Slightly confusing Enthalpy of hydrogenation of Benzene question

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JanaALEVEL
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a(iii)
I don't understand how they just assumed enthalpy change of hydrogenation of C=C outside the benzene ring is =to the enthalpy change of hydrogenation of the C=C outside the ring.
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charco
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(Original post by JanaALEVEL)
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PART C
I don't understand how they just assumed enthalpy change of hydrogenation of C=C outside the benzene ring is =to the enthalpy change of hydrogenation of the C=C outside the ring.
If the same bonds are broken and formed you would expect the same energy change
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JanaALEVEL
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(Original post by charco)
If the same bonds are broken and formed you would expect the same energy change
But shouldn't the bond enthalpy be influenced by the environment ?
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charco
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(Original post by JanaALEVEL)
But shouldn't the bond enthalpy be influenced by the environment ?
... and that is precisely what the question is all about. The fact that the enthalpies are very different suggests that the environment is very different and hence the bond enthalpies. This is due to delocalisation, that essentially stabilises the molecule.
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JanaALEVEL
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(Original post by charco)
... and that is precisely what the question is all about. The fact that the enthalpies are very different suggests that the environment is very different and hence the bond enthalpies. This is due to delocalisation, that essentially stabilises the molecule.
That isn't my question, I understand that. I'm talking about the C=C part
The C=C in arenes shouldn't have the same bond enthalpy as the C=C which is attached to the arene
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charco
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false
(Original post by JanaALEVEL)
That isn't my question, I understand that. I'm talking about the C=C part
The C=C in arenes shouldn't have the same bond enthalpy as the C=C which is attached to the arene
There has been no previous mention of a C=C attached to an arene!

You are comparing an aromatic (kekule) double bond and an aliphatic ring double bond.

If the kekule structure were 'correct' then you would expect them to undergo hydrogenation with the same energy change. But they dont, and this tells you that the kekule structure is inadequate to describe the bonding.
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JanaALEVEL
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(Original post by charco)
false

There has been no previous mention of a C=C attached to an arene!

You are comparing an aromatic (kekule) double bond and an aliphatic ring double bond.
Is benzene not an arene ? Doesn't the structure in (iii) include a C=C attached to Benzene ?
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CuriosityYay
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4H2 is the answer because you need 3 to hydrogenate the benzene ring and 1 to hydrogenate the double bond and this gives ethyl cyclohexane

Hope that helps!

P.S: you should have made it clear you are asking about question (iii). You said part C and there is no part C.
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JanaALEVEL
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(Original post by CuriosityYay)
4H2 is the answer because you need 3 to hydrogenate the benzene ring and 1 to hydrogenate the double bond and this gives ethyl cyclohexane

Hope that helps!

P.S: you should have made it clear you are asking about question (iii). You said part C and there is no part C.
oh I didn't notice, will fix it now.
I know how to get the number of moles of H2 needed, what I don't know how to get is the last part of (iii), the suggest a value for the enthalpy part
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CuriosityYay
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(Original post by JanaALEVEL)
oh I didn't notice, will fix it now.
I know how to get the number of moles of H2 needed, what I don't know how to get is the last part of (iii), the suggest a value for the enthalpy part
(-208) + (-120) = -328
Hydrogenation of benzene ring + hydrogenation of one C=C double bond
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JanaALEVEL
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(Original post by CuriosityYay)
(-208) + (-120) = -328
Hydrogenation of benzene ring + hydrogenation of one C=C double bond
Okay.. ctrl c ctrl v
I don't understand how they just assumed enthalpy change of hydrogenation of C=C outside the benzene ring is =to the enthalpy change of hydrogenation of the C=C outside the ring.
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CuriosityYay
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(Original post by JanaALEVEL)
Okay.. ctrl c ctrl v
I don't understand how they just assumed enthalpy change of hydrogenation of C=C outside the benzene ring is =to the enthalpy change of hydrogenation of the C=C outside the ring.
Because a C=C double bond is a C=C double bond. It is the same whether it is in a cyclic compound or not. Plus this enthalpy change is the only knowledge given in an exam, so you should use.
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JanaALEVEL
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(Original post by CuriosityYay)
Because a C=C double bond is a C=C double bond. It is the same whether it is in a cyclic compound or not.
Ok correct me if I'm wrong but this statement (or 2 if you want to get technical) is incorrect.
First of all I assume that by 'it' you're referring to the bond enthalpy because that's what we're discussing now.
The enthalpy change of C=C in a cyclic compound isn't the same as the enthalpy change of C=C in another compound, bond enthalpies are influenced by the environment , the strength of the bond is influenced by what else is around it, hence, the bond enthalpy is influenced by it's environment
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CuriosityYay
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(Original post by JanaALEVEL)
Ok correct me if I'm wrong but this statement (or 2 if you want to get technical) is incorrect.
First of all I assume that by 'it' you're referring to the bond enthalpy because that's what we're discussing now.
The enthalpy change of C=C in a cyclic compound isn't the same as the enthalpy change of C=C in another compound, bond enthalpies are influenced by the environment , the strength of the bond is influenced by what else is around it, hence, the bond enthalpy is influenced by it's environment
You are right because the carbons in the cyclic compound are both attached to a hydrogen and another carbon but in the alkene one of them is attached to two hydrogens and the other is attached to a hydrogen and a benzene ring.

And as you know we use 'mean' bond enthalpies in enthalpy change calculations because they are a mean for the same bond in different environments.

But this is an exam and this is the only information given so if you want the mark you gotta let go of some principals and just use the value from the different environment
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cephid
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The important point to understand is that benzene is stabilised by resonance. Were benzene cyclohexa1,3,5-triene with no additional electronic interaction between C-C=C (ie, no resonance) then enthalpy changes could be predicted on the basis of hydrogenation of cyclohexene (-3*120 kJ/mol). However, benzene is resonance stabilised, thus more stable than the hypothetical triene by an amount 3*120 - 208 = 152 kJ/mol
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nathanwmarch
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Any explanations including “it’s just an exam, use the values” are missing the point... there is actually a rational explanation.

The ethenyl C=C bond in phenylethene does interact with the aromatic system of the phenyl ring, so if you were to selectively hydrogenate that single bond, it would probably be a little different to the mean C=C enthalpy value... but it’s not a part of the aromatic system, so it’s not as resistant to hydrogenation as the aromatic ring. There is difference between aromaticity and conjugation. The ethenyl C=C is conjugated, but still close to a regular C=C bond, it is not aromatic.

See image for comparison of enthalpy changes:
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