Aromaticity cycloheptatrienyl cation and

Quinones, as oxidation products of aromatic systems, are capable of being reduced back to aromatic systems and some quinones are used particularly for this purpose. For example, as shown in Scheme 6.79, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) can be used to oxidize 1,2,3,4-tetrahydronaphthalene (tetraUn, CioFli2) to naphthalene (CioHg) while it is reduced to the corresponding phenol, 2,3-dichloro-5,6-dicyanohydroquinone (DDQ H2). Further, while reduced to the same product, DDQ is also capable of oxidizing 1,3,5-cycloheptatriene to the corresponding, aromatic cycloheptatrienyl cation and, if the oxidation is carried out in perchloric acid, the perchlorate of the cation is an isolable salt (Scheme 6.79). 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is we are not comparing the stability of the two to each other Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic Cycloheptatrienyl cation although aromatic is still a carbocation and reasonably reac tive toward nucleophiles Its special stability does not imply a rock like passivity but rather a much greater ease of formation than expected on the basis of the Lewis struc ture drawn for it A number of observations indicate that cycloheptatrienyl cation is far more stable than most other carbocations To emphasize its aromatic nature chemists often write the structure of cycloheptatrienyl cation m the Robinson circle m a ring style... 

Because charged ring systems can satisfy the Hiickel criteria for aromaticity, they are highly stabilized compared to other nonaromatic cations or anions. Examples include the cyclopropenyl cation, cycloheptatrienyl cation, and cyclopenta-dienyl anion. 

When we say cycloheptatriene is not aromatic but cycloheptatrienyl cation is, we are not comparing the stability of the two to each other. Cycloheptatriene is a stable hydrocarbon but does not possess the special stability required to be called aromatic. Cycloheptatrienyl cation, although aromatic, is still a carbocation and reasonably reactive toward... 

Benzene is the smallest member of the class of aromatic cyclic polyenes following Hiickel s (4 + 2) rule. Most of the 4n ir systems are relatively reactive anti- or nonaromatic species. Hiickel s rule also extends to aromatic charged systems, such as the cyclopentadienyl anion, cycloheptatrienyl cation, and cyclooctatetraene dianion. 

Figure 11.14 shows a molecular orbital diagrfflTt for cycloheptatrienyl cation. There are seven tt MOs, three of which are bonding and contain the six tt electrons of the cation. Cycloheptatrienyl cation is a Hiickel (4n + 2) system and is an aromatic ion. 

According to the Hiickel criteria for aromaticity, a molecule must be cyclic, conjugated (that is, be nearly planar and have ap orbital on each carbon) and have 4n + 2 tt electrons. Nothing in this definition says that the number of p orbitals and the number of nr elections in those orbitals must be the same. In fact, they can he different. The 4n + 2 rule is broadly applicable to many kinds of molecules and ions, not just to neutral hydrocarbons. For example, both the cydopentadienyl anion and the cycloheptatrienyl cation are aromatic. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

Problem 15.10 I Show the relative energy levels of the seven 77 molecular orbitals of the cvclohepta-trienyl system. Tel) which of the seven orbitals are filled in the cation, radical, and anion, and account for the aromaticity of the cycloheptatrienyl cation. 

Other kinds of substances besides benzene-like compounds can also be aromatic. For example, the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic ions. Pyridine, a six-membered, nitrogen-containing heterocycle, is aromatic and resembles benzene electronically. Pyrrole, a hve-membered heterocycle, resembles the cyclopentadienyl anion. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

Problem 10.6 Account for aromaticity observed in (o) 1,3-cyclopentadienyl anion but not 1,3-cyclopen-tadiene (b) 1,3,5-cycloheptatrienyl cation but not 1,3,5-cycloheptatriene (c) cyclopropenyl cation (d) the heterocycles pyrrole, furan and pyridine. 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

A special case of carbocation stability arises where the cation complies with the Hiiekel (4n+2) rule governing aromatic structures. Of these, the best known and most useful is the cycloheptatrienyl cation, more frequently referred to as the tropylium ion. For an informative and wide ranging account of structures, stabilities, properties and reactions of almost every type of carbocation, reference should be made to the series of monographs edited by Olah and Schleyer (18). 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

We can also possible to get aromatic ring. The cydopentadienyl anion and the cycloheptatrienyl cation are both aromatic. Both are cyclic and planar, containing six n electrons, and all the atoms in the ring are sp2 hybridised. 

Fig. (a) Cydopentadienyl anion (b) cycloheptatrienyl cation. Bicyclic and polycyclic systems can also be aromatic. 

Carbazole, like most aromatic amines, oxidizes readily via electron transfer. We recognized early that electron transfer may be an important initiation process for polymerizing the N-vinyl derivative. Some years ago we showed (29) that cycloheptatrienyl cation could act as an efficient one-electron transfer reagent, producing the appropriate cation radicals from reactive amines such as phenothiazine and tetramethyl-p-phenylene-diamine. It was also suggested that the product of the reaction between cycloheptatrientyl cation and carbazole itself was the carbazole cation radical. However, our recent work (21) has demonstrated that one-electron oxidation of carbazole leads directly to the 3,3-dicarbazoyl cation radical (VII). 

The cycloheptatrienyl cation is called the tropylium ion. This aromatic ion is much less reactive than most carbocations. Some tropylium salts can be isolated and stored for months without decomposing. Nevertheless, the tropylium ion is not necessarily as stable as benzene. Its aromaticity simply implies that the cyclic ion is more stable than the corresponding open-chain ion. 

The cycloheptatrienyl cation. This cation is aromatic (see the energy diagram above), and is frequently found at m/z 91 in the mass spectra of alkylbenzenes. (p. 728)... 

Draw just the bonding ir-MO s for the cycloheptatrienyl cation. Draw the energy diagram to show the relative energies of all the MO s, and show which orbitals the electrons would occupy in the ground state. Predict whether this ion is aromatic, antiaromatic, or nonaromatic. 

The cycloheptatrienyl cation has six n electrons (a Hiickel number) and is aromatic. 

Non-benzoid aromaticism. From the Hiiekel theory it follows that such systems as cyclopropenyl cation, cyclo-pentadienyl anion, cycloheptatrienyl cation (also called tropylium cation) and others must exhibit aromatic properties. 

Figure 15.6 Generation of the cycloheptatrienyl cation, radical, and anion. Only the six---electron cation is aromatic.

Figure 29.10. Aromatic compounds with 6 w electrons. Configuration of IT electrons in cyclopentadienyl anion, benzene, and cycloheptatrienyl cation.

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