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I have been doing practice problems in General Organic Chemistry. I got some questions that asked me to compare the Heat of Combustion of some molecules. I couldn't get any of those questions and the logic behind their answers. I tried googling but it didn't help. The search results were not direct. So, I'd like to know the factors on which the heat of hydrogenation depends. I am not looking for an answer that describes it quantitatively but rather qualitatively.

Here are some example questions:

Which one of the following has the smallest heat of combustion? enter image description here

The answer is (c). But why is that?

Rank the following substances in order of decreasing heat of combustion (maximum $\rightarrow$ minimum) enter image description here

The answer is 2>4>1>3. But again, why is that?

I knew that the heat of combustion depends on the number of carbon atom. But in these two examples the number of carbon atoms are same. Is there anything I need to watch out for?

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  • $\begingroup$ I guess you're asking about effect of delocalisation on the heat of hydrogenation? $\endgroup$ – Mithoron Apr 23 '16 at 13:57
  • $\begingroup$ @Mithoron Not just that. But some other common dependence of heat of hydrogenation on other things too. $\endgroup$ – manshu Apr 23 '16 at 15:54
  • $\begingroup$ That may be too broad, it would help if you'd rather specified this. $\endgroup$ – Mithoron Apr 23 '16 at 15:58
  • $\begingroup$ @Mithoron I added some examples. I hope it's alright now. :)...And oh..I also changed the heat of hydrogenation to heat of combustion. $\endgroup$ – manshu Apr 23 '16 at 16:30
  • $\begingroup$ Och no now it looks like homework :D Rollback, quickly! ;) $\endgroup$ – Mithoron Apr 23 '16 at 17:23
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For isomers differences in the heat of combustion are due to differences in the internal energy of the educts. If intermolecular interaction (lattice energy etc.) can be neglected it reflects differences in the internal energy of the molecules.

In your first example C has the smallest heat of combustion since it has the lowest internal Van der Waals strain.

In the second example the order is given by the ring strain, and the Van der Waals strain between the two methyl groups at the cyclobutane ring.

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