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I'm told that heat of hydrogenation (HOH) is directly proportional to number of π bonds and inversely proportional to stability. So, is the aromaticity responsible for this?

Also, what is the general approach to the problems like this? Say, I encountered napthalene and some hydrocarbon which contains about 2 rings and have π bonds less than naphthalene, just like stated above – then how shall I decide?

Is there any specific rule like if a compound will be aromatic then it's HOH will be reduced by this amount or something like that?

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  • $\begingroup$ "Heat of hydrogenation of alkenes is a measure of the stability of carbon-carbon double bonds. All else being the same, the smaller the numerical value of heat of hydrogenation of an alkene, the more stable the double bond therein." — See chem.libretexts.org/Ancillary_Materials/Reference/… $\endgroup$
    – SteffX
    Commented Jan 29, 2019 at 13:03
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    $\begingroup$ Yeah I know that but how do I measure how much stable.Say I told you to give me an order of HOH of benzene, 1,3-cyclopentadiene and cyclopentene then how can I rate that whose HOH will be higher than whom.See the main thing which I'm getting stuck is that HOH of 3 π bond will be more than that of 2 π bond and so on.But see here aromaticity comes into play and so HOH of benzene is less than 1,3-cyclopentadiene but more than cyclopentene.Why HOH of benzene not lowest among all these coz of Aromaticity.I hope that you might be getting my doubt. $\endgroup$
    – Abner Alfred Thompson
    Commented Jan 29, 2019 at 14:29

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Yes. The relatively smaller Heat of hydrogenation (HOH) for benzene as compared to that for 1,3-cyclohexadiene is due to the aromaticity of the first. Analyzing the thermochemistry is indeed among the first and perhaps more intuitive ways to present and quantifies aromaticity itself.

A) Cycloexene HOH = -120 kJ/mol

B) 1,4-Cycloexadiene HOH = -240 kJ/mol

C) 1,3-Cyclohexadiene HOH = -232 kJ/mol

D) Benzene HOH = -208 kJ/mol

While for B the HOH is about double as compared to that of cyclohexane and thus is according to the assertion in the question, the conjugation in C does results in a "lower than expected" value.

The 8 kJ/mol difference is the energy of resonance of the two conjugated double bonds.

The situation is even more striking for D as this difference from "an expected value" amounts now to about 152 kJ/mol. This is again a measure of stability and its distinctively high value is a manifestation of aromaticity.

The HOH of benzene is even less than that of cyclohexadienes. This shouldn't come as surprise though, as for there are not really double bonds.

In other words, while hydrogenation of individual double bond is an exothermic reaction, hydrogenation of benzene to cycloexadienes is an endothermic process.

Once you disrupt aromaticity, then you can think again in terms of individual double bonds (although delocalisation may occur to little extent, as in C). And this fact should answer the question "why HOH of benzene is higher than that of cyclopentene?", too.

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    $\begingroup$ @HarryHolmes yes. You can use heat of hydrogenation to conclude that. Ethane is more stable than ethene, the logic is the same. Just benzene is more stable than expected, and by a good amount. $\endgroup$
    – Alchimista
    Commented Feb 19, 2021 at 9:24
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    $\begingroup$ @HarryHolmes yes. This is however in an scale that doesn't take into account the energetic content by amount of atoms. To give you the point: a long saturated hydrocarbon is inherently more stable than a shorter one (unless entropy kicks in). But this does not really means that octane is more "chemically robust" than pentane. Same would be for a non conjugated diene respect to the alkene of corresponding length. Not to say that, as usual, thermodynamic stability and reactivity shall not be confused. $\endgroup$
    – Alchimista
    Commented Feb 20, 2021 at 9:04
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    $\begingroup$ @HarryHolmes look at this link. Out of a defined chemical process you cannot get to much by comparing different things. It is in the meaning you want/can put into the term. Cyclohexene is more stable than benzene simply because it is more saturated, ie more energy is released upon its formation. It is quit obvious. Here is the link chemistry.stackexchange.com/questions/146533/… $\endgroup$
    – Alchimista
    Commented Feb 21, 2021 at 10:39
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    $\begingroup$ In particular my comment: Take it as an energy content. Of course in ideal case it would be better to have a "normalised stability", which in some cases is possible. For instance benzene is more stable than cyclohexane as per pi electrons. I agree that comparing different things isn't really necessary, but I have no examples in mind in which we do this comparison other than in a vague way. When it comes to quantities, we analyse reactions or transformations in which elemental composition is globally the same. $\endgroup$
    – Alchimista
    Commented Feb 21, 2021 at 10:45
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    $\begingroup$ @HarryHolmes yes it is. You can isolate it in normal conditions, put it in sort of container, keep it. It is stable. Tough, its mixture with oxygen is not that stable and transform to water and CO2, soon or later. If it does it fast or not, and if it needs some initiation, that is about so called kinetic stability, ie the effective reactivity. The latter part isn't relevant to this thread here, but it can be confusing for starter readers. It is also addressed in the thread I linked, tough again is independent of the issue "what therm. stability is?". $\endgroup$
    – Alchimista
    Commented Feb 21, 2021 at 12:01
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Perhaps a diagram as to why the heat of hydrogenation (ΔHho) of benzene is less than the ΔHho of 1,3-cyclohexadiene would be helpful. The heats of formation (ΔHfo) of the relevant species except the hypothetical cyclohexatriene are available at the NIST site. The ΔHfo of cyclohexatriene is estimated as ~56 kcal/mol above the standard state. This value is obtained by tripling the ΔHho of cyclohexene to cyclohexane. One might have also averaged this ΔHho with that of 1,3-cyclohexadiene to obtain the indeterminant ΔHho of cyclohexatriene to 1,3-cyclohexadiene. Not only is the exact ΔHfo of cyclohexatriene uncertain, but, as a result, so is the resonance stabilization of benzene (-36.2 kcal/mol). The ΔHfo of benzene places it ~5 kcal/mol below that of 1,3-cyclohexadiene. Because both of these compounds are hydrogenated to cyclohexane, their difference in heat of hydrogenation is likewise ~5 kcal/mol. In other words, the ΔHho of 1,3-cyclohexadiene is greater than the ΔHho of benzene by ~5 kcal/mol. Incidently, the ΔHfo of 1,4-cyclohexadiene has been reported at NIST as ranging between 24-26 kcal/mol.

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  1. National Institute of Standards and Technology, U. S. Department of Commerce
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