Benzenes - Kekule and stability

Hi,

Can someone explain this business with Arenes, about how Benzenes can become Kekule benzenes?

I don't understand the stability and how it's represented on the ethalpy diagram.

Thanks.

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Reply 1

EierVonSatan

arenes such as bezene have a conjugated (joined up) pi system, kekule incorrectly theorized that benzene had alternating double bonds, so his was just his model. So theres no such thing as a 'kekule benzene' and although its often drawn with 3 single and 3 double bonds, the bond lengths are all equal

Reply 2

GlassVase

So the normal benzene with the delocalised electrons (1 from each carbon) is more or less stable then kekule's benzene?

Also, why is it higher on an enthalpy profile diagram?

Reply 3

EierVonSatan

the delocalised benzene is more stable than kekules model predicted, the 'real' benzene the one with a delocalised structure is definatly lower than kekules model on an enthalpy profile

Reply 4

forgottenromeo

Kekules model is an actual substance its 1,3,5 cyclo hexene. the higher enthalpy profile
is caused by greater stability in the molecule and an increase in bonds to be over come.

Reply 5

Caliowin

Kekule structure consisted alternative single and double bonds, which switched.

This was proved to be incorrect as,

- it should undergo electrophilic addition- but kekule's structure didn't
- x ray diffraction showed equal electron density around each carbon, indicating all the bonds are the same length, and the length being halfway between a single and a double bond.

Benzene is more stable as each carbon uses one of its electrons to for a pi bond, which are delocalised around the whole ring, making it more stable than other alkenes.

That's all I can remember right now, hope its helpful in some way.

Reply 6

EierVonSatan

forgottenromeo

Kekules model is an actual substance its 1,3,5 cyclo hexene. the higher enthalpy profile
is caused by greater stability in the molecule and an increase in bonds to be over come.

no. there is no substance with the structure that kekule predicted. Also if something is higher on an energy profile diagram, it is more unstable

Reply 7

charco

Study Forum Helper

Kekule had a dream about a snake eating its tail ... I hope this is useful in some way

Reply 8

EierVonSatan

charco

Kekule had a dream about a snake eating its tail ... I hope this is useful in some way

haha

Reply 9

forgottenromeo

The kekule structure is feasible as its found in penicillins and when they break down it reacts with hydrogen to form it, its just not the structure of benzene.

Reply 10

EierVonSatan

forgottenromeo

The kekule structure is feasible as its found in penicillins and when they break down it reacts with hydrogen to form it, its just not the structure of benzene.

Trust me its not a real structure, penicillins mode of action is to do with the beta-lactam ring, the aromatic group is just a side chain variant. There are other known C6H6 structures, but cyclohexatriene isn't one of them :s-smilie:

Reply 11

forgottenromeo

... There is no reason for it not to exsist, and by experiment it can be proved to exsist. I said nothing about the mode of action of penicillin i just said that the triple double bonds in a cyclo hexene exsist in it

Reply 12

EierVonSatan

forgottenromeo

Best of luck :smile:

Reply 13

charco

Study Forum Helper

EierVonSatan

Best of luck :smile:

Reply 14

HawkAhriman

forgottenromeo

it doesn't seem like you're quite getting the difference between 3 double bonds, which is perfectly possible, and 3 ALTERNATING double bonds, which...just...doesn't happen.

Reply 15

forgottenromeo

Its plausible though, if you look at certain mechanisms for reactions double bonds breaking and reforming does happen.

Reply 16

HawkAhriman

no...it's...really not. Bonds don't just break and reform alternatingly for no reason. that's not how it works.

Reply 17

forgottenromeo

And if the substance was in a vacuum then there would be no reason for it to happen, but it is also under the effect of various radiations. Not to mention the entire reason they are called electron clouds.

Reply 18

RosiePosiePuddingAndPie

I think you're a bit confused, forgottenromeo. I'll post the bit in my Chemical Ideas book (the Salters syllabus book) where it talks about the Kekule structure...

You can think of the delocalised structure of benzene as halfway between the two extreme structures:

These are sometimes called Kekule structures because they were first proposed by August Kekule in 1865. Neither of these forms actually exists, and the delocalised arrangement is often represented by drawing a circle inside the ring.

Reply 19

forgottenromeo

You can't rely on texts books, some of the things in them are wrong but we still have to answer in some of the cases, such as the exsistance of mesosomes in prokaryotic cells.