enthalpy change for hydrogenation + benzene

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TheGreaterGood
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Slightly confused about stability and the diagram

Please see attached file to see exactly what I am struggling with

will give rep if I haven't repped u b4r (won't let me rep pl I've recently repped b4r )
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thegodofgod
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(Original post by TheGreaterGood)
Slightly confused about stability and the diagram

Please see attached file to see exactly what I am struggling with

will give rep if I haven't repped u b4r (won't let me rep pl I've recently repped b4r )
Basically, the Kekulé structure of benzene is the theoretical molecule, cyclohexa-1,3,5-triene. That is, a 6 carbon ring with alternate C=C bonds (C6H6).

Now, if you have a molecule of cyclohexene, (one C=C bond, C6H10), the enthalpy change when you hydrogenate the molecule (to form cyclohexane, no C=C bonds, C6H12) is -120 kJmol-1. This means, that theoretically, it would take -120 kJmol-1 to hydrogenate each C=C bond.

Therefore, you would expect that it would take 3 x -120 kJmol-1 = -360 kJmol-1 to fully hydrogenate cyclohexa-1,3,5-triene. However, this is clearly not the case, as the enthalpy change for benzene is only -208 kJmol-1, 152 kJmol-1 less than you would expect.

This means that the structure of benzene as we know it (with the delocalised ring of pi electrons, and not the alternate double bonds) is more thermodynamically stable than cyclohexa-1,3,5-triene.
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TheGreaterGood
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(Original post by thegodofgod)
Basically, the Kekulé structure of benzene is the theoretical molecule, cyclohexa-1,3,5-triene. That is, a 6 carbon ring with alternate C=C bonds (C6H6).

Now, if you have a molecule of cyclohexene, (one C=C bond, C6H10), the enthalpy change when you hydrogenate the molecule (to form cyclohexane, no C=C bonds, C6H12) is -120 kJmol-1. This means, that theoretically, it would take -120 kJmol-1 to hydrogenate each C=C bond.

Therefore, you would expect that it would take 3 x -120 kJmol-1 = -360 kJmol-1 to fully hydrogenate cyclohexa-1,3,5-triene. However, this is clearly not the case, as the enthalpy change for benzene is only -208 kJmol-1, 152 kJmol-1 less than you would expect.

This means that the structure of benzene as we know it (with the delocalised ring of pi electrons, and not the alternate double bonds) is more thermodynamically stable than cyclohexa-1,3,5-triene.
Thanks but

I don't understand how having a smaller enthalpy change means it is more stable as I thought if it had a big enthalpy change that would mean more energy is needed to change it into something else which would make me think big enthalpy change= very very stable. However the molecule that has an enthakpy change of 360 is less stable than benze which has a enthalpy change of 208. That is what is confusing
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charco
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(Original post by TheGreaterGood)
Thanks but

I don't understand how having a smaller enthalpy change means it is more stable as I thought if it had a big enthalpy change that would mean more energy is needed to change it into something else which would make me think big enthalpy change= very very stable. However the molecule that has an enthakpy change of 360 is less stable than benze which has a enthalpy change of 208. That is what is confusing
Take a look at this interactive graph on the hydrogenation of benzene, it may help your understanding.

Remember that the Kekulé structure is hypothetically three double bonds.

Hover over the data on the RHS of page 2 to see the energy profiles.
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TheGreaterGood
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(Original post by charco)
Take a look at this interactive graph on the hydrogenation of benzene, it may help your understanding.

Remember that the Kekulé structure is hypothetically three double bonds.

Hover over the data on the RHS of page 2 to see the energy profiles.
Thanks. I feel that has cleared things up :dance:
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