Organic Chemistry

600 CHAPTER 15 Aromaticity • Reactions of Benzene

Why is the of cyclopentadiene so much lower than that of ethane? To answer

this question, we must look at the stabilities of the anions that are formed when the

compounds lose a proton. (Recall that the strength of an acid is determined by the sta-

bility of its conjugate base: The more stable its conjugate base, the stronger is the acid;

see Section 1.18.) All the electrons in the ethyl anion are localized. In contrast, the

anion that is formed when cyclopentadiene loses a proton fulfills the requirements for

aromaticity: It is cyclic and planar, each atom in the ring has a porbital, and the

cloud has three pairs of delocalized electrons. The negatively charged carbon in the

cyclopentadienyl anion is hybridized because if it were hybridized, the ion

would not be aromatic. The resonance hybrid shows that all the carbons in the cy-

clopentadienyl anion are equivalent. Each carbon has exactly one-fifth of the negative

charge associated with the anion.

As a result of its aromaticity, the cyclopentadienyl anion is an unusually stable

carbanion. This is why cyclopentadiene has an unusually low In other words, it is

the stability conveyed by the aromaticity of the cyclopentadienyl anion that makes the

hydrogen much more acidic than hydrogens bonded to other carbons.

PROBLEM 7

Predict the relative values of cyclopentadiene and cycloheptatriene.

PROBLEM 8

a. Draw arrows to show the movement of electrons in going from one resonance contribu-

tor to the next in

1. the cyclopentadienyl anion


2. pyrrole
   b. How many ring atoms share the negative charge in


3. the cyclopentadienyl anion?


4. pyrrole?

Another example of the influence of aromaticity on chemical reactivity is the un-

usual chemical behavior exhibited by cycloheptatrienyl bromide. Recall from

Section 2.9 that alkyl halides tend to be relatively nonpolar covalent compounds—

they are soluble in nonpolar solvents and insoluble in water. Cycloheptatrienyl bro-

mide, however, is an alkyl halide that behaves like an ionic compound—it is insoluble

in nonpolar solvents, but readily soluble in water.

+

covalent

cycloheptatrienyl bromide

Br

ionic

cycloheptatrienyl bromide

tropylium bromide

Br−

pKa

sp^3

pKa.

δ− δ−

δ− δ−

δ−

resonance contributors of the cyclopentadienyl anion

resonance hybrid

− −

−

−−

sp^2 sp^3

p

p

pKa

Tutorial:

Aromaticity and acidity