kekule's model vs Benzene Watch

Leah.J
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Discuss the evidence that the better representation of Benzene is with a delocalized ring of electrons :

As part of my answer, can I say that if Kekule's model was the actual structure of Benzene, you would expect to find 4 di halo benzene isomers but there are only 3 ?

I don't think I phrased it very well, here's a picture of what I mean.
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Could someone correct the statement please ?
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MIKESPIKE10
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you need to talk about both kekule's model and the actual benzene in terms of electron systems.

First start by discussing the three main differences (equal bond lengths in benzene, resistance to electrophilic addition reactions and enthalpy of hydrogenation being less exothermic and therefore more energetically stable than predicted)

then talk about what these findings must mean about the electron system. So talk about electron density being lower because of resistance to electrophilic addition reactions meaning it would better fit a delocalised pi system, then discuss equal length bonds suggesting that each carbon loses an electron from their p- orbitals

How many marks is it?
Hope this helps
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Leah.J
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(Original post by MIKESPIKE10)
you need to talk about both kekule's model and the actual benzene in terms of electron systems.

First start by discussing the three main differences (equal bond lengths in benzene, resistance to electrophilic addition reactions and enthalpy of hydrogenation being less exothermic and therefore more energetically stable than predicted)

then talk about what these findings must mean about the electron system. So talk about electron density being lower because of resistance to electrophilic addition reactions meaning it would better fit a delocalised pi system, then discuss equal length bonds suggesting that each carbon loses an electron from their p- orbitals

How many marks is it?
Hope this helps
6, I never thought the resistance of Benzene to electrophilic addition meant that it's electron density was lower, I thought it was resistant to it because it would disturb it's stability (?) .
I know the other points but is the isomer thing incorrect ?
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Kian Stevens
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The delocalised model is better for three main reasons:
  • All bonds in benzene are of the same length (they're actually intermediate in length between a C-C bond and a C=C bond)
  • Benzene doesn't decolourise things such as bromine water
  • The enthalpy of hydrogenation of benzene is less exothermic than that predicted for cyclohexatriene

If the Kekule structure was correct, then the above points should be different:
  • There would be two different types of bond -- C-C and C=C -- hence two different bond lengths
  • Benzene would decolourise bromine water etc.
  • The enthalpy of hydrogenation of benzene would be equal to what is predicted for cyclohexatriene

But why is this the case?
Well, the Kekule structure implies benzene is an alkene. Benzene is not an alkene...
If benzene was an alkene, it would exhibit the properties of one, which I have stated in the second bullet-point list

What actually happens is benzene constantly resonates between both Kekule structures
This resonance implies that there's electron density delocalised all around the molecule, above and below the ring (this is what the benzene ring actually is), instead of just being localised to one area via \pi-bonds -- this electron density delocalisation occurs as all \pi-bonds are exactly parallel to each other

Now, since benzene doesn't have any localised \pi-bonds, its bonds are actually an intermediate between a C-C bond and a C=C bond: this is why all bonds in benzene are of the same length, which is intermediate between a C-C bond and C=C bond
This benzene ring is extremely stable, thanks to its extra 'resonance stability', and this means that breaking it is very thermodynamically unfavourable: this is why the enthalpy of hydrogenation of benzene is less exothermic than predicted
What constitutes 'breaking' the ring? Well, pretty much any electrophilic addition reaction: benzene's stability resists these, and so electrophilic substitution reactions are favoured instead: this is why benzene doesn't decolourise bromine water
Last edited by Kian Stevens; 3 weeks ago
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Leah.J
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(Original post by Kian Stevens)
The delocalised model is better for three main reasons:
  • All bonds in benzene are of the same length (they're actually intermediate in length between the a C-C bond and a C=C bond)
  • Benzene doesn't decolourise things such as bromine water
  • The enthalpy of hydrogenation of benzene is less exothermic than that predicted for cyclohexatriene

If the Kekule structure was correct, then the above points should be different:
  • There are two different types of bond -- C-C and C=C -- hence two different bond lengths
  • Benzene would decolourise bromine water etc.
  • The enthalpy of hydrogenation of benzene would be equal to what is predicted for cyclohexatriene

But why is this the case?
Well, the Kekule structure implies benzene is an alkene. Benzene is not an alkene...
If benzene was an alkene, it would exhibit the properties of one, which I have stated in the second bullet-point list

What actually happens is benzene constantly resonates between both Kekule structures
This resonance implies that there is electron density delocalised all around the molecule, above and below the ring, instead of just being localised to one bond via \pi-bonds (this is what the benzene ring actually is) -- this electron density delocalisation occurs as all \pi-bonds are exactly parallel to each other

Now, since benzene doesn't have any localised \pi-bonds, its bonds are actually an intermediate between a C-C bond and a C=C bond: this is why all bonds in benzene are of the same length, which is intermediate between a C-C bond and C=C bond
This benzene ring is extremely stable, thanks to its extra 'resonance stability', and this means that breaking it is very thermodynamically unfavourable: this is why the enthalpy of hydrogenation of benzene is less exothermic than predicted
What constitutes 'breaking' the ring? Well, pretty much any electrophilic addition reaction: benzene's stability resists these, and so electrophilic substitution reactions are favoured instead: this is why benzene doesn't decolourise bromine water

(Also, your isomers point is incorrect... Two of the isomers you've drawn are exactly the same and so three should be observed for the 1,2-, 1,3-, and 1,4- positions... You've drawn two 1,2- isomers)
which isomers are exactly the same ?
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BobbJo
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(Original post by Leah.J)
Discuss the evidence that the better representation of Benzene is with a delocalized ring of electrons :

As part of my answer, can I say that if Kekule's model was the actual structure of Benzene, you would expect to find 4 di halo benzene isomers but there are only 3 ?

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Could someone correct the statement please ?
You are correct. There are 4 different isomers according to Kekule's model. In one ortho isomer, the 2 bromine atoms are across a C=C double bond while in the other, they are across a C-C single bond. In reality there are only 3 isomers.

Other points have been discussed by other posts.
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Kian Stevens
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(Original post by BobbJo)
You are correct. There are 4 different isomers according to Kekule's model. In one ortho isomer, the 2 bromine atoms are across a C=C double bond while in the other, they are across a C-C single bond. In reality there are only 3 isomers.

Other points have been discussed by other posts.
Correct, I didn't spot that...
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Kian Stevens
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Also, the lower electron density in benzene isn't what causes benzene to resist electrophilic addition reactions, it's the thermodynamics behind it which dictates that

The lower electron density just means that catalysts (you might have seen them called halogen carriers) are required, in order to generate an electrophile, as benzene can't do it by itself like an alkene would
For example, the halogenation of benzene requires a catalyst, as benzene wouldn't be able to induce a dipole in the halogen

However, if benzene has directing (activating) groups attached to it -- e.g. an hydroxyl group (this would be phenol) -- catalysts are made redundant, as the increased electron density means a dipole can now be induced... Phenol can therefore be halogenated by reacting it with just halogens, without the need of a catalyst
Regardless of this though, a substitution reaction would still take place, as the thermodynamics are still dictating that
Last edited by Kian Stevens; 3 weeks ago
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Y-NR
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https://prnt.sc/ntkp7y
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Leah.J
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(Original post by Kian Stevens)
Also, the lower electron density in benzene isn't what causes benzene to resist electrophilic addition reactions, it's the thermodynamics behind it
The lower electron density just means that catalysts (you might have seen them called halogen carriers) are required, in order to generate an electrophile, as benzene can't do it by itself like an alkene would
For example, the halogenation of benzene requires a catalyst, as benzene wouldn't be able to induce a dipole in the halogen

However, if benzene has directing (activating) groups attached to it -- e.g. an hydroxyl group (this would be phenol) -- catalysts are made redundant, as the increased electron density means a dipole can now be induced... Phenol can therefore be halogenated by reacting it with just halogens, without the need of a catalyst
However, an addition reaction would still take place, as the thermodynamics are still dictating that
Okay thank you!
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MIKESPIKE10
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surely it partly is related to low electron density because it isn't nucleophilic enough or have a high enough electron density to induce a dipole in a neighbouring molecule?
(Original post by Kian Stevens)
Also, the lower electron density in benzene isn't what causes benzene to resist electrophilic addition reactions, it's the thermodynamics behind it which dictates that

The lower electron density just means that catalysts (you might have seen them called halogen carriers) are required, in order to generate an electrophile, as benzene can't do it by itself like an alkene would
For example, the halogenation of benzene requires a catalyst, as benzene wouldn't be able to induce a dipole in the halogen

However, if benzene has directing (activating) groups attached to it -- e.g. an hydroxyl group (this would be phenol) -- catalysts are made redundant, as the increased electron density means a dipole can now be induced... Phenol can therefore be halogenated by reacting it with just halogens, without the need of a catalyst
Regardless of this though, an addition reaction would still take place, as the thermodynamics are still dictating that
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Kian Stevens
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(Original post by MIKESPIKE10)
surely it partly is related to low electron density because it isn't nucleophilic enough or have a high enough electron density to induce a dipole in a neighbouring molecule?
That's literally what I said?
I gave two examples of when there's low electron density, and when there's high enough electron density to induce a dipole, yet both scenarios undergo electrophilic substitution regardless

Thermodynamics drives the entirety of Chemistry, this especially...
Last edited by Kian Stevens; 3 weeks ago
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MIKESPIKE10
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I think there's a contradiction, it is one of the reasons for current benzene model resisting electrophilic addition
"Also, the lower electron density in benzene isn't what causes benzene to resist electrophilic addition reactions, it's the thermodynamics behind it which dictates that"
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MIKESPIKE10
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^^
(Original post by Kian Stevens)
That's literally what I said?
I gave two examples of when there's low electron density, and when there's high enough electron density to induce a dipole, yet both scenarios undergo electrophilic addition regardless

Thermodynamics drives the entirety of Chemistry, this especially...
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Leah.J
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If I didn't add a halogen carrier would Benzene still react but a lower rate ? Or would it not occur at all ?
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MIKESPIKE10
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it wouldn't react at all...
also in terms of your drawings, surely you would desaturate your double bonds as it is an addition reaction instead of a substitution reaction?
(Original post by Leah.J)
If I didn't add a halogen carrier would Benzene still react but a lower rate ? Or would it not occur at all ?
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Kian Stevens
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(Original post by MIKESPIKE10)
I think there's a contradiction, it is one of the reasons for current benzene model resisting electrophilic addition
"Also, the lower electron density in benzene isn't what causes benzene to resist electrophilic addition reactions, it's the thermodynamics behind it which dictates that"
Regardless of the amount of electron density, benzene will still favour electrophilic substitution reactions because it means that the benzene ring is maintained, which is a favourable process due to the stability of the ring... Hence, benzene's resistance to electrophilic addition is due to the thermodynamics behind it

Electrophilic addition reactions with the same reactants would mean that the benzene ring would have to be broken, which is thermodynamically unfavourable, regardless of how much electron density there is

(Original post by Leah.J)
If I didn't add a halogen carrier would Benzene still react but a lower rate ? Or would it not occur at all ?
I believe it wouldn't react at all
Benzene simply doesn't have enough electron density to form a dipole, and thus an electrophile, by itself in a halogen
Last edited by Kian Stevens; 3 weeks ago
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MIKESPIKE10
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I thought benzene favoured electrophilic substitution reactions and resisted electrophilic addition reactions?
(Original post by Kian Stevens)
Regardless of the amount of electron density, benzene will still favour electrophilic addition reactions because it means that the benzene ring is maintained, which is a favourable process due to the stability of the ring... Hence, benzene's resistance to electrophilic addition is due to the thermodynamics behind it

Electrophilic substitution reactions with the same reactants would mean that the benzene ring would have to be broken, which is thermodynamically unfavourable, regardless of how much electron density there is


I believe it wouldn't react at all
Benzene simply doesn't have enough electron density to form a dipole, and thus an electrophile, by itself in a halogen
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Kian Stevens
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(Original post by MIKESPIKE10)
I thought benzene favoured electrophilic substitution reactions and resisted electrophilic addition reactions?
I see what you mean, you're right
I accidentally put addition instead of substitution, so I've updated the comment you linked and will update the previous one too
Apologies
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Leah.J
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Hi, last question, are halogen carriers also lewis acids ? And does that mean that the rxn between the halogen carrier and say the halogen, where an electrophile is formed, is an acid base rxn ?
(Original post by Kian Stevens)
Regardless of the amount of electron density, benzene will still favour electrophilic substitution reactions because it means that the benzene ring is maintained, which is a favourable process due to the stability of the ring... Hence, benzene's resistance to electrophilic addition is due to the thermodynamics behind it

Electrophilic addition reactions with the same reactants would mean that the benzene ring would have to be broken, which is thermodynamically unfavourable, regardless of how much electron density there is


I believe it wouldn't react at all
Benzene simply doesn't have enough electron density to form a dipole, and thus an electrophile, by itself in a halogen
(Original post by MIKESPIKE10)
it wouldn't react at all...
also in terms of your drawings, surely you would desaturate your double bonds as it is an addition reaction instead of a substitution reaction?
I just saw this, when I desaturate the double bonds, how would I draw the last 2 kekule structures ?
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