Conjugated cyclohexatriene

The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

The heat of hydrogenation of the cyclic diene (8) is very nearly twice that of cyclohexene (7), and the heat of hydrogenation of the three double bonds in a Kekule structure might thus be expected to be of the order of 3 x -120 kJ ( — 28-6 kcal)mol = -360 kJ ( — 85-8 kcal) mol but when real benzene is hydrogenated only —208 kJ (-49-8 kcal) mol are evolved. Real benzene is thus thermodynamically more stable than the hypothetical cyclohexatriene by 151 kJ (36 kcal) mol this compares with only 17 kJ (4 kcal)mol by which a conjugated diene i stabilised, with respect to its analogue in which there is no interaction between the electrons of the double bonds. 

When a 6-carbon ring contains 2 donble bonds, it is called cychhexadiene, but when it has 3 double bonds, it is not called cyclohexatriene this is because a 6-carbon ring with three double bonds takes on a special kind of stability. The double bonds become completely conjugated and no longer behave as double bonds. The ring, known as a benzene ring, is said to be aromatic. 

There have been two general approaches to determining the amount of stabilization that results from aromatic delocalization. One is to use experimental thermodynamic measurements. Bond energies, as was mentioned in Chapter 1, are nearly additive when there are no special interactions between the various bond types. Thus, it is possible to calculate such quantities as the heat of combustion or heat of hydrogenation of cyclohexatriene by assuming that it is a compound with no interaction between the conjugated double bonds. For example, a very simple... 

When the conjugation is continued around the 1,3-cyclohexadiene ring to give what we might call 1,3,5-cyclohexatriene, matters change (Fig. 13.2). None of the reactions shown in Figure 13.1 is easy, and this molecule appears to be remarkably unreactive. It is clearly in a different class from 1,3-cyclohexadiene and the acyclic dienes. 

In Summary Acyclic extended conjugated systems show increasing thermodynamic stability but also kinetic reactivity because of the many sites open to attack by reagents and the ease of formation of delocalized intermediates. In contrast, the cyclohexatriene benzene is unusually stable and unreactive. 

Benzene has four degrees of unsaturation (see Table 11-6), satisfied by a 1,3,5-cyclohexatriene structure (one ring, three double bonds) first proposed by Kekuld and Loschmidt, yet it does not exhibit the kind of reactivity expected for a conjugated triene. In other words, its structure does not seem to correlate with its observed function. However, benzene is not completely umeactive. For example, it reacts with bromine, albeit only in the... 

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