18. Aromaticity

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Which of the following compounds could be protonated without destroying its aromaticity?

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Explain why each compound is aromatic, antiaromatic, or nonaromatic. (d)

How would you convert the following compounds to aromatic compounds? (d)

Explain why each compound is aromatic, antiaromatic, or nonaromatic. (g)

Explain why each compound is aromatic, antiaromatic, or nonaromatic. (a)

(a) Explain how pyrrole is isoelectronic with the cyclopentadienyl anion. (b) Specifically, what is the difference between the cyclopentadienyl anion and pyrrole?

Determine which of the heterocyclic amines just shown are aromatic. Give the reasons for your conclusions.

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For each heterocyclic compound, (iii) are any of the rings aromatic? Explain (a)

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.

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The following molecules and ions are grouped by

similar structures. Classify each as aromatic, antiaromatic,

or nonaromatic. For the aromatic and antiaromatic species,

give the number of pi electrons in the ring.

(b) <IMAGE>

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Anions of hydrocarbons are rare, and dianions of hydrocarbons are extremely rare. The following hydrocarbon reacts with two equivalents of butyllithium to form a dianion of formula [C8H6]2-. Propose a structure for this dianion, and suggest why it forms so readily.

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Some of the following compounds show aromatic properties, and others do not. Predict which ones are likely to be aromatic, and explain why they are aromatic. (a)

The ribonucleosides that make up ribonucleic acid (RNA)

are composed of d-ribose (a sugar) and

four heterocyclic “bases.” The general structure of a

ribonucleoside is shown here.

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The four heterocyclic bases are cytosine, uracil, guanine,

and adenine. Cytosine and uracil are called pyrimidine bases

because their structures resemble pyrimidine.

Guanine and adenine are called purine bases because

their structures resemble purine.

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that are aromatic? (Consider moving a proton from nitrogen

to a carbonyl group to form a phenolic derivative.)

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Consider the following compound, which has been synthesized

and characterized:

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(a) Assuming this molecule is entirely conjugated, do you

expect it to be aromatic, antiaro- matic, or nonaromatic?

(b) Why was this molecule synthesized with three

tert-butyl substituents? Why not make the unsubstituted

compound and study it instead?

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The following molecules and ions are grouped by similar structures. Classify each as aromatic, antiaromatic, or nonaromatic. For the aromatic and antiaromatic species, give the number of pi electrons in the ring.

(g) <IMAGE>

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Hexahelicene seems a poor candidate for optical activity

because all its carbon atoms are sp2 hybrids and

presumably flat. Nevertheless, hexahelicene has been

synthesized and separated into enantiomers. Its optical rotation

is enormous: [α]D = 3700°. Explain why hexahelicene is

optically active, and speculate as to why the rotation is so large.

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