Organic Chemistry

Cycloheptatrienyl bromide is an ionic compound because its cation is aromatic.
The alkyl halide is notaromatic in the covalent form because it has an hybridized
carbon, so it does nothave an uninterrupted ring of porbital-bearing atoms. In the
ionic form, however, the cycloheptatrienyl cation (also known as the tropylium cation)
isaromatic because it is a planar cyclic ion, all the ring atoms are hybridized
(which means that each ring atom has a porbital), and it has three pairs of delocalized
electrons. The stability associated with the aromatic cation causes the alkyl halide to
exist in the ionic form.

PROBLEM-SOLVING STRATEGY

Which of the following compounds has the greater dipole moment?

Before attempting to answer this kind of question, make sure that you know exactly what the

question is asking. You know that the dipole moment of these compounds results from the un-

equal sharing of electrons by carbon and oxygen. Therefore, the more unequal the sharing, the

greater is the dipole moment. So now the question becomes, which compound has a greater

negative charge on its oxygen atom? Draw the structures with separated charges, and deter-

mine their relative stabilities. In the case of the compound on the left, the three-membered

ring becomes aromatic when the charges are separated. In the case of the compound on the

right, the structure with separated charges is not aromatic. Because being aromatic makes a

compound more stable, the compound on the left has the greater dipole moment.

PROBLEM 9

Draw the resonance contributors of the cyclooctatrienyl dianion.

a. Which of the resonance contributors is the least stable?

b. Which of the resonance contributors makes the smallest contribution to the hybrid?

C+

C+ O−

O−

C

O

C

O

p

sp^2

sp^3

Section 15.4 Some Chemical Consequences of Aromaticity 601

δ+

δ+

δ+

δ+

δ+

δ+

δ+

resonance contributors of the cycloheptatrienyl cation

resonance hybrid

++

+

+

+

+ +