Benzene delocalisation energy

One final word of caution may be sounded on the delocalisation energies discussed in 3.2a and tabulated in Table 3-1. They have been calculated without reference whatever to any changes in the ex-framework which may be required to go from a hypothetical Kekule-structure to an actual, delocalised , conjugated system—changes which, energetically, may sometimes be quite large. For example, to convert a benzene Kekule-... 

The viewpoint expressed by Glendening et al. [62] is that the resonance between the structures is the key factor for delocalisation. We find this as well. When there is no resonance in cyclobutadiene (and benzene), the molecule becomes asymmetric. Just resonance is not enough, however. Both benzene and square cyclobutadiene have large resonance energies. 

The majority of the reactions of benzene are substitution reactions and not. as might be expected, addition reactions. The reason is that the continuous cloud of electrons above and below the carbon hexagon is very stable and it takes energy to break it. The preferred reaction is to replace a hydrogen atom so that the delocalised ring structure is kept intact. This is best achieved by substitution reactions. Addition across the double bonds would destroy the delocalised electron cloud of the ring. These addition reactions are not very common for benzene and similar compounds, although they are possible. 

Benzene is the archetypal example of a compound that displays aromatic properties. Aromatic compounds are characterised by a special stability over and above that which would be expected as a result of the delocalisation of the double bonds in a linear system. Typically, this extra stability is associated with the closed loop of six electrons, the aromatic sextet, as occurs in benzene itself. However, larger and smaller loops are possible. So long as there are (4n+2)7i electrons (where n is an integer from zero, upwards) present in (at least three) adjacent p sub-orbitals that form a closed circuit, then the resultant molecule will be aromatic. It is also possible for heteroatoms to form part of the cyclic structure, and for the structure to be charged. Furthermore, aromatic compounds, in contrast to unsaturated compounds, tend to undergo substitution reactions more readily than addition reactions. This is because it is usually thermodynamically favourable to preserve the aromatic stability rather than release the energy contained in the double bonds. 

Attack at the 3-position 6 is not so bad as the intermediate cation 7 is no longer delocalised onto the electron-deficient nitrogen atom but is not as stable as the benzene intermediate 5 and the slow step is very slow because the HOMO of pyridine is lower in energy than the HOMO of benzene. 

For benzene, the molecular orbital theory states that the six p-orbitals combine to give six molecular orbitals. The three lower-energy molecular orbitals are bonding molecular orbitals, and these are completely filled by the six electrons (which are spin-paired). There are no electrons in the (higher-energy) antibonding orbitals, and hence benzene has a closed bonding shell of delocalised Jt-electrons. 

The delocalisation of electrons means that benzene is unusually stable. It has a heat of hydrogenation which is approximately I50k.lmol 1 less than that which would be expected for a cyclic conjugated triene. This is known as the resonance energy. 

It has been suggested that cyclic polyalkenes having 4n ir-electrons may not only lack special aromatic stability but may indeed suffer destabilisation consequent upon delocalisation of the ir-electrons, and have been called antiaromatio. [28]. The difference between aromatic benzene and antiaromatic cyclobutadiene has been pictorial ly represented by energy diagrams such as the following ... 

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