Hydrogenation alkenes, stability

The pattern of alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion... 

In Section 12.3 we learned that hydrogen adds to alkenes to form alkanes, and that the heat released in this reaction, the heat of hydrogenation, can be used as a measure of alkene stability. 

Evidence for the extra stability of conjugated dienes comes from mea surements of heats of hydrogenation (Table 14.1). We saw earlier inthedit cussion of alkene stabilities (Section 6.7) that alkenes of similar substitution pattern have remarkably similar AHgydrog values. Monosubstituted alkenes such as 1-butene have values for near -126 kJ/mol... 

Hydrogenation Relative Stability of Double Bonds Comparing alkenes and alkanes. 

These features of a hydrogenation reaction allow us to compare the stabilities and reactivities of any two alkenes that would yield the same product upon hydrogenation. Thus we explain the greater stability of trans alkenes relative to cis alkenes in terms of nonbonded interaction strain. In ar-2-butene, the two —CH3 groups are sufficiently close to each other that there is repulsion between their electron clouds. This repulsion is reflected in the larger heat of hydrogenation (decreased stability) of d -2-butene compared with that of frawfr2-butene (approximately 4.2 kJ/mol). 

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