Hexatrienes conjugation compared with benzene

We reach a similar conclusion when comparing benzene with the open-chain conjugated tr-iene (Z)-l,3,5-hexatriene. Here we compare two real molecules, both conjugated trienes, but one is cyclic and the other is not. The heat of hydrogenation of... 

A cyclic conjugated system containing An + 2)ji electrons has an extra stability over that of a comparable number of isolated double bonds. This extra stabilization, known as aromaticity, leads to a characteristic pattern of reactivity which distinguishes the reactions of benzene (1.8) from, for example, the linear hexatriene (1.9) or cyclooctatetraene (1.10) An electrons, n = 2). The aromatic sextet may arise not just from the overlap of three double bonds as in benzene (1.8) or pyridine (1.11) but also from the participation of the lone pair of electrons on a heteroatom. Thus pyrrole (1.12), with effectively six n-electrons, shows some aromatic character. In allene (1.13) the double bonds are at 90° to each other and conjugation does not occur. 

We have just examined the atomic orbital picture of benzene. Now let us look at the molecular orbital picture, comparing the six tt molecular orbitals of benzene with those of 1,3,5-hexatriene, the open-chain analog. Both sets are the result of the contiguous overlap of six p orbitals, yet the cyclic system differs considerably from the acyclic one. A comparison of the energies of the bonding orbitals in these two compounds shows that cyclic conjugation of three double bonds is better than acyclic conjugation. 

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