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 \u2013 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
\u2211
 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 \u2014 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 \u201cbenzene.\u201d 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 \u2014 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 \u201ccyclohexatriene,\u201d we should expect the C \u2013 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 \u2013 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 \u2013 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\ufffd \ufffdH\ufffd
Catalytic Pt
 \ufffd \ufffd28.6 kcal mol\ufffd1 (\ufffd120 kJ mol\ufffd1)
2 H2\ufffd \ufffdH\ufffd
Catalytic Pt
\ufffd \ufffd54.9 kcal mol\ufffd1 (\ufffd230 kJ mol\ufffd1)
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\ufffd
Catalyst
\ufffdH\ufffd \ufffd ?
\ufffd\ufffdH\ufffd 3 (\ufffdH\ufffd of hydrogenation of ) \ufffd 3 (resonance correction in )
\ufffd (3
 \ufffd \ufffd28.6) \ufffd (3 \ufffd 2.3) kcal mol\ufffd1 
\ufffd \ufffd85.8 \ufffd 6.9 kcal mol\ufffd1
\ufffd \ufffd78.9 kcal mol\ufffd1 (\ufffd330 kJ mol\ufffd1) 
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\u2212Csp2
H1s\u2212Csp2
\u3c0 \u3c0
Cloud\u3c0
 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 \u2013 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