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Topic: Heat of hydrogenation  (Read 2554 times)

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Offline shon1802

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Heat of hydrogenation
« on: November 13, 2016, 03:27:56 PM »
Hello everyone,
I have to rank the heats of hydrogenation of the following compounds below.


Undoubtedly, II is the most stable, because it has the the largest number of alkyl groups, and the lowest number of hydrogens.
Then I'd say I, followed by III, because of the number of hydrogens, but I'm not sure.
Regarding IV and V - as far as I understand, the only relevant difference is that one has 2 of its H's in cis configuration, and in the other one (V) they are attached to the same carbon. I'd guess V is less stable...
Any ideas?
Thanks
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Offline AWK

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Re: Heat of hydrogenation
« Reply #1 on: November 13, 2016, 06:58:54 PM »
IV is completely different from others. Why?
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Offline shon1802

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Re: Heat of hydrogenation
« Reply #2 on: November 14, 2016, 09:11:41 AM »
Quote from: AWK on November 13, 2016, 06:58:54 PM
IV is completely different from others. Why?

It's aromatic, but I don't see how it's relevant...
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Offline orgopete

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Re: Heat of hydrogenation
« Reply #3 on: November 14, 2016, 10:02:46 AM »
Let's do this differently. Compare I, III, and V. Which would have the greater heat of hydrogenation? Compare IV and V? Compare I and II?
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Offline Corribus

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Re: Heat of hydrogenation
« Reply #4 on: November 14, 2016, 10:13:45 AM »
Question is a little ambiguous - do they mean hydrogenation of only a single double bond, or complete hydrogenation to saturation?
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Offline Enthalpy

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Re: Heat of hydrogenation
« Reply #5 on: November 14, 2016, 01:11:28 PM »
Can the question be answered by mere qualitative considerations? I had to cheat using a table to rank III vs IV.
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Offline AWK

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Re: Heat of hydrogenation
« Reply #6 on: November 15, 2016, 12:06:37 PM »
Quote from: shon1802 on November 14, 2016, 09:11:41 AM
Quote from: AWK on November 13, 2016, 06:58:54 PM
IV is completely different from others. Why?

It's aromatic, but I don't see how it's relevant...
Every organic chemistry textbook discusses difference in energy of hydrogenation between hypothetic cyclohexatriene and benzene.
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