Hydrogenation, heat of, for

Fleats of hydrogenation can be used to estimate the stability of double bonds as structural units, even in alkenes that are not isomers. Table 6.1 lists the heats of hydrogenation for a representative collection of alkenes. 

The heat of hydrogenation for allene (Problem 6.37) to yield propane is -295 kj/mol, and the heat of hydrogenation for a typical monosubstituted alkene such as pcopene is - 126 kj/mol. Is allene more stable or less stable than you might expect for a diene Explain. 

Table 14.1 Heats of Hydrogenation for Some Alkenes and Dienes...

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."...

The heats of hydrogenation for a number of unsaturated compounds are available from the work of Kistiakowsky and collaborators, and in a few cases from accurate heats of formation of the monomer and the reference compound can be estimated... 

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. 

Gas-phase structure and solution-phase NMR of 75c theoretical heats of hydrogenation for 75d and 76d... 

When a molecule has two isolated double bonds, the heat of hydrogenation is close to the sum of the heats of hydrogenation for the individual double bonds. For example, the heat of hydrogenation of penta-1,4-diene is -252 kJ/mol (-60.2 kcal/mol), about twice that of pent-l-ene. 

The heat of hydrogenation for a typical diene is 2 x (Af/°hydrog of an alkene) = -252 kJ/mol. Thus, allene, with Af/°hydrog = -295 kJ/mol is 43 kJ/mol higher in energy than a typical diene and is less stable. 

The PM3-derived heats of hydrogenation for the formation of the saturated pyrrolidine, phospholane, arsolane, and... 

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... 

Problem 14.2 Use the data in Table 14.1 to calculate an expected heat of hydrogenation for allene, H2C=C=CH2, The measured value is —298 kJ/mol (-71.3 kcal/mol). Rank a conjugated diene, a nonconjugated diene, and an allene in order of stability. 

If we were to measure what are called heats of Hydrogenation for... 

Cis-acyclic olefins formed more stable complexes than trans-acyclic olefins and was attributed to relief of strain on complexation the order of stability is the same as the order of heats of hydrogenation for which a similar explanation was given. The importance of strain was further exemplified when trans-cyclo-octene was found to form a much more stable complex than cis-cyclooctene ... 

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