Cycloheptatrienyl anion, and

Following the instructions for drawing the tt molecular orbital energy levels of the compounds shown in Figure 15.2, draw the tt molecular orbital energy levels for the cyclohep-tatrienyl cation, the cycloheptatrienyl anion, and the cyclopropenyl cation. For each compound, show the distribution of the tt electrons. Which of the compounds are aromatic Which are andaromatic ... 

R. Breslow and W. Chu, Thermodynamic determination of pkfs of weak hydrocarbon acids using electrochemical reduction data. Triarylmethyl anions, cycloheptatrienyl anion, and triphenyl- and trialkylcyclopropenyl anions, J. Am. Chan. Soc. 95 (1973), pp. 411 18. 

PRACTICE PROBLEM 14.5 Apply the polygon-and-circle method to the cycloheptatrienyl anion and cation and... 

An interesting structural question involves the contribution of a dipolar structure which pictures the molecule as the fusion of a cyclopentadienide anion and a cycloheptatrienyl cation ... 

Even though resonance tells us that the negative charge in cycloheptatrienyl anion can be shared by all seven of its carbons, this delocalization offers little in the way of stabilization. Indeed with eight tt electrons, cycloheptatrienyl anion is antiaromatic and relatively unstable. 

Section 11.21 Species with six tt electrons that possess special stability include certain ions, such as cyclopentadienide anion and cycloheptatrienyl cation. 

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 10.30 Design a table showing the structure, number of tt electrons, energy levels of tt MO s and electron distribution, and state of aromaticity of (a) cyclopropienyl cation, b) cyclopropenyl anion, (c) cyclobutadiene, (d) cyclobutadienyl dication, (c) cyclopentadienyl anion, (/) cyclopentadienyl cation, (g) benzene, (h) cycloheptatrienyl anion, (/) cyclooctatetraene, (/ ) cyclooctatetraenyl dianion. ... 

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. 

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. 

Although the tropylium ion forms easily, the corresponding anion is difficult to form because it is antiaromatic. Cycloheptatriene (pKa = 39) is barely more acidic than propene (piTa = 43), and the anion is very reactive. This result agrees with the prediction of Htickel s rule that the cycloheptatrienyl anion is antiaromatic if it is planar. 

Compare the piQ of cyclopentadiene with that of cycloheptatriene. Whilst the anion of the former has 6 7t electrons (which makes it isoelectronic with benzene), the anion of the latter has 8 ti electrons. Remember that on p. 176 we saw how 4n n electrons made a compound anti-aromatic The cycloheptatrienyl anion does have 4 7t electrons but it is not anti-aromatic because it isn t planar. However, it certainly isn t aromatic either and its pKa of around 36 is about the same as that of propene. This contrasts with the cyclopropenyl anion, which must be planar since any three points define a plane. Now the compound is anli-aromatic and this is reflected in the very high pKit (about 62). Other compounds may become aromatic on losing a proton. We looked at fluorene a few pages back now you will see that fluorene is acidic because its anion is aromatic (14 n electrons). 

Both the cycloheptatrienyl radical and the anion arc reactive and difficult to prepare. The six-7r-electron cation, however, is extraordinarily stable. In fact, the cycloheptatrienyl cation was first prepared more than a century ago by reaction of Br2 with cycloheptatriene (Figure 15.7), although its structure was not recognized at the time. 

The cyclooctadiendiynes 93 and 94 are planar conjugated eight-electron systems (the four extra triple-bond electrons do not participate), which nmr evidence show to be antiaromatic.There is evidence that part of the reason for the lack of planarity in 78 itself is that a planar molecular would have to be antiaromatic. The cycloheptatrienyl anion (61) also has eight electrons, but does not behave like an aromatic system. The bond lengths for a series of molecules containing the cycloheptatrienide anion have recently been published. " The NMR spectrum... 

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