aromatic substitution

[capn jazz]

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Are we still responsible for aromatic substitution on the MCAT? As in, whether substituents are O/P directors or M directors, or activating or deactivating?

 

[FuturaDocta]

Are we still responsible for aromatic substitution on the MCAT? As in, whether substituents are O/P directors or M directors, or activating or deactivating?

I would know it since it is a relatively simple concept. 🙂

 

[G1SG2]

Are we still responsible for aromatic substitution on the MCAT? As in, whether substituents are O/P directors or M directors, or activating or deactivating?

Read my signature

 

[capn jazz]

Well in THAT case, you guys asked for it!

So you have a substitued ethyl benzoate (benzene with a COOEt group, and another substituent, with location on benzene unspecified). The question (which is kind of complicated and involves a graph) essentially is asking about the electronic effects of having either an EWG or an EDG as the substituent when the sub'd ethyl benzoate undergoes base-catalyzed hydrolysis.

They claim that an EWG INCREASES the rxn rate by stabilizing the negative charge that is involved in a base-catalyzed hydrolysis.

I always remembered aromatic substitutions as EWG are deactivating while EDG are activating - is that only for acid-catalyzed substitutions? Is the fact that it's a base-catalyzed substitution the reason that an EWG stabilizes the rxn?

Help!

 

[G1SG2]

Well in THAT case, you guys asked for it!

So you have a substitued ethyl benzoate (benzene with a COOEt group, and another substituent, with location on benzene unspecified). The question (which is kind of complicated and involves a graph) essentially is asking about the electronic effects of having either an EWG or an EDG as the substituent when the sub'd ethyl benzoate undergoes base-catalyzed hydrolysis.

They claim that an EWG INCREASES the rxn rate by stabilizing the negative charge that is involved in a base-catalyzed hydrolysis.

I always remembered aromatic substitutions as EWG are deactivating while EDG are activating - is that only for acid-catalyzed substitutions? Is the fact that it's a base-catalyzed substitution the reason that an EWG stabilizes the rxn?

Help!

You have to look at what the EWG does. In electrophilic aromatic substitution, there's a positively charged arenium ion intermediate. The EWG groups would be bad for that type of reaction because they would exacerbate the + charge on the arenium ion by withdrawing electrons/electron density instead of the EDGs which donate electrons and help out the + charge a bit. But it seems to be correct-if the EWG can stabilize the NEGATIVE charge involved in the reaction you mentioned just as good as the EDG can stabilize the positive charge in the arenium ion, then that would definitely increase the rate of the reaction. Just look at the role the EWG plays in the type of reaction you're faced with-in regular electrophilic aromatic substitution, EWG are deactivating, as you said, but that's because they DESTABILIZE the positive charge on the arenium ion. In this case, they STABILIZE the negative charge by withdrawing electrons/electron density. Hope this helps.

 

[capn jazz]

I guess what I'm confused about is the fact that I'm blanking on how a base-catalyzed substitution involves a - charge. I get why I was confused about electrophilic substitution versus this but can anyone think of an example or a mechanism?

 

[loveoforganic]

If an EWG does anything, it's inductively, so I'd think the effect would be quite minimal as the substituent would be at minimum 3 sigma bonds away from the negative charge, and inductive effect decreases significantly with distance.

 

[G1SG2]

I guess what I'm confused about is the fact that I'm blanking on how a base-catalyzed substitution involves a - charge. I get why I was confused about electrophilic substitution versus this but can anyone think of an example or a mechanism?

It does involve a negative charge. There are two types of aromatic substitutions-electrophilic aromatic substitutions and NUCLEOPHLIC aromatic substitutions. I'll try to give you a mechanism that I remember and you can try to draw it out. Picture a 2,3 dinitrochlorobenzene molecule. You have OH-, a strong base, that you want to substitute for the Cl. Since it's a strong base, it will attack at the 1 position (where the chlorine is at). This is a SLOW step, since you're losing aromaticity. The electrons from the pi bond that you broke are now on the 2ND CARBON (where the 1st NO2 substituent is hanging out, so now your first carbon is an sp3 carbon with both Cl and OH). Thus, the negative charge get's delocalized around the resonance stabilized intermediate due to your two very nice EWG (NO2). The negative delocalized around the ring and gets a chance to stop by at carbons 2 and 3 where the NO2 groups are. Step 2-the Cl leaves (from carbon 1) and the electrons that are hanging at the 2nd carbon now return aromaticity (this is a fast step-you're regaining aromaticity).

So there you have it-nucleophilic aromatic substitution in which you have a negatively charged intermediate which is resonance stabilized via EWGs like NO2.

 

[G1SG2]

If an EWG does anything, it's inductively, so I'd think the effect would be quite minimal as the substituent would be at minimum 3 sigma bonds away from the negative charge, and inductive effect decreases significantly with distance.

No no, in nucleophilic aromatic substitution, the negative charge is RIGHT where the EWGs are, so they are resonance stabilized, and resonance kicks butt (or as my orgo professor used to say)

 

[capn jazz]

Thank you! That makes perfect sense. Sometimes I wish I had taken the MCAT right after OChem... I used to know this stuff!

 

[loveoforganic]

No no, in nucleophilic aromatic substitution, the negative charge is RIGHT where the EWGs are, so they are resonance stabilized, and resonance kicks butt (or as my orgo professor used to say)

If you add base to that compound aren't you going to preferentially hydrolyze the ester as opposed to breaking aromaticity?

 

[G1SG2]

If you add base to that compound aren't you going to preferentially hydrolyze the ester as opposed to breaking aromaticity?

That can happen too, I suppose the yield wouldn't be too great and the reaction would have to be controlled. Then again, that's also the case with electrophilic aromatic substitution in which you break aromaticity in the the beginning to get that arenium ion.

 

[boaz]

This kind of thread pops up every few days.

Aromatic chemistry is NOT tested on the MCAT. Take a look at the AAMC's outline and you won't find any of this.