Vollhardt  Capítulo 15 (Benzenos e Aromaticidade)

Vollhardt Capítulo 15 (Benzenos e Aromaticidade)


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temperatures as low as 22008C to give the two products shown. 
Explain mechanistically.
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H H
H H
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What happens when we fuse aromatic to antiaromatic 
rings? The general answer is that the resulting system 
tries to distort its antiaromatic substructures to minimize 
the destabilizing contributions of cyclic 4n p circuits. 
A case in point is the juxtaposition of benzene with 
cyclobutadiene, as in the series of molecules 
shown below, which was studied by one of your 
authors (P. V.).
 C h a p t e r 1 5 695
1,3,5,7-Cyclooctatetraene is nonplanar and nonaromatic
Let us now examine the properties of the next higher cyclic polyene analog of benzene, 
1,3,5,7-cyclooctatetraene, another 4n p cycle (n 5 2). Is it antiaromatic, like 1,3-
cyclobutadiene? First prepared in 1911 by Willstätter,* this substance is now readily 
available from a special reaction, the nickel-catalyzed cyclotetramerization of ethyne. 
It is a yellow liquid (b.p. 152 8C) that is stable if kept cold but that polymerizes when 
heated. It is oxidized by air, catalytically hydrogenated to cyclooctane, and subject to 
electrophilic additions and to cycloaddition reactions. This chemical reactivity is diag-
nostic of a normal polyene (Section 14-7).
Spectral and structural data confi rm the ordinary alkene nature of cyclooctatetraene. 
Thus, the 1H NMR spectrum shows a sharp singlet at d 5 5.68 ppm, typical of an alkene. 
The molecular-structure determination reveals that cyclooctatetraene is actually nonplanar 
molecules A \u2013 C, which contribute strongly to the respective 
overall structures. As shown for biphenylene A below, other 
resonance contributors are less (or un-) important. Here, 
because of symmetry, the extent of bond alternation is 
equal on both sides of the cyclobutadiene core. However, 
in phenylenes B and C, the outside rings, in order to pre-
serve their own aromaticity, \u201cgang up\u201d to maximize bond 
alternation in the center benzene, rendering it increasingly 
cyclohexatriene-like (see Section 15-2). An abbreviated, but 
quite descriptive, resonance picture is shown below.
Major Minor
Illustrative Resonance Forms of C
is that expected for three minimally interacting double 
bonds, as in the \u201chypothetical\u201d cyclohexatriene discussed 
in Section 15-2!
Hydrocarbons like the phenylenes are not only of funda-
mental interest in efforts to understand aromaticity, but also 
constitute potential building blocks for a new generation of 
organic electronic materials (see also Chemical Highlight 14-2). 
They are fl at, like graphite (Chemical Highlight 15-1), but 
they have activated electronic structures, a prerequisite for 
electron mobility.
These conclusions are based on the observed spectral, 
X-ray structural, and chemical properties of A \u2013 C. For 
example, the d value for the center \u201caromatic\u201d hydrogens 
in B is 6.13 ppm, refl ecting the presence of a diminished 
benzenoid ring current (Section 15-4). The extent of bond 
alternation between single (1.50 Å) and double (1.34 Å) 
bonds is maximal in the center cyclohexatriene of C, and 
both B and C undergo addition reactions typical of alkenes 
(Chapter 12). Most convincingly, the DH8hydrogenation of 
C [283.0 to 284.2 kcal mol21 (2347 to 2352 kJ mol21)] 
1 5 - 6 O t h e r C y c l i c P o l y e n e s : H ü c k e l \u2019 s R u l e
4 HC CH
70%
1,3,5,7-Cyclooctatetraene:
nonaromatic
Ni(CN)2,
70°C,
15\u201325 atm
q
*Professor Richard Willstätter (1872 \u2013 1942), Technical University, Munich, Germany, Nobel Prize 1915 
(chemistry).
696 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
Only cyclic conjugated polyenes containing (4n 1 2) pi electrons 
are aromatic
Unlike cyclobutadiene and cyclooctatetraene, certain higher cyclic conjugated polyenes are 
aromatic. All of them have two properties in common: They contain (4n 1 2) p electrons, 
and they are suffi ciently planar to allow for delocalization.
The fi rst such system was prepared in 1956 by Sondheimer*; it was 1,3,5,7,9,11,13,15,17-
cyclooctadecanonaene, containing 18 p electrons (4n 1 2, n 5 4). To avoid the use of such 
cumbersome names, Sondheimer introduced a simpler system for naming cyclic conjugated 
Exercise 15-13
On the basis of the molecular structure of 1,3,5,7-cyclooctatetraene, would you describe its double 
bonds as conjugated (i.e., does it exhibit extended p overlap)? Would it be correct to draw two 
resonance forms for this molecule, as we do for benzene? (Hint: Build molecular models of the 
two forms of 1,2-dimethylcyclooctatetraene.)
126.1°
117.6°
1.48 Å
1.34 Å
H
Figure 15-17 The molecular 
structure of 1,3,5,7-
cyclooctatetraene. Note the 
alternating single and double 
bonds of this nonplanar, 
nonaromatic molecule.
Exercise 15-14
Cyclooctatetraene A exists in equilibrium with less than 0.05% of a bicyclic isomer B, which is 
trapped by Diels-Alder cycloaddition to 2-butenedioic anhydride (maleic anhydride, Table 14-1) 
to give compound C.
B
A C
[
[
H
H
O
O
O
O
O O
What is isomer B? Show a mechanism for the A y B y C interconversion. (Hint: Work back-
ward from C to B and review Section 14-9.)
*Professor Franz Sondheimer (1926 \u2013 1981), University College, London.
and tub-shaped (Figure 15-17). The double bonds are nearly orthogonal (perpendicular) and 
not conjugated. Conclusion: The molecule is nonaromatic.
 C h a p t e r 1 5 697
polyenes. He named completely conjugated monocyclic hydrocarbons (CH)N as [N]annulenes, 
in which N denotes the ring size. Thus, cyclobutadiene would be called [4]annulene; benzene, 
[6]annulene; cyclooctatetraene, [8]annulene. The fi rst almost unstrained aromatic system in 
the series after benzene is [18]annulene.
[18]Annulene
(1,3,5,7,9,11,13,15,17-Cyclooctadecanonaene)
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H H
1 5 - 6 O t h e r C y c l i c P o l y e n e s : H ü c k e l \u2019 s R u l e
Exercise 15-15
The three isomers of [10]annulene shown here have been prepared and exhibit nonaromatic behavior. 
Why? (Hint: Build models!)
trans,cis,trans,cis,cis-
[10]Annulene
trans,cis,cis,cis,cis-
[10]Annulene
cis,cis,cis,cis,cis-
[10]Annulene
[18]Annulene is fairly planar and shows little alternation of the single and double bonds. 
The extent of the delocalization of its p electrons is pictured in the electrostatic potential 
map in the margin. Like benzene, it can be described by a set of two equivalent resonance 
forms. In accord with its aromatic character, the molecule is relatively stable and undergoes 
electrophilic aromatic substitution. It also exhibits a benzenelike ring-current effect in its 
1H NMR spectrum (see Problems 64 and 65).
Since the preparation of [18]annulene, many other annulenes have been made. As long 
as they are planar and delocalized, those with (4n 1 2) p electrons, such as benzene and 
[18]annulene, are aromatic, whereas those with 4n p electrons, such as cyclobutadiene and 
[16]annulene, are antiaromatic. When cyclic delocalization is prohibited by angle or steric 
strain, such as in cyclooctatetraene or [10]annulene (Exercise 15-15), the systems are non-
aromatic. Of course, cyclic polyenes in which there is no contiguous array of p orbitals are 
not annulenes and therefore also nonaromatic.
Cyclobutadiene:
planar, antiaromatic
Benzene:
planar, aromatic
Cyclooctatetraene:
nonplanar \ufffd nonaromatic
[10]Annulene:
nonplanar \ufffd nonaromatic
[12]Annulene:
planar \ufffd antiaromatic
The Annulenes and Other Cyclic Polyenes
[18]Annulene: aromatic
698 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
[14]Annulene:
planar \ufffd aromatic
[18]Annulene: