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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 Jan 29 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 Jan 29 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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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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