Structure and Stability of Benzene

Although benzene is clearly unsaturated, it is much less reactive than typical alkenes and fails to undergo the usual alkene addition reactions. Cyclohexene, for instance, reacts rapidly with Br2 and gives the addition product 1,2-dihromo-cyclohexane, hut benzene reacts only slowly with Br2 and gives the substitution product CeHsBr. 

We can get a quantitative idea of benzene s stability by measuring heats of hydrogenation (Section 7.5). Cyclohexene, an isolated alkene, has AH°hydrog = -118 kj/mol (-28.2 kcal/mol), and cyclohexa-1,3-diene, a conjugated diene, has AH°hydrog = 230 kJ/mol (-55.0 kcal/mol). As noted in Section 8.12, this value for cyclohexa-1,3-diene is a bit less than twice that for cyclohexene because conjugated dienes are more stable than isolated dienes. 

FIGURE 9.1 A comparison of the heats of hydrogenation for cyclohexene, cyclohexa-1,3-diene, and benzene. Benzene is 150 kj/mol (36 kcal/mol) more stable than might be expected for cyclohexatriene.  

Further evidence for the unusual nature of benzene is that all its carbon-carbon bonds have the same length—139 pm—intermediate between typical single (154 pm) and double (134 pm) bonds. In addition, an electrostatic 

Because all six carbon atoms and all six p orbitals in benzene are equivalent, it s impossible to define three localized TThonds in which a given p orbital overlaps only one neighboring p orbital. Rather, each p orbital overlaps equally well with both neighboring p orbitals, leading to a picture of benzene in which the six 7T electrons are completely delocalized around the ring. In resonance terms (Sections 2.4 and 2.5), benzene is a hybrid of two equivalent forms. Neither form is correct by itself the true structure of benzene is somewhere in between the two resonance forms but is impossible to draw with our usual conventions. 

Carrying the process one step further, we might expect AH hydrog for cyclo-hexatriene (benzene) to be a bit less than 356 kJ/mol, or three times the 

Further evidence for the unusual nature of benzene is that all its carbon-carbon bonds have the same length—139 pm—intermediate between typical single (154 pm) and double (134 pm) bonds. In addition, an electrostatic potential map shows that the electron density in all six C C bonds is identical. Thus, benzene is a planar molecule with the shape of a regular hexagon. All C-C-C bond angles are 120°, all six carbon atoms are sp -hybridized, and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

Chemists sometimes represent the two benzene resonance forms by using a circle to indicate the equivalence of the carbon-carbon bonds. This kind of representation has to be used carefully, however, because it doesn t indicate the number of tt electrons in the ring. (How many electrons does a cucle represent ) In this book, benzene and other aromatic compounds will be represented by a single line-bond stmcture. We ll be able to keep count of it electrons this way but must be aware of the limitations of the drawings. 

Alternative representations of benzene. The circle representation must be used carefully since it doesn t indicate the number of tt electrons in the ring. 

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One explanation for the structure and stability of benzene and other arenes is based on resonance according to which benzene is regarded as a hybrid of the two Kekule structures... 

Structure and Stability of Benzene Molecular Orbital Theory 521... 

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