψ3
Ψ4
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…
NON
H
N
H
10e-, aromatic 12e-, antiaromatic
6e-, aromatic (the oxygen contribues 2 e-)10e-, aromatic (the nitrogen
contributes 2 e-)
nonaromatic
sp3 carbon
12e-, antiaromatic
nonaromatic (see below)
HH
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 themolecule 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 ofelectrons shown below is more likely to become protonated)? Clearly explain yourchoice.
O
O
Me Me
O
4-pyrone acetone
O
OH
O
OH
6e-, aromatic
more basic
less basic
Me Me
OH
Me Me
OH
+ other resonance structures
only one resonance structure stabilizes the protonated form
multiple resonance structures stabilize the protonated form; the aromatic resonance structure is particularly stable
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 explainyour reasoning.
NN N
HH
pyridine pyrole pyrolidine
N
H
most basic pyrolidine has a very basic sp3-hydrbridized lone pair. Aromaticity is not a consideration.
N
N
H
pyridine is less basic than pyrolidine becuase the lone pair is in an sp2-hydrbidized orbital**. However, protonation of the nitrogen does notdisurpt the aromatic π-system.
least basic pyrole is not very basic becuase the lone pair on nitrogen is involved in π-bonding with the aromatic system. Protonation disrupts the aromaticity.
** Why does hybridization matter? That's an excellent question. Can you think of any other topics that we have discussed where this is true?
