emaan779
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Please help me understand a statement.

What I know is that benzene is thermodynamically more stable than what is expected by Kekule's structure. The enthalpy of hydrogenation of benzene is -208kJmol-1 whereas according to Kekule's structure, it should be -360kJmol-1. Hence, benzene is more stable than expected.

Statment : "the difference in these two values is called the resonance energy. The lower value suggests that benzene is more stable than expected as more energy is needed to overcome the delocalised electron ring system".

Surely it should say "the higher value (-208kJmol-1) suggests that benzene is more stable than expected, as LESS energy is needed to overcome the delocalised electron rinf system".

If not please explain why. Thanks!
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h3rmit
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(Original post by emaan779)
Please help me understand a statement.

What I know is that benzene is thermodynamically more stable than what is expected by Kekule's structure. The enthalpy of hydrogenation of benzene is -208kJmol-1 whereas according to Kekule's structure, it should be -360kJmol-1. Hence, benzene is more stable than expected.

Statment : "the difference in these two values is called the resonance energy. The lower value suggests that benzene is more stable than expected as more energy is needed to overcome the delocalised electron ring system".

Surely it should say "the higher value (-208kJmol-1) suggests that benzene is more stable than expected, as LESS energy is needed to overcome the delocalised electron rinf system".

If not please explain why. Thanks!
Technically, as -208 is negative it's the higher number but the magnitude of the energy you get out is what's important, so -208 kJ/mol is also the lower value (in chemistry terms).

More energy is required to overcome the delocalised electron ring system, not less.
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emaan779
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(Original post by h3rmit)
Technically, as -208 is negative it's the higher number but the magnitude of the energy you get out is what's important, so -208 kJ/mol is also the lower value (in chemistry terms).

More energy is required to overcome the delocalised electron ring system, not less.
Ah I see. Thanks very much.
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emaan779
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(Original post by h3rmit)
Technically, as -208 is negative it's the higher number but the magnitude of the energy you get out is what's important, so -208 kJ/mol is also the lower value (in chemistry terms).

More energy is required to overcome the delocalised electron ring system, not less.
By the way, please would you be able to tell me what "resonance energy" is since the textbook also says "as the resonance energy increases, the stability increases". I've googled it but I can't find a definition that an A level student like myself can understand... It'll help me to understand the stability of benzene better. Thanks
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h3rmit
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(Original post by emaan779)
By the way, please would you be able to tell me what "resonance energy" is since the textbook also says "as the resonance energy increases, the stability increases". I've googled it but I can't find a definition that an A level student like myself can understand... It'll help me to understand the stability of benzene better. Thanks
http://www.chem.ucalgary.ca/courses/...resenergy.html
https://en.wikipedia.org/wiki/Conjugated_system

Edit:

https://www2.chemistry.msu.edu/facul...jml/react3.htm is also helpful

Resonance energy seems to be a measure of how the stability of a conjugated system, which is a system with overlapping p orbitals and/or alternating double and single bonds, in which the electrons don't belong to a single bond but are shared across all adjacent aligned p-orbitals (like benzene, but doesn't have to be aromatic), diverges from the stability of the hypothetical molecule with isolated C=C bonds/no conjugation (like the Kekule model of benzene, or cyclohexa-1,3,5-triene). The enthalpy change of hydrogenation is a good indicator of the stability, and a more negative enthalpy = less stable.

Cyclohexene has 1 double bond, so multiplying the enthalpy change of hydrogenation for cyclohexene by 3 gives the enthalpy change of hydrogenation for cyclohexa-1,3,5-triene (ignoring the effects of conjugation) or the energy needed to break 3 isolated C=C bonds (and form 3 C=H bonds). When you compare this enthalpy change to benzene though, the values aren't the same, showing there's some funky stuff going on, which is due to the delocalised electrons moving around to form resonance structures. Resonance stabilisation, where the electrons move around the whole conjugated region leads to the activating and directing effects of substituents, like hydroxyls in phenol. Anyway, the difference in the enthalpy change of hydrogenation between the hypothetical non-conjugated molecule, and the conjugated molecule is the resonance energy - it shows you the magnitude of the effect of the resonance stabilisation.


In just benzene, each electron pair moves to the next carbon-carbon single bond, so the double bonds "shift" one carbon, so it only has two resonance structures, as opposed to phenol's four.
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