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Why total heat of hydrogenation of 1,3-cyclohexadiene is more than that of benzene?
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Posted by
Aaron Weinberger
Why total heat of hydrogenation of 1,3-cyclohexadiene is more than that of benzene?
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.
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.
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 isnt 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.More
@HarryHolmes yes. This is however in an scale that doesnt 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.More
@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 More
@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.More
@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 isnt 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?".More
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.
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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