16. Homework Chapter 16 .pdf - 16-5 Does the MO energy...

16-5
Does the MO energy diagram of cyclooctatetraene (Figure 16-8) appear to be a particularly stable or unstable
configuration? Explain.
16-7
Classify the following compounds as aromatic, antiaromatic, or nonaromatic
16-8
One of the following compounds is much more stable than the other two. Classify each as aromatic, antiaromatic, or
nonaromatic.
16-9
(a) Use the polygon rule to draw an energy diagram (as in Figures 16-5 and 16-7) for the MOs of a planar
cyclooctatetraenyl system.

(b) Fill in the eight pi electrons for cyclooctatetraene. Is this electronic configuration aromatic or antiaromatic? Could the
cyclooctatetraene system be aromatic if it gained or lost electrons?
*(c) Draw pictorial representations (as in Figures 16-4 and 16-6) for the three bonding MOs and the two nonbonding MOs
of cyclooctatetraene. The antibonding MOs are difficult to draw, except for the all-antibonding MO.
16-10
(a) Draw the molecular orbitals for the cyclopropenyl case. (Because there are three p orbitals, there must be three MOs:
one all-bonding MO and one degenerate pair of MOs.)
(b) Draw an energy diagram for the cyclopropenyl MOs. (The polygon rule is helpful.) Label each MO as bonding,
nonbonding, or antibonding, and add the nonbonding line. Notice that it goes through the approximate average of the
MOs.
(c) Add electrons to your energy diagram to show the configuration of the cyclopropenyl cation and the cyclopropenyl
anion. Which is aromatic and which is antiaromatic?
16-12
Explain why each compound or ion should be aromatic, antiaromatic, or nonaromatic.

(f) the [20]annulene dication
16-15
The polarization of a carbonyl group can be represented by a pair of resonance structures:
Cyclopropenone and cycloheptatrienone are more stable than anticipated. Cyclopentadienone, however, is relatively
unstable and rapidly undergoes a Diels–Alder dimerization. Explain.

16-17
Show which of the nitrogen atoms in purine are basic, and which one is not basic. For the nonbasic nitrogen, explain why
its nonbonding electrons are not easily available to become protonated.
16-21
(a) Draw all the Kekulé structures of anthracene and phenanthrene.
(b) Propose mechanisms for the two additions shown.
(c) In Chapter 8, most of the additions of bromine to double bonds gave entirely anti stereochemistry. Explain why the
addition to phenanthrene gives a mixture of syn and anti stereochemistry.
(d) When the product from (c) is heated, HBr is evolved and 9-bromophenanthrene results. Propose a mechanism for this
dehydrohalogenation.
16-24
Name the following compounds:

16-26