Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)


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the sun in this wavelength region. Some 
individuals cannot tolerate PABA because of an allergic skin reaction, and many recent sun 
creams use other compounds for protection from the sun (\u201cPABA-free\u201d).
Exercise 15-5
The thermal ring opening of cyclobutene to 1,3-butadiene is exothermic by about 10 kcal mol21 
(Section 14-9). Conversely, the same reaction for benzocyclobutene, A, to compound B (shown 
in the margin) is endothermic by the same amount. Explain.
\ufffdH\ufffd ~ \ufffd10 kcal mol\ufffd1
A
B
Suntan lotions are applied to pro-
tect the skin from potential cancer-
causing high-energy UV radiation. 
They contain substances, such 
as PABA, that absorb light in the 
damaging region of the electro-
magnetic spectrum.
Figure 15-6 The distinctive 
 ultraviolet spectra of benzene: 
lmax(e) 5 234(30), 238(50), 
243(100), 249(190), 255(220), 
261(150) nm; and 1,3,5-hexatriene: 
lmax(e) 5 247(33,900), 258(43,700), 
268(36,300) nm. The extinction 
coeffi cients e of the absorptions
of 1,3,5-hexatriene are very much 
larger than those of benzene; 
therefore the spectrum at the right 
was taken at lower concentration.
COOH
NH2
4-Aminobenzoic acid
(p-Aminobenzoic acid, PABA)
Benzene
1,3,5-Hexatriene
230 240 250 260 270 280
A
bs
or
ba
nc
e 
(A
)
230 240 250 260 270 280
A
bs
or
ba
nc
e 
(A
)
Wavelength (nm) Wavelength (nm)
UV UV
 C h a p t e r 1 5 683
The infrared spectrum reveals substitution patterns in 
benzene derivatives
The infrared spectra of benzene and its derivatives show characteristic bands in three 
regions. The fi rst is at 3030 cm21 for the phenyl \u2013 hydrogen stretching mode. The second 
ranges from 1500 to 2000 cm21 and includes aromatic ring C \u2013 C stretching vibrations. 
Finally, a useful set of bands due to C \u2013 H out-of-plane bending motions is found between 
650 and 1000 cm21.
690\u2013710
730\u2013770
690\u2013710
750\u2013810
735\u2013770 790\u2013840
Typical Infrared C\u2013H Out-of-Plane Bending Vibrations
for Substituted Benzenes (cm\ufffd1)
R R R R
R
R
R
Their precise location indicates the specifi c substitution pattern. For example, 1,2-
dimethylbenzene (o-xylene) has this band at 738 cm21, the 1,4 isomer at 793 cm21; the 
1,3 isomer (Figure 15-7) shows two absorptions in this range, at 690 and 765 cm21.
1 5 - 4 S p e c t r a l C h a r a c t e r i s t i c s o f t h e B e n z e n e R i n g
The mass spectrum of benzene indicates stability
Figure 15-8 depicts the mass spectrum of benzene. Noticeable is the lack of any signifi cant 
fragmentation, attesting to the unusual stability of the cyclic six-electron framework (Sec-
tion 15-2). The (M 1 1)1\u2022 peak has a relative height of 6.8%, as expected for the relative 
abundance of 13C in a molecule made up of six carbon atoms.
The NMR spectra of benzene derivatives show the effects of
an electronic ring current
1H NMR is a powerful spectroscopic technique for the identifi cation of benzene and its 
derivatives. The cyclic delocalization of the aromatic ring gives rise to unusual deshielding, 
which causes the ring hydrogens to resonate at very low fi eld (d < 6.5 \u2013 8.5 ppm), even lower 
than the already rather deshielded alkenyl hydrogens (d < 4.6 \u2013 5.7 ppm, see Section 11-4).
CH3
CH2 CH3
Allylic:
 \ufffd 1.68 ppm\ufffd
Benzylic:
 \ufffd 2.35 ppm\ufffd
CH
Chemical Shifts of
Allylic and Benzylic
Hydrogens
P O
Figure 15-7 The infrared 
 spectrum of 1,3-dimethylbenzene 
(m-xylene). There are two C \u2013 H 
stretching absorptions, one due to 
the aromatic bonds (3030 cm21), 
the other to saturated C \u2013 H bonds 
(2920 cm21). The two bands at 
690 and 765 cm21 are typical of 
1,3-disubstituted benzenes.
0
100
Tr
an
sm
itt
an
ce
 (%
 
)
4000 3500 3000 2500 2000 1500 1000 600 cm \u22121
Wavenumber
H
H
H
H
H
H
H
H C
C
HH
IR
684 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
The 1H NMR spectrum of benzene, for example, exhibits a sharp singlet for the six equiv-
alent hydrogens at d 5 7.27 ppm. How can this strong deshielding be explained? In a simpli-
fi ed picture, the cyclic p system with its delocalized electrons may be compared to a loop of 
conducting metal. When such a loop is exposed to a perpendicular magnetic fi eld (H0), the 
electrons circulate (called a ring current), generating a new local magnetic fi eld (hlocal). This 
induced fi eld opposes H0 on the inside of the loop (Figure 15-9), but it reinforces H0 on the 
outside \u2014 just where the hydrogens are located. Such reinforcement results in deshielding. The 
effect is strongest close to the ring and diminishes rapidly with increasing distance from it. 
Thus, benzylic nuclei are deshielded only about 0.4 to 0.8 ppm more than their allylic coun-
terparts, and hydrogens farther away from the p system have chemical shifts that do not differ 
much from each other and are similar to those in the alkanes.
H H
hlocal
hlocalhlocal
hlocal
Ring current
of circulating 
electrons
Induced local
magnetic field,
hlocal
External field, H0
Opposes H0 in this
region of space
Strengthens
H0 in this
region of space
Strengthens
H0 in this
region of space
Opposes H0 in this
region of space
Figure 15-9 The p electrons of 
benzene may be compared to 
those in a loop of conducting 
metal. Exposure of this loop of 
electrons to an external magnetic 
fi eld H0 causes them to circulate. 
This \u201cring current\u201d generates a 
 local fi eld, reinforcing H0 on the 
outside of the ring. Thus, the 
 hydrogens resonate at lower fi eld.
Figure 15-8 The mass spectrum 
of benzene reveals very little 
 fragmentation.
R
el
at
iv
e 
ab
u
n
da
nc
e
0 20 40 60 10080 120
78
0
100
m/z
M\ufffd\u2022
Molecular ion
50
MS
 C h a p t e r 1 5 685
Whereas benzene exhibits a sharp singlet in its NMR spectrum, substituted derivatives 
may have more complicated patterns. For example, introduction of one substituent renders 
the hydrogens positioned ortho, meta, and para nonequivalent and subject to mutual coupling. 
An example is the NMR spectrum of bromobenzene, in which the signal for the ortho hydro-
gens is shifted slightly downfi eld relative to that of benzene. Moreover, all the protons are 
coupled to one another, thus giving rise to a complicated spectral pattern (Figure 15-10).
Figure 15-11 shows the NMR spectrum of 4-(N,N-dimethylamino)benzaldehyde. The 
large chemical shift difference between the two sets of hydrogens on the ring results in a 
near-fi rst-order pattern of two doublets. The observed coupling constant is 9 Hz, a typical 
splitting between ortho protons.
1 5 - 4 S p e c t r a l C h a r a c t e r i s t i c s o f t h e B e n z e n e R i n g
Figure 15-11 300-MHz 1H NMR 
spectrum of 4-(N,N-dimethylamino) 
benzaldehyde (p-dimethylamino-
benzaldehyde). In addition to the 
two aromatic doublets (J 5 9 Hz), 
there are singlets for methyl 
(d 5 3.09 ppm) and the aldehydic 
hydrogens (d 5 9.75 ppm).
Figure 15-12 300-MHz 1H NMR 
spectrum of 1-methoxy-2,4-
dinitrobenzene (2,4-dinitroanisole).
A fi rst-order spectrum revealing all three types of coupling is shown in Figure 15-12. 
1-Methoxy-2,4-dinitrobenzene (2,4-dinitroanisole) bears three ring hydrogens with different 
chemical shifts and distinct splittings. The hydrogen ortho to the methoxy group appears at 
1H NMR
456789 3 2 1 0
ppm ( )\u3b4
1 H 2 H 2 H
6 H
(CH3)4Si
CHO
H
H
H
H
N(CH3)2
7.77.8
9.79.8
6.66.76.8
ppm
ppm
ppm
456789 3 2 1 0
ppm ( )\u3b4
1 H 1 H
1 H
3 H
(CH3)4Si
8.78.8
8.48.5
7.27.3
NO2
H
H
NO2
OCH3
H
1H NMR
ppm
ppm
ppm
1H NMR
6789
ppm ( )\u3b4
2 H 3 H
7.4
ppm
ppm7.6 7.5
7.27.3
Br
H
H
H
H
H
Figure 15-10 A part of the 
300-MHz 1H NMR spectrum of 
bromobenzene,