Stability of the Benzene Ring

A single bond has a bond order of 1 and a double bond a bond order of 2. 

A double-headed arrow, is used to connect resonance structures. 

It is emphasized that the circle inside a ring represents exclusively six n -electrons. 

The three Dewar structures 5, 11 and 12 (Dewar benzene) are also considered to contribute to the resonance hybrid (according to valence bond theory, approximately 20% in total) and to the extra stability. Dewar benzene has now been prepared. It is a bent, non-planar molecule and is not aromatic. It gradually reverts to benzene at room temperature. The Ladenburg structure, prismane i6), is an explosive liquid. Dewar benzene and prismane are valence isomers of benzene. 

The double bond in but-1-ene, CH pH.,CH=CH, is fixed between C-1 and C-2 that is. it is localized. However, the double bonds in buta-1,3-diene, 

=CH-CH=CH. are not localized, but are spread over the whole molecule, and are said to be delocalized  

X-ray crystallographic analysis indicated that benzene is a planar, regular hexagon in which all the carbon-carbon bond lengths are 139 pm, intermediate between the single C-C bond in ethane (154 pm) and the C=C bond in ethene (134 pm), and therefore all have some double bond character. Thus the representation of benzene by one Kekule structure is unsatisfactory. The picture of benzene according to valence bond the-  

Although the canonical forms for benzene are imaginary and do not exist, the structure of benzene will be represented by one of the Kekule structures throughout this book. This is common practice. A circle within a hexagon as in 10, symbolic of the Jt-cloud, is sometimes used to represent benzene. 

The current understanding of the structure of benzene is based on molecular orbital (MO) theory. The six carbon atoms of benzene are sp2 hybridized. The three sp2 hybrid orbitals of each carbon atom, which are arranged at angles of 120°, overlap with those of two other carbon atoms and with the s orbital of a hydrogen atom to form the planar a-bonded skeleton of the benzene ring. The p orbital associated with each carbon contains one electron and is perpendicular to the plane of the ring. 

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In this process, it is very uncommon to detect any benzene rings brealdng up. As in the olefins plant, the thermal stability of the benzene ring is demonstrated by its survival under these severe operating conditions, especially the high temperatures. 

Benzene is unusually stable and it is the delocalised electrons that account for this stability. The presence of the delocalised electrons also explains why benzene does not undergo addition reactions. Addition reactions would disrupt the electron delocalisation and so reduce the stability of the ring. Substitution reactions, on the other hand, can occur without any such disruption and the stability of the benzene ring is maintained. The delocalised electrons in the % molecular orbital make benzene susceptible to attack by electrophiles (electron pair acceptors). As a result, benzene undergoes electrophilic substitution reactions and some of these are outlined at the top of the next page. Note that the electrophiles are shown in red, the reagents in blue and the reaction names in green. 

An extreme example of the stabilization of an enol by electron de-localization is benzenol (phenol), which exists 100% in the enol form. In this case the extra stability of the benzene ring is the important factor ... 

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... 

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. 

Stability of the benzene ring. Heats of hydrogenation and combustion... 

For the two carbonyl compounds given, only acetophenone has an a-hydrogen which is readily ionized. The benzaldehyde a-hydrogen is not acidic due to resonance stabilization of the benzene ring. Thus, the only carban-ions formed in significant concentration will be... 

The unique bonding characteristics of aromatics, the stability of the benzene ring system, and the varied and often complex chemistry of arenes and hetero-arenes have fascinated chemists for close to two centuries. Benzene rings are omnipresent in organic chemistry and they find important applications in the pharma, agrochemical, and polymer fields. New applications of aromatics include sectors such as functional materials and molecular machines. 

Thus, only dehydrogenation of cyclohexane is possible to any extent at these temperatures, this in point of fact being due to the far greater stabilization of the benzene ring by resonance compared with that of the cyolopentadiene and cycloheptatriene rings. 

Resonance re-z9n-9n(t)s, r9z-n9n(t)s (15c) n. In chemistry, the periodic cycling of electrons from one atom of a molecule or ion to another atom of the same molecule or ion. Thus, given atoms remain in a fixed spatial arrangement with their electrons oscillating between atoms so as to satisfy two (or more) possible structural formulas. Resonance was first conceived to account for the outstanding stability of the benzene ring and it took almost a century for researchers to prove its reality. A large-amplitude vibration of a mechanical or electrical system caused by a relatively small... 

Unsubstituted aromatic hydrocarbons (see Sections 23-5 and 23-6) are quite resistant to oxidation by chemical oxidizing agents. The reactions of strong oxidizing agents with aikyibenzenes illustrate the stability of the benzene ring system. Heating toluene with a ba-... 

The reaction leading from benzene and an electrophile to the arenium ion is highly endothermic, because the aromatic stability of the benzene ring is lost. The reaction leading from the arenium ion to the substituted benzene, by contrast, is highly exothermic because it restores aromaticity to the system. 

Therefore, the rule that tells us to add the electrophile to the s] carbon bonded to the most hydrogens predicts that approximately equal amounts of the two products will be formed. When the reaction is carried out, however, only one of the products is obtained. (Notice that the stability of the benzene ring prevents its double bonds from undergoing electrophilic addition reactions.)... 

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