1.4- and 1,3-cyclohexadiene

Summarizing all of the above, it would appear that 1,3- and 1,4-cyclohexadiene have nearly identical enthalpies of formation. Does this mean that the 1,3-isomer is destabilized and/or that the 1,4-isomer is stabilized Let us accept an enthalpy of formation of ca 105 kJmoD1 for the enthalpy of formation of both isomers. In the absence of any stabilization or destabilization, we would expect the cyclohexene disproportionation reaction 25... 

Our uncertainty is derived in part from the lack of a measured enthalpy of vaporization, cf Reference 67. However, what triggered our skepticism is the observation that the isomeric 1,2-and 1,4-dihydronaphthalenes have reported enthalpies of formation that differ by ca 13 kJ mol-1 while the corresponding species lacking the benzene ring, the isomeric 1,3- and 1,4-cyclohexadienes, are almost isoenergetic (see Section V.D of this chapter). From J. F. Liebman, in The Cyclophanes (Eds. P. M. Keehn and S. M. Rosenfeld), Academic Press, New York, 1983,... 

Michael and Hart10 found that the reaction of OH radicals (formed by pulse radiolysis of aqueous solutions saturated with N20) with 1,3- and 1,4-cyclohexadienes leads to formation of an intermediate absorbing at 310 nm. In the case of 1,4-cyclohexadiene, another band at A, < 240 nm was also found. In this system there are both H atoms and OH radicals, however the yield of the OH radicals is 10 times higher than that of the H- atoms. Michael and Hart10 assumed that the band at 310 nm is due to CeWi ... 

Von Sonntag and coworkers14 repeated Michael and Hart s study of the reaction of OH radical with 1,3- and 1,4-cyclohexadienes and extended it. They found that in the case of 1,4-cyclohexadiene, 50% of the OH radicals abstract an hydrogen atom, while only about 25% of the OH radicals abstract an hydrogen atom from 1,3-cyclohexadiene. The remaining OH radicals probably add to the double bond. The addition to the double bond was confirmed by final products analysis in the case of the 1,4-isomer. When N20-saturated aqueous solution of 1,4-cyclohexadiene (10-2 M) together with lower (10-4 M) concentration of the thiol (1,4-dithiothreitol) was y-radiolysed, it was found that 4-hydroxycyclohexene was produced with a yield of 0.29 prnol J 1, i.e. a yield of 50% of the OH radicals (equation 9). 

Okada and coworkers78 studied the /-radiolysis of 1,3- and 1,4-cyclohexadienes. From... 

The VEEL spectrum of 1,3-cyclohexadiene on Pt(lll) at 95 K has already been discussed in relation to the thermal evolution of cyclohexene adsorbed on that surface (Section VI.A.l and ref 240). Palazov and Shopov et al. (241, 242) reported that cyclohexadiene adsorbed on Ni/SiOz at room temperature decomposes into benzene on evacuation. SER spectra of 1,3-and 1,4-cyclohexadiene have been obtained indicating 77-complexes on a gold electrode (83), and the general results have been discussed earlier in Section VI.A.2. 

B2C14 can add to dienes (1,3-butadiene, 1,3- and 1,4-cyclohexadienes) to yield either mono- or diadducts depending on the reagent reactant ratio.474-477 1,3-Cyclohexadiene, for example, forms the unstable l,2,3,4-tetrakis(dichloroboryl)cy-clohexane with high selectivity when it reacts with a threefold excess of B2C14.476 Naphthalene yields a similar diadduct.476 477... 

Vinylcyclohexene formed a rather stable intermediate which persisted even after a day at room temperature. The structure is thought to be 16, arising from isomerization of the internal double bond. An alternative possibility, 17, is considered less likely because 1,3- and 1,4-cyclohexadiene gave much more reductive elimination product after a day. 

The hydroformylation of conjugated dienes with unmodified cobalt catalysts is slow, since the insertion reaction of the diene generates an tj3-cobalt complex by hydride addition at a terminal carbon (equation 10).5 The stable -cobalt complex does not undergo facile CO insertion. Low yields of a mixture of n- and iso-valeraldehyde are obtained. The use of phosphine-modified rhodium catalysts gives a complex mixture of Cs monoaldehydes (58%) and C6 dialdehydes (42%). A mixture of mono- and di-aldehydes are also obtained from 1,3- and 1,4-cyclohexadienes with a modified rhodium catalyst (equation ll).29 The 3-cyclohexenecarbaldehyde, an intermediate in the hydrocarbonylation of both 1,3- and 1,4-cyclo-hexadiene, is converted in 73% yield, to the same mixture of dialdehydes (cis.trans = 35 65) as is produced from either diene. 

It is interesting to note that the archival enthalpies of formation of 1,2- and 1,4-dihydronaphthalene differ by 12.7 2.3 kJ mol-1 and those of their 1-ring de-benzoated counterparts 1,3- and 1,4-cyclohexadiene have been suggested to be nearly identical (see Reference 8). 

Hohoyd and coworkers studied the attachment of excess electrons to 1,3-butadiene in n-hexane solution, and the detachment of an electron from the butadiene anion. It was found that the equilibrium constant K for equation 25 increases rapidly with pressure and decreases with increasing temperature, as was found earlier for other molecules with negative electron affinities in non-polar solvents. At —7°C attachment is observed at 1 bar. At high pressure it was found that the rate of the attachment is diffusion-controlled. Freeman and coworkers measured the free-ion yields in several liquid hydrocarbons, three of which were cyclic dienes, as a function of temperature. At room temperature they measured free-ion yields of 7.5 nmol and 23 nmolJ for 1,3- and 1,4-cyclohexadiene,... 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

On hydrogenation under mild conditions, 1,3- and 1,4-cyclohexadiene yield cyclohexane as the sole product. To date, the hydrogenation has not been stopped at the cyclohexene stage. The 1,3 isomer is reduced more rapidly than the 1,4 isomer. 

Finally, the interesting study of Pinelo et al on the ozonolysis of 1,3- and 1,4-cyclohexadiene shall be mentioned here. The obtained experimental and theoretical results demonstrated that these reactions predominantly do not follow the long-accepted Criegee mechanism. Instead, the reaction of O3 with 1,4-cyclohexadiene led to the essentially... 

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