Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)

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at carbon 1.

1-Iodo-2-methylbenzene
(o-Iodotoluene)

2,4,6-Tribromophenol 1-Bromo-3-ethenylbenzene
(m-Bromostyrene)

CH2PCHOHCH3
Br BrI

Br

Br

A number of the common names for aromatic compounds refer to their fragrance and
natural sources. Several of them have been accepted by IUPAC. As before, a consistent
logical naming of these compounds will be adhered to as much as possible, with common
names mentioned in parentheses.

Aromatic Flavoring Agents

Methyl 2-hydroxybenzoate
(Methyl salicylate,

oil of wintergreen flavor)

4-Hydroxy-3-methoxybenzaldehyde
(Vanillin, vanilla flavor)

5-Methyl-2-(1-methylethyl)phenol
(Thymol, thyme flavor)

CH3OH
OH OCH3

OHCOOCH3
CH(CH3)2CHO

The generic term for substituted benzenes is arene. An arene as a substituent is referred
to as an aryl group, abbreviated Ar. The parent aryl substituent is phenyl, C6H5. The
C6H5CH2 – group, which is related to the 2-propenyl (allyl) substituent (Sections 14-1 and
22-1), is called phenylmethyl (benzyl) (see examples in margin).

Phenylmethanol
(Benzyl alcohol)

trans-1-(4-Bromophenyl)-
2-methylcyclohexane

CH2OH

CH3

Br

∑

 C h a p t e r 1 5 677

Exercise 15-1

Write systematic and common names of the following substituted benzenes.

(a)

Cl

NO2

(b)

CH3
D

(c)

OH
NO2

NO2

Exercise 15-2

Draw the structures of (a) (1-methylbutyl)benzene; (b) 1-ethenyl-4-nitrobenzene (p-nitrostyrene);
(c) 2-methyl-1,3,5-trinitrobenzene (2,4,6-trinitrotoluene — the explosive TNT; see also Chemical
Highlight 16-1).

Exercise 15-3

The following names are incorrect. Write the correct form. (a) 3,5-Dichlorobenzene;
(b) o-aminophenyl fl uoride; (c) p-fl uorobromobenzene.

1 5 - 2 S t r u c t u r e a n d R e s o n a n c e E n e r g y o f B e n z e n e

In Summary Simple monosubstituted benzenes are named by placing the substituent name
before the word “benzene.” For more highly substituted systems, 1,2-, 1,3-, and 1,4- (or
ortho-, meta-, and para-) prefi xes indicate the positions of disubstitution. Alternatively, the
ring is numbered, and substituents labeled with these numbers are named in alphabetical
order. Many simple substituted benzenes have common names.

15-2 Structure and Resonance Energy of Benzene:
A First Look at Aromaticity

Benzene is unusually unreactive: At room temperature, benzene is inert to acids, H2, Br2,
and KMnO4 — reagents that readily add to conjugated alkenes (Section 14-6). The reason
for this poor reactivity is that the cyclic six-electron arrangement imparts a special stabil-
ity in the form of a large resonance energy (Section 14-7). We shall fi rst review the evi-
dence for the structure of benzene and then estimate its resonance energy by comparing
its heat of hydrogenation with those of model systems that lack cyclic conjugation, such
as 1,3-cyclohexadiene.

The benzene ring contains six equally overlapping p orbitals
If benzene were a conjugated triene, a “cyclohexatriene,” we should expect the C – C bond
lengths to alternate between single and double bonds. In fact, experimentally, the benzene
molecule is a completely symmetrical hexagon (Figure 15-1), with equal C – C bond lengths of
1.39 Å, between the values found for the single bond (1.47 Å) and the double bond (1.34 Å)
in 1,3-butadiene (Figure 14-6).

Figure 15-2 shows the electronic structure of the benzene ring. All carbons are sp2
hybridized, and each p orbital overlaps to an equal extent with its two neighbors. The
resulting delocalized electrons form circular p clouds above and below the ring. The

1.09 Å
1.39 Å

120°
120°

120°

HH

Figure 15-1 The molecular
structure of benzene. All six C – C
bonds are equal; all bond angles
are 1208.

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678 C h a p t e r 1 5 B e n z e n e a n d A r o m a t i c i t y

 symmetrical structure of benzene is a consequence of the interplay between the s and
p electrons in the molecule. The symmetrical s frame acts in conjunction with the delo-
calized p frame to enforce the regular hexagon.

Benzene is especially stable: heats of hydrogenation
A way to establish the relative stability of a series of alkenes is to measure their heats of
hydrogenation (Sections 11-5 and 14-5). We may carry out a similar experiment with
 benzene, relating its heat of hydrogenation to those of 1,3-cyclohexadiene and cyclohexene.
These molecules are conveniently compared because hydrogenation changes all three into
cyclohexane.

The hydrogenation of cyclohexene is exothermic by 228.6 kcal mol21, a value expected
for the hydrogenation of a cis double bond (Section 11-5). The heat of hydrogenation of
1,3-cyclohexadiene (DH 8 5 254.9 kcal mol21) is slightly less than double that of cyclo-
hexene because of the resonance stabilization in a conjugated diene (Section 14-5); the
energy of that stabilization is (2 3 28.6) 2 54.9 5 2.3 kcal mol21 (9.6 kJ mol21).

H2� �H�
Catalytic Pt

 � �28.6 kcal mol�1 (�120 kJ mol�1)

2 H2� �H�
Catalytic Pt

� �54.9 kcal mol�1 (�230 kJ mol�1)

Armed with these numbers, we can calculate the expected heat of hydrogenation of benzene,
as though it were simply composed of three double bonds like that of cyclohexene, but with
the extra resonance stabilization of conjugation as in 1,3-cyclohexadiene.

3 H2�
Catalyst

�H� � ?

��H� 3 (�H� of hydrogenation of ) � 3 (resonance correction in )
� (3

 � �28.6) � (3 � 2.3) kcal mol�1
� �85.8 � 6.9 kcal mol�1
� �78.9 kcal mol�1 (�330 kJ mol�1)

Figure 15-2 Orbital picture of the bonding in benzene. (A) The s framework is depicted as
straight lines except for the bonding to one carbon, in which the p orbital and the sp2 hybrids are
shown explicitly. (B) The six overlapping p orbitals in benzene form a p-electron cloud located
above and below the molecular plane. (C) The electrostatic potential map of benzene shows the
relative electron richness of the ring and the even distribution of electron density over the six
 carbon atoms.

A

C HH C
C

H
C

H

H

H
C C

H

H H

H

H

H

B C

Benzene bond Cloud

Csp2−Csp2

H1s−Csp2

π π

Cloudπ

 C h a p t e r 1 5 679

Now let us look at the experimental data. Although benzene is hydrogenated only
with diffi culty (Section 14-7), special catalysts carry out this reaction, so the heat of
hydrogenation of benzene can be measured: DH 8 5 249.3 kcal mol21, much less than the
278.9 kcal mol21 predicted.

Figure 15-3 summarizes these results. It is immediately apparent that benzene is much
more stable than a cyclic triene containing alternating single and double bonds. The differ-
ence is the resonance energy of benzene, about 30 kcal mol21 (126 kJ mol21). Other terms
used to describe this quantity are delocalization energy, aromatic stabilization, or simply
the aromaticity of benzene. The original meaning of the word aromatic has changed with
time, now referring to a thermodynamic property rather than to odor.

In Summary The structure of benzene is a regular hexagon made up of six sp2-hybridized
carbons. The C – C bond length is between those of a single and a double bond. The electrons
occupying the p orbitals form a p cloud above and below the plane of the ring. The struc-
ture of benzene can be represented by two equally contributing cyclohexatriene resonance
forms. Hydrogenation to cyclohexane releases about 30 kcal mol21 less energy than is
expected on the basis of nonaromatic models. This difference is the resonance energy of
benzene.

15-3 Pi Molecular Orbitals of Benzene
We have just examined the atomic orbital picture of benzene. Now let us look at the
molecular orbital picture, comparing the six p molecular orbitals of benzene with those of