OChem Destroyer 2011 #137

[Wahoowah]

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Problem 137. Question asked with molecule had smallest heat of hydrogenation.

There were five forms of pentene and 1 butene. I though it was butene because I had learned that larger alkenes always have higher imf, boiling pt, and heat of hydrogenation, but the answer was 2 methyl-2butene (the most substituted pentene). Does substitution outweigh molecular weight in this problem?

 

[Jay0689]

Are you sure you aren't mixing up heat of hydrogenation with heat of combustion? I think the butene would have the lower heat of combustion because it's smaller, but with heat of hydrogenation, why would the # of carbons matter much? The thing that matters is how stable that double bond is. If it is low energy, then hydrogenation will not be able to release as much heat as if it was a high energy, unstable alkene.

 

[mh0000]

are you sure you aren't mixing up heat of hydrogenation with heat of combustion? I think the butene would have the lower heat of combustion because it's smaller, but with heat of hydrogenation, why would the # of carbons matter much? The thing that matters is how stable that double bond is. If it is low energy, then hydrogenation will not be able to release as much heat as if it was a high energy, unstable alkene.

+1

 

[Wahoowah]

That makes sense. For some reason I grouped hydrogenation with those other trends. Thanks!