Delocalisation addition reactions

R groups in which the C=0 group is conjugated with C=C (1,4-addition can also compete here, cf. p. 200), or with a benzene ring, also exhibit slower addition reactions than their saturated analogues. This is because the stabilisation, through delocalisation, in the initial carbonyl compounds (7 and 8) is lost on proceeding to the adducts (9 and 10), and to the transition states that precede them ... 

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. 

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. 

Although [14]annulene supports a diamagnetic ring current and appears to have significant delocalisation of its ir-electrons, it undergoes a cyclo-addition reaction with maleic anhydride [46]. 

The reaction of the diene with a free radical produces an allyl radical having unpaired electron delocalised over more than one carbon atom. The allyl free radical can undergo 1, 2, or 1, 4 addition. 

Electrophilic addition to die alkene with the formation of a 5-membered ring occurs on anodic oxidation of 44 [150]. Reaction of the intermediate delocalised carbonium ion with the adjacent hydroxyl function in 45 also results in the formation of a 5-merabered ring [151],... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

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