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why is methylbenzene more reactive than benzene?

Reply 20

EierVonSatan

21

DeanK2

What!!

What is the point of learning delocalisation if we don't use it in organic chemistry?

It's an important concept but is also limited in its own way to an understanding of mechanisms :borat:

Reply 21

Mr Snips

17

EierVonSatan

Methylbenzene (toluene) is more reactive because the methyl group is electron donating (positive inductive effect) and thus makes the benzene ring more electron rich and so a better nucleophile to attack electrophiles in electrophilic substitution reactions.

This thread desperately needed EierVonSaten

Reply 22

EierVonSatan

21

Mr Snips

This thread desperately needed EierVonSatan

Reply 23

ChemistBoy

16

DeanK2

What!!

What is the point of learning deloocalization if we don't use it in organic chemistry?

Don't worry, we do it properly in physical chemistry where a mechanism actually has some relation to what actually goes on.

Reply 24

EierVonSatan

21

ChemistBoy

Don't worry, we do it properly in physical chemistry where a mechanism actually has some relation to what actually goes on.

I'll not have you poison young minds with reality! :mad:

Reply 25

ChemistBoy

16

EierVonSatan

I'll not have you poison young minds with reality! :mad:

Bloody curly arrows! I wish electrons were like that it would make life simpler.
I actually had to put some curly arrows in my thesis, I mean, I'm supposed to be a respectable physical chemist... I had to put an extra equation in just to cancel it out.

Reply 26

EierVonSatan

21

ChemistBoy

Bloody curly arrows! I wish electrons were like that it would make life simpler.
I actually had to put some curly arrows in my thesis, I mean, I'm supposed to be a respectable physical chemist... I had to put an extra equation in just to cancel it out.

And yet, its more scientific than ''well we have this electron cloud which rearranges to form this electron cloud [gets out supercomputer]...so yes the drug goes through the benzene ring...oh no, wait...[bashes supercomputer]...stupid computer :rant:

''

Reply 27

ChemistBoy

16

EierVonSatan

And yet, its more scientific than ''well we have this electron cloud which rearranges to form this electron cloud [gets out supercomputer]...so yes the drug goes through the benzene ring...oh no, wait...[bashes supercomputer]...stupid computer :rant:

''

Nah, I've seen delocalised benzene rings, with an STM. I've also seen people do reactions with an STM - that is awesome.

Reply 28

DeanK2

Good old fashioned physical chemistry - that's where the real stuff is.

Reply 29

EierVonSatan

21

ChemistBoy

Nah, I've seen delocalised benzene rings, with an STM. I've also seen people do reactions with an STM - that is awesome.

Sounds quite cool actually, but I never really got into nanochemistry to be honest, may have something to do with Julie Macpherson and her quite disturbing love of AFM.

Reply 30

ChemistBoy

16

EierVonSatan

Sounds quite cool actually, but I never really got into nanochemistry to be honest, may have something to do with Julie Macpherson and her quite disturbing love of AFM.

I've never really got on with AFM myself, although when we eletrosprayed some polymers on to silicon we did get some pretty patterns. STM allows you to see electron distributions on an atomic scale - combined with spectroscopy it is such a brilliant technique. On its own it is a bit like covering a load of lemons and oranges in a blanket and then trying to find the grapefruit.

Reply 31

mgfrost

OP

Charzhino

Methyl groups are electron releasing. They release electrons through a conjugated effect so it increases the partial negative charge on benzene in the delocalised structure.

WOW! this thread really exploded. Thanks for everyone's contribution to my question, though it deteriorated towards the end.

I quoted the above post because I think it best answers my question, although I would request that Charzhino posts again to explain why methyl gorups are electron releasing.

As for the rest of the explainations, I would like to point out that the methyl group activates the ring, making it more succeptable to electrophilic, not nucleophilic attack. Also seeing as the CH3 has replaced a Hydrogen, this is unlikely to be more electron "pushing" than the hydrogen was.

Perhaps, for completness of the thread Charzhino could give a brief outline of the conjugating effect he mentioned.

Many thanks to everyone :smile:

Reply 32

Charzhino

19

mgfrost

Perhaps, for completness of the thread Charzhino could give a brief outline of the conjugating effect he mentioned.

Ye the thread got a bit off topic towards the end lol.

Basically, alkyl groups attached to benzene activate the ring and direct subsitution towards the ortho and para positions.
Alkyl groups donate electrons weakly by the overlap of one of the C-H Sigma bonds with the Pi system in the benzene ring. This is known as sigma conjugation.
The donated electrons from the C-H bond comes from the small difference in electronegativty between carbon and hydrogen.
As for why this particularly happens I'm not too sure. May be the opposite reason to why electron withdrawing groups like NO2 like to withdraw electrons if you know what I mean.

Reply 33

EierVonSatan

21

Charzhino

Alkyl groups donate electrons weakly by the overlap of one of the C-H Sigma bonds with the Pi system in the benzene ring. This is known as sigma conjugation.

Hmmm, is this what you've been taught?

Reply 34

mgfrost

OP

EierVonSatan

Hmmm, is this what you've been taught?

I've been tipped off that you are among the best chemists on this forum. Please break it down for me - really simply. Why does the presence of a methyl group make the ring more succeptable to electrophilic attack, such as nitrogenation?? Everyone seems a bit confused over why, and have given me how.

:confused:

Reply 35

DeanK2

mgfrost

I've been tipped off that you are among the best chemists on this forum. Please break it down for me - really simply. Why does the presence of a methyl group make the ring more succeptable to electrophilic attack, such as nitrogenation?? Everyone seems a bit confused over why, and have given me how.

:confused:

I have always assumed the 'why' to be because of hyperconjugation.

Reply 36

EierVonSatan

21

The truth is that there's no evidence for hyperconjugation in neutral hydrocarbons, but it offers the most reasonable explanation of what's going on.

Why is tough, ultimatley why do molecules do anything? To reach a more stable state, the methyl group pushes its electron density into the ring to achieve the lowest energy ground state in electronic terms. Its like trying to explain why benzene doesn't have alternating single and double bonds - there's just a more stable configuration it can adopt.

Reply 37

Charzhino

19

EierVonSatan

Hmmm, is this what you've been taught?

I got this directly from my first year organic chemistry notes.

Reply 38

ChemistBoy

16

Charzhino

I got this directly from my first year organic chemistry notes.

It's a postulate, nothing more. I think the problem with conceptualizing it is that we are taught to think of electron densities in a far more localized and geometrically constricted way than is reality. Valence level spectroscopy shows us that these outlying molecular orbitals are far from discrete and that electron density can shift from the precise shapes that linear combination of atomic orbitals tells us in order to minimize energy.

Reply 39

Rozzo89

12

The methly substituent as correctly said acts as an activating group,more approprately its anortho-para directing group which enhances electron density at the 2,4,6 position - or ortho/para positions. It does this predominatelyby the inductive effect of the methyl group into the ring.

why is methylbenzene more reactive than benzene?

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