Heats of Hydrogenation and Benzene

[golgiapparatus88]

I need help with this. I've searched on the forum but I can find this example. My understanding is that heats of hydrogenation is the energy released from hydrogenation of an alkene. The more stable a compound is, a higher amount of energy is needed to break the double bond. So conjugated systems have lower heats of hydrogenation (lower as in, more energy is released).

What confuses me is benzene. If you look at 1-cyclohexene and 1,3-cyclohexadiene, the 1,3 molecule releases more energy due to an increased stability. To be exact, the McMurry organic textbook says cyclohexene released -118kJ/mol and 1,3 releases -230kj/mol. When the third pi bond is added, benzene is created. The expected value is -356kJ/mol, the actual value is -206kJ/mol.

So why does the actual value get higher (more positive)? Since it's more stable, shouldn't it have a lower heat of hydrogenation (more negative)? It was my understanding that the more stable the compound, the lower the heat of hydrogenation.

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[Rabolisk]

I need help with this. I've searched on the forum but I can find this example. My understanding is that heats of hydrogenation is the energy released from hydrogenation of an alkene. The more stable a compound is, a higher amount of energy is needed to break the double bond. So conjugated systems have lower heats of hydrogenation (lower as in, more energy is released).

What confuses me is benzene. If you look at 1-cyclohexene and 1,3-cyclohexadiene, the 1,3 molecule releases more energy due to an increased stability. To be exact, the McMurry organic textbook says cyclohexene released -118kJ/mol and 1,3 releases -230kj/mol. When the third pi bond is added, benzene is created. The expected value is -356kJ/mol, the actual value is -206kJ/mol.

So why does the actual value get higher (more positive)? Since it's more stable, shouldn't it have a lower heat of hydrogenation (more negative)? It was my understanding that the more stable the compound, the lower the heat of hydrogenation.

You understanding is wrong. Let's say that cyclohexane has enthalpy of 0. Of course, enthalpy of H2 is 0 too. If cyclohexene gets hydrogenated (i.e. reacts with nH2, n = 1 for this molecule) and its enthalpy change is -118 kJ/mol, that means that its enthalpy is 118 kJ/mol. Cyclohexene's enthalpy is 118 kJ/mol higher than cyclohexane. By the same reasoning, 1,3-cyclohexadiene has an enthalpy of 230 kJ/mol. Benzene, however, only has an ethalpy of 206 kJ/mol. Its enthalpy is lower than the expected value. Since lower enthalpy means higher stability (chemical reactions proceed toward lower enthalpy and higher entropy) benzene is more stable than the theoretical 1,3,5-cyclohexatriene. This extra loss of enthalpy or stabilization is due to the aromatic delocalization of the pi electrons. More negative enthalpy change means LESS not MORE stable. Ignoring entropy effects, the most stable molecule would be one whose enthalpy change is positive with respect to any chemical reaction, which is the opposite of what you were thinking.

 

[PiBond]

I need help with this. I've searched on the forum but I can find this example. My understanding is that heats of hydrogenation is the energy released from hydrogenation of an alkene. The more stable a compound is, a higher amount of energy is needed to break the double bond. So conjugated systems have lower heats of hydrogenation (lower as in, more energy is released).

What confuses me is benzene. If you look at 1-cyclohexene and 1,3-cyclohexadiene, the 1,3 molecule releases more energy due to an increased stability. To be exact, the McMurry organic textbook says cyclohexene released -118kJ/mol and 1,3 releases -230kj/mol. When the third pi bond is added, benzene is created. The expected value is -356kJ/mol, the actual value is -206kJ/mol.

So why does the actual value get higher (more positive)? Since it's more stable, shouldn't it have a lower heat of hydrogenation (more negative)? It was my understanding that the more stable the compound, the lower the heat of hydrogenation.

The more heat released, the more negative the value. Benzene releases less heat (has a lower heat of hydrogenation) because of its aromatic properties which gives it remarkable stability.

 

[ilovemcat]

Consider these 4 different types of cyclohexenes:

#1: Cyclohexene
#2: 1,4-Cyclohexadiene
#3: 1,3- Cyclohexadiene
#4 1,3,5-Cyclohexatriene (Benzene)

It's found that a single alkene bond will release some amount of energy per pi-bond (say -30kcal/mole).
If you were to double the number of pi bonds, twice as much energy would be released (-60kcal/mole). This is the case of #2.

However, if you moved the pi-bonds to the 1,3 position (as opposed to the 1,4 position in compound 2), the electrons in these pi-bonds can delocalize. We say the system is conjugated and this delocalization imparts stability. Therefore, what we find now is rather than this cyclohexene (#3), releasing the -60kcal/mol we expected, it will instead release a little less than that (say -55kcal/mol), because the system is more stable.

Now for your question. Benzene has 3 pi bonds and so one might expect that -90kcal/mole of energy will released. But instead, experimentally, it's found that much less energy is actually released, say -70kcal/mole. A greater amount of energy is released (than compounds #1, 2, and 3) because we have more pi bonds, but not as much as we expected due to the stability of this molecule. And unlike 1,3-cyclohexadiene which has a slight deviation from the expected value, Benzene has a much larger deviation because it has both aromatic stability and conjugation. Only some cyclic molecules are aromatic (they have to obey Huckel's Rules) - Benzene is one of them.

Regardless, energy is released in each scenario because a cyclohexane is more stable than any cyclohexene. The more stable the alkene, the less energy will be released.

By the way, I just made those values up to explain the discrepancy in the numbers. Those numbers might not be anywhere near the expected values. The general trend though is the same.

 

[ilovemcat]

Pfft, late as always lol. Grr

 

[GreenRabbit]

I need help with this. I've searched on the forum but I can find this example. My understanding is that heats of hydrogenation is the energy released from hydrogenation of an alkene. The more stable a compound is, a higher amount of energy is needed to break the double bond. So conjugated systems have lower heats of hydrogenation (lower as in, more energy is released).

What confuses me is benzene. If you look at 1-cyclohexene and 1,3-cyclohexadiene, the 1,3 molecule releases more energy due to an increased stability. To be exact, the McMurry organic textbook says cyclohexene released -118kJ/mol and 1,3 releases -230kj/mol. When the third pi bond is added, benzene is created. The expected value is -356kJ/mol, the actual value is -206kJ/mol.

So why does the actual value get higher (more positive)? Since it's more stable, shouldn't it have a lower heat of hydrogenation (more negative)? It was my understanding that the more stable the compound, the lower the heat of hydrogenation.

The simplest explanation is this: you think that -206kJ/mol is greater than -356kJ/mol, when it's not. The magnitude is far less, so it has a lower heat of hydrogenation as per your understanding. The sign simply indicates the direction energy is moving, e.g. it's negative so the compound is losing energy.