1.3- Cyclohexadiene, heat hydrogenation

FIGURE 11.2 Heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, a hypothetical 1,3,5-cyclohexa-triene, and benzene. All heats of hydrogenation are in kilojoules per mole. 

We can get a quantitative idea of benzene s stability by measuring heats of hydrogenation (Section 6.6). Cyclohexene, an isolated alkene, has ff ydrog = -118 kj/mol (-28.2 kcal/mol), and 1,3-cyclohexadiene, a conjugated diene, has A/Chydrog = 230 kj/mol (-55.0 kcal/mol). As noted in Section 14.1, this value for 1,3-cyclohexadiene is a bit less than twice that for cyclohexene because conjugated dienes are more stable than isolated dienes. 

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

One way to investigate the stability of benzene is to compcire the cimount of heat produced by the reactions of benzene to similar compounds that are not aromatic. For example, a simple comparison of the heat of hydrogenation for a series of related compounds allows us to see the difference. Figure 6-6 shows the hydrogenation of cyclohexane, 1,3-cyclohexadiene, and benzene, which make a suitable set because all three yield cyclohexane. 

The heat of hydrogenation of cyclohexene is about -120 kJ/mol (kilojoules per mole). If the reaction of one double bond releases this amount of energy, then the reaction of two double bonds (1,3-cyclohexadiene) should release about twice this cimount of energy. The classical, three double-bond benzene should... 

B-3. Hydrogenation of cyclohexene releases 120 kJ/mol (28.6 kcal/mol) of heat. Which of the following most likely represents the observed heat of hydrogenation of 1,3-cyclohexadiene ... 

The impurities can be grouped into two categories lights (water, cyclohexene, cyclohexadiene) and heavies (phenol, dicyclohexyl-ether, cyclohexenyl- cyclohexanone). To limit their amount, the conversion is kept around 80% with a selectivity of about 98%. The hot reactor effluent is cooled in countercurrent with the feed in FEHE, and finally for phase separation in the heat exchanger (E-2) at 33 °C. The simple flash (S-2) can ensure a sharp split between hydrogen, recycled to hydrogenation reactor, and a liquid phase sent to separation. 

Heats of hydrogenation, which were used in Section 16.9 to show that conjugated dienes are more stable than isolated dienes, can also be used to estimate the stability of benzene. Equations [1]—[3] compare the heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, and benzene, all of which give cyclohexane when treated with excess hydrogen in the presence of a metal catalyst. 

Use the data in Figure 15.2 to calculate the heat of hydrogenation, 5//hydn>R. for the partial hydrogenation of benzene to yield 1,3-cyclohexadiene. Is the reaction exothermic or endothermic ... 

Deprotection takes place by catalytic hydrogenation in the presence of palladium under a hydrogen pressure of a few atmospheres. This reforms the hydroxyl groups and toluene is easily separated. Hydrogenolysis by transfer does not require any special equipment. An alcohol solution is heated at reflux with the protected sugar in the presence of palladium over charcoal with, as hydrogen donor, cyclohexene or cyclohexadiene which aromatizes in the reaction. 

Problem 10.2 (a) How do the following heats of hydrogenation (AHh, kJ/mol) show that benzene is not the ordinary triene 1,3,5-cyclohexatriene Cyclohexene, -119.7 1,4-cyclohexadiene, 239.3 1,3-cyclohexadiene, -231.8 and benzene, 208.4. (b) Calculate the delocalization energy of benzene, (c) How does the delocalization energy of benzene compare to that of 1,3,5-hexatriene (AHh = - 336.8 kJ/mol) Draw a conclusion about the relative reactivities of the two compounds. 

---