l.S. various AHH values values allow assigning observed AHH12 and 4H as in the b
ID: 533392 • Letter: L
Question
l.S. various AHH values values allow assigning observed AHH12 and 4H as in the below. Using the standard value of 27.4 kcal/mol for a cis-double bond allows the calculation of the heats of hydrogenation and gives a value for the "strain" associated with each ring. For example, the AHH2 of cis-cyclooctene is only 23.0 kcal/mol, indicating an increase of 27.4 23.0 4.4 kcal/mol of strain on going to cyclooctane. The relatively high 4HH2 for Irans-cyclooctene reflects the release of strain on reduction to cycloocuane. The "strain" for each compound is a combination of total strain minus any stabilization for conjugation. The contribution of conjugation can be seen by comparing the conjugated I,3- isomer with the unconjugated 14- and 1,5-isomers of cyclooctadiene and is about 4 t 1kcal/mol. Since the "strain" for cyclooctatetraene is similar o the other systems, there is no evidence of any major stabilization by conjugation.Explanation / Answer
A stable alkene(less energy content) when undergo catalytic hydrogenation it releases lesser amount of heat of hydrogenations.
On the other hand an unstable alkene releases high amount of heat of hydrogenations.
For example a cycloalkene with trans double bond is unstable and undergo hydrogenation releasing more heat of hydrogenations.
As per given in the chart the heat of hydrogenation of transcyclooctatetraene is -32.2 kcal/mol.
Then the trans cyclooctene gets relief from strain
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