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Chapter 16: 16-5P (page 809)

Does the MO energy diagram of cyclooctatetraene appear to be a particularly stable or unstable configuration? Explain

Short Answer

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It is a qualitative tool that is represented in the form of a diagram that explains the molecular orbital theory in which bonding in molecules takes place in a linear combination of atomic orbitals.

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01

Molecular orbital diagram

It is a qualitative tool that is represented in the form of a diagram that explains the molecular orbital theory in which bonding in molecules takes place in a linear combination of atomic orbitals.

02

MO energy diagram

The cyclooctatetraene molecule has 8 pi electrons. Of which six are paired but two electrons are unpaired in nonbonding orbitals due to which the configuration of cyclooctatetraene is unstable. The MO diagram of cyclooctatetraene can be represented as follows:

MO energy diagram of cyclooctatetraene

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Most popular questions from this chapter


(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 andanti stereochemistry.

(d) When the product from (c) is heated, HBr is evolved, and 9-bromophenanthrene results. Propose a mechanism for this dehydrohalogenation.

Question: The benzene ring alters the reactivity of a neighboring group in the benzylic position much as a double bond alters the reactivity of groups in the allylic position.

Benzylic cations, anions, and radicals are all more stable than simple alkyl intermediates.

  1. Use resonance forms to show the delocalization (over four carbon atoms) of the positive charge, unpaired electron, and negative charge of the benzyl cation, radical, and anion.
  2. Toulene reacts with bromine in the presence of light to give benzyl bromide. Propose a mechanism for this reaction.

    c. Which of the following reactions will have the faster rate and give the better yield? Use a drawing of the transition state to explain your answer.

(a) Draw the molecular orbitals for the cyclopropenyl case. H H H (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?

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.

Cyclopropenone CycloheptatrienoneCyclopentadienone

(a) Use the polygon rule to draw an energy diagram 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 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
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