Heats of Formation and Hydrogenation of Alkenes
Heats of Formation
The heats of formation (ΔHfo in kcal/mol) of selected alkenes are shown on the right. Figure 1 illustrates that 1,2-dialkyl substituted alkenes of the (Z)-configuration are less stable (less negative value) than their (E)-stereoisomers. Moreover, that difference is consistently ~1 kcal/mol. The (Z)-isomers in Fig.1 differ by one methylene group. As with alkanes themselves, increasing the chain length by a methylene group makes the the heat of formation more negative by ~5 kcal/mol. The same is true of the (E)-isomers. This progression is readily seen in the series of 1-alkenes in Fig.2.
Fig. 3 shows a series of six carbon alkenes of increasing double bond alkyl substitution. The more substitution; the more negative heat of formation and, therefore, more stability.
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Heats of Hydrogenation
The illustration below shows the five C6 alkenes of Fig. 3 in a Standard State Diagram. The alkenes 1-hexene and the two 2-hexene stereoisomers form the same alkane, n-hexane, upon hydrogenation. The most stable alkene of the trio, (E)-2-hexene, liberates the smallest amount of heat upon hydrogenation. The difference in the heat of formation is equal to the difference in the heat of hydrogenation for these three straight chain alkenes. While it is true that increased alkyl group substitution lowers the heat of formation of each alkene and reduces the heat of hydrogenation, respectively, the two branched alkenes, 2-methyl-2-pentene and 2,3-dimethyl-2-butene, each give different products upon hydrogenation and different from n-hexane.
