Benzene heat of hydrogenation for

The huge difference between the hypothetical and observed heats of hydrogenation for benzene carmot be explained solely on the basis of re.sonance and conjugation. 

A comparison between the observed and hypothetical heats of hydrogenation for benzene... 

Based on the data in Figure 18.1, estimate the heat of hydrogenation for benzene. (Note that it takes an exotic rhodium catalyst or extreme conditions to reduce benzene to cyclohexane.)... 

Figure 15.2 A comparison of the heats of hydrogenation for cyclohexene, 1,3-cyclo-hexadiene, and benzene. Benzene is 150 kJ/mol (36 keal/mol) more stable than might be expected for "cyclohexatriene."...

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. 

An energy diagram comparing the heats of hydrogenation for cyclohexene, cyclohexadiene, and benzene. 

We calculated the heat of hydrogenation of the unreal, unknown, hypothetical molecule 1,3,5-cyclohexatriene by using values for the heats of hydrogenation of the real molecules cyclohexene and 1,3-cyclohexadiene.The heat of hydrogenation of 1,3-cyclohexadiene was calculated quite accurately, so we can be rather confident that our estimate for 1,3,5-cyclohexatriene won t be far off. That s important, because we can never make the cyclic triene to check the calculation. Any synthetic route will inevitably produce the much more stable real molecule, benzene. The heat of hydrogenation for the real molecule, benzene, is far lower than our estimate. 

Figure 15-3 Heats of hydrogenation provide a measure of benzene s unusual stability. Experimentai vaiues for cyciohexene and 1,3-cyciohexadiene allow us to estimate the heat of hydrogenation for the hypotheticai 1,3,5-cyciohexatriene. Comparison with the experimental AH° for benzene gives a value of approximateiy 29.6 kcai mol for the aromatic resonance energy.

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. 

I Benzene is unusually stable, having a heat of hydrogenation 150 kj/mol less negative than we might expect for a conjugated cyclic triene. 

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