Benzenes Unusual Stability

HMO theory is named after its developer, Erich Huckel (1896-1980), who published his theory in 1930 [9] partly in order to explain the unusual stability of benzene and other aromatic compounds. Given that digital computers had not yet been invented and that all Hiickel s calculations had to be done by hand, HMO theory necessarily includes many approximations. The first is that only the jr-molecular orbitals of the molecule are considered. This implies that the entire molecular structure is planar (because then a plane of symmetry separates the r-orbitals, which are antisymmetric with respect to this plane, from all others). It also means that only one atomic orbital must be considered for each atom in the r-system (the p-orbital that is antisymmetric with respect to the plane of the molecule) and none at all for atoms (such as hydrogen) that are not involved in the r-system. Huckel then used the technique known as linear combination of atomic orbitals (LCAO) to build these atomic orbitals up into molecular orbitals. This is illustrated in Figure 7-18 for ethylene. 

Energies for hydrogenation reactions are often employed to probe for unusual stability or instability. For example, the observation that the first step in the hydrogenation of benzene (to 1,3-cyclohexadiene) is slightly endothermic while the remaining two steps (to cyclohexene and then to cyclohexane) are strongly exothermic, i.e. 

The greatest theoretical interest has been devoted to the six-membered boron-nitrogen heterocycles, apparently due to the unusual stability of these compounds, and to their formal similarity to benzene. Most work in the field has been reviewed (77HC(30)38l), in which some unpublished material is also presented. 

Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

Many other compounds discovered in the nineteenth century seemed to be related to benzene. These compounds also had low hydrogen-to-carbon ratios as well as pleasant aromas, and they could be converted to benzene or related compounds. This group of compounds was called aromatic because of their pleasant odors. Other organic compounds without these properties were called aliphatic, meaning Tatlike. As the unusual stability of aromatic compounds was investigated, the term aromatic came to be applied to compounds with this stability, regardless of their odors. 

Visualizing benzene as a resonance hybrid of two Kekule structures cannot fully explain the unusual stability of the aromatic ring. As we have seen with other conjugated systems, molecular orbital theory provides the key to understanding aromaticity and predicting which compounds will have the stability of an aromatic system. 

The unusual stability of the benzene ring dominates the chemical reactions of benzene and naphthalene. Both compounds resist addition reactions which lead to destruction of the aromatic ring. Rather, they undergo substitution reactions, discussed in detail in Chapter 11, in which a group or atom replaces an H... 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

The unusual stability of the aromatic sextet suggests that benzene will be resistant to oxidation and reduction of the ring, since both processes will destroy the aromaticity. Although this is generally the case, both types of reaction are possible under certain conditions. This chapter is restricted to benzene and its derivatives, but other aromatic systems are more easily oxidized and reduced (see Chapter 12). 

Considering benzene as the hybrid of two resonance structures adequately explains its equal C—C bond lengths, but does not account for its unusual stability and lack of reactivity towards addition. 

The low heat of hydrogenation of benzene means that benzene is especially stable, even more so than the conjugated compounds introduced in Chapter 16. This unusual stability is characteristic of aromatic compounds. 

Kekule s structure, then, accounts satisfactorily for facts (a), ( ), and (c) in Sec. 10.3. But there are a number of facts that are still not accounted for by this structure most of these unexplained facts seem related to unusual stability of the benzene ring. The most striking evidence of this stability is found in the chemical reactions of benzene. 

Finally, the unusual stability of benzene is not unusual at all it is what one would expect of a hybrid of equivalent structures. The 36 kcal of energy that l enzene does not contain—compared with cyclohexatriene—is resonance energy. It is the 36 kcal of resonance energy that is responsible for the new set of proper-fws we c gll aromatic properthst ... 

A double bond that is separated from a benzene ring by one single bond is said to be conjugated with the ring. Such conjugation confers unusual stability on... 

The unusual stability of benzene (and other aromatic molecules) means that it undergoes substitution rather than addition reactions (cf. alkenes/alkynes). This is because substitution reactions lead to the formation of products which retain the stable aromatic ring. 

The literal meaning of aromaticity is fragrance, but the word has a special meaning in chemistry. Aromaticity has to do with the unusual stability of the compound benzene and its derivatives, as well as certain other unsaturated ring componnds. The stmctnres of these componnds are often shown to contain donble bonds, bnt they do not actually behave like double bonds. For example, reagents snch as bromine react with benzene by substitntion rather than addition. Benzene and its derivatives had long been referred to as aromatic becanse of their distinctive odors. 

The Kekule structures of benzene account for the molecular formula of benzene and for the number of isomers obtained as a result of substitution. However, they fail to account for the unusual stability of benzene and for the observation that the double bonds of benzene do not undergo the addition reactions characteristic of alkenes. That benzene had a six-membered ring was confirmed in 1901, when Paul Sabatier (Section 4.11) found that the hydrogenation of benzene produced cyclohexane. This, however, still did not solve the puzzle of benzene s structure. 

Because benzene and cyclohexatriene have different energies, they must be different compounds. Benzene has six delocalized tt electrons, whereas hypothetical cyclohexatriene has six localized tt electrons. The difference in their energies is the resonance energy of benzene. The resonance energy tells us how much more stable a compound with delocalized electrons is than it would be if its electrons were localized. Benzene, with six delocalized tt electrons, is 36 kcal/mol more stable than hypothetical cyclohexatriene, with six localized tt electrons. Now we can understand why nineteenth-century chemists, who didn t know about delocalized electrons, were puzzled by benzene s unusual stability (Section 7.1). 

Cyclobutadiene has two pairs of rr electrons, and cyclooctatetraene has four pairs of tt electrons. Unlike benzene, these compounds are not aromatic because they have an even number of pairs of rr electrons. There is an additional reason why cyclooctatetraene is not aromatic—it is not planar but, instead, tub-shaped. Earlier, we saw that, for an eight-membered ring to be planar, it must have bond angles of 135° (Chapter 2, Problem 28), and we know that sp carbons have 120° bond angles. Therefore, if cyclooctatetraene were planar, it would have considerable angle strain. Because cyclobutadiene and cyclooctatetraene are not aromatic, they do not have the unusual stability of aromatic compounds. 

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