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From Chemistry Libretexts:

Heat of hydrogenation (symbol: $ΔH_\mathrm{hydro}$) of an alkene is the standard enthalpy of catalytic hydrogenation of an alkene.

Does the heat of hydrogenation include only the double bonds in an alkene or does it also include other forms of unsaturation, such as rings?

For an example, while discussing the heat of hydrogenation of cyclooctatetraene (which is non-aromatic), would we say that the heat of hydrogenation is equal to just the energy required to hydrogenate the four double bonds, or do we also account for the energy required to open the ring and convert it into octane?

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    $\begingroup$ It is not that easy to open a ring. $\endgroup$ – Ivan Neretin Feb 26 at 8:31
  • $\begingroup$ Would you say that cyclohexane is an alkene? $\endgroup$ – Karl Feb 26 at 19:08
  • $\begingroup$ No it is the enthalpy of reaction for every reaction A + nH2 - - - > AH2n. $\endgroup$ – Alchimista Feb 27 at 8:08
  • $\begingroup$ @Alchimista the reaction you have stated would imply the ring being broken, which is exactly what my question is about. $\endgroup$ – Aniruddha Deb Feb 27 at 12:02
  • $\begingroup$ The point is simply that first there is a reaction then you can name it and refer to it. If I would be able to hydrogenate cycloexAne I will certainly speak of its enthalpy of hydrogenation. $\endgroup$ – Alchimista Feb 28 at 7:43
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"Heat of hydrogenation" is a subset of "heat of reaction" which is nothing but the standard enthalpy change ($\Delta H^{\circ}$) in the system as reactants are converted to products. In catalytic hydrogenation, as hydrogen molecules are added to atoms bearing double/triple bonds, the enthalpy change which accompanies the reaction is called as "heat of hydrogenation".

Now coming to your question:

Does the heat of hydrogenation include only the double bonds in an alkene or does it also include other forms of unsaturation, such as rings?

Heat of hydrogenation just refers to the enthalpy change required to convert double and triple bonds to single bonds in a molecule. It doesn't account for ring opening. This is because opening a ring involves breaking at least one carbon-carbon covalent bond which requires a great deal of energy. But that doesn't mean rings cannot be broken. If they could be broken, then the enthalpy change is plainly called as "heat of reaction" but not "heat of hydrogenation" since it would lead to ambiguities.

Thus, heat of hydrogenation of cyclooctatetraene is just the enthalpy change associated with those four double bonds, even though cyclooctane has a Index of hydrogen deficiency (degree of unsaturation) as one.

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