EAS Donating Group question

[FeralisExtremum]

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In an EAS reaction, why are -OH groups more donating than -OR groups? Shouldn't the alkyl chain make it even more donating for the same reason that alkoxides are stronger bases than deprotonated alcohols?

 

[DAT Destroyer]

In an EAS reaction, why are -OH groups more donating than -OR groups? Shouldn't the alkyl chain make it even more donating for the same reason that alkoxides are stronger bases than deprotonated alcohols?

Great question.....Both groups can donate electrons......OH donates by resonance,,,,,,while groups like Methyl or Ethyl donate by Hyperconjugation. Studies have shown Resonance is the more important factor....and as such will Almost ALWAYS trump hyperconjugation. Also.....NH2 and OH both are very activating.....if you examine the resonance forms.......these small atoms are in the SAME PERIOD as carbon.....thus the orbital SIZE is similar...this translates into high Stability. Thanks for an exciting question !!!!

With that said.....Now I ASK YOU a question.......Which halogen and WHY..... is most activating if sitting on a benzene ring for EAS reactuons ?

Good work Feralis!

Dr. Romano

 

[FeralisExtremum]

Great question.....Both groups can donate electrons......OH donates by resonance,,,,,,while groups like Methyl or Ethyl donate by Hyperconjugation. Studies have shown Resonance is the more important factor....and as such will Almost ALWAYS trump hyperconjugation. Also.....NH2 and OH both are very activating.....if you examine the resonance forms.......these small atoms are in the SAME PERIOD as carbon.....thus the orbital SIZE is similar...this translates into high Stability. Thanks for an exciting question !!!!

With that said.....Now I ASK YOU a question.......Which halogen and WHY..... is most activating if sitting on a benzene ring for EAS reactuons ?

Good work Feralis!

Dr. Romano

Thank you Dr. Romano, that cleared it up for me! Just out of curiosity, why doesn't the -OR group also donate via resonance? Can it not donate its lone pairs like -OH can?

As for your question, it's a very tough one and I'm not entirely sure. Intuitively I would want to say that Fluorine being the most electronegative and therefore deactivating would be the worst at activating, and that Iodine being the least electronegative would subsequently be the best at activating. But as you just mentioned, resonance is an important factor here - and resonance is why halogens are ortho/para directors despite being inductively withdrawing - their extra resonance intermediate has added stability. So I'm thinking that it may be the halogen which forms the most stable resonance intermediate here that ends up being the most activating, rather than just defaulting to the inductive rule. I'm also not sure if Fluorine can participate in this resonance because I've read that it can't form double bonds (is this true?). It's a really great question and I'm curious what the answer is!

Also, on the subject of EAS reactions - if you have two withdrawing groups of different strength on a benzene ring, which one ends up directing the majority of the product? The weaker or stronger withdrawing group?

 

[DAT Destroyer]

The OR group....i.e.....methoxy, ethoxy etc.......can indeed donate electrons by resonance. This is why an OR group is more activating than an R group......recall.....methoxy groups are more reactive than a methyl group in EAS reactions. Believe it or not.....I have seen many PhDs get this wrong about the halogens ! Fluorine is the most reactive of the halides......Since the 1950s it was observed that fluorobenzene is almost as reactive as benzene !!!!! Why ? Resonance has to do with part of it,,,,but F and C.....are similar in size......Carbon and Fluorine BOTH use 2p orbitals,,,,,the fit is very efficient. Iodine is Huge......uses 5p orbitals against 2p on carbon.....thus lease effective, poor resonance contributor ..,,,,,thus least reactive ! As a rule,,,,,,,,If two groups are on a ring.....the more activating group does the directing ! What if they both are deactivating ? LOL.....Good luck......you get a mixture of products that can vary with conditions....and horrible yields ! Keep up the great work......

Good luck guys,,,,,study hard !!!

Dr. Romano

 

[FeralisExtremum]

Wow, I would never have been able to figure that out on my own. Thanks again Dr. Romano!

 

[PocketRocket]

The OR group....i.e.....methoxy, ethoxy etc.......can indeed donate electrons by resonance. This is why an OR group is more activating than an R group......recall.....methoxy groups are more reactive than a methyl group in EAS reactions. Believe it or not.....I have seen many PhDs get this wrong about the halogens ! Fluorine is the most reactive of the halides......Since the 1950s it was observed that fluorobenzene is almost as reactive as benzene !!!!! Why ? Resonance has to do with part of it,,,,but F and C.....are similar in size......Carbon and Fluorine BOTH use 2p orbitals,,,,,the fit is very efficient. Iodine is Huge......uses 5p orbitals against 2p on carbon.....thus lease effective, poor resonance contributor ..,,,,,thus least reactive ! As a rule,,,,,,,,If two groups are on a ring.....the more activating group does the directing ! What if they both are deactivating ? LOL.....Good luck......you get a mixture of products that can vary with conditions....and horrible yields ! Keep up the great work......

Good luck guys,,,,,study hard !!!

Dr. Romano

Sorry but just to clarify... Fluorine is more activating than Iodine because the electrons and orbitals in fluorine and carbon are, in a way, on the same "level" and therefore fluorine is able to contribute electron density to benzene better than iodine???

Also, I thought that the reason why OH is more activating than OR is because of electronegativity. The difference in electronegativity in OH(O and H, farther on periodic table) is greater than in OR(O and C, closer on the periodic tale) so the oxygen in OH will attract more electrons to it self than the oxygen in OR. Therefore, when attached to a benzene, Benzene-OH, the O will have more electrons to contribute. Does that make sense?