Hard Q on Withdrawing/Donating Affects and Effects

[GoldenTuth]

Okay I am getting seriously pissed off and confused about the donating/withdrawing affects and effects.

Reading my notes and destroyer and some other stuff it looks like donating and withdrawing effects contradict their effect on acidity. I won't show exact examples of contradictions because I lost them, but I'll go over again and let authors know.

So let's use carboxylic acid as example, please tell me if I am thinking correctly here.

Q1: What is benzene ring?

Simple carboxylic acid with a methyl group has pKa of almost 5
But, carboxylic acid with a benzene ring, Benzoic Acid, has a pKa of like 4

SO, is benzene ring donating or withdrawing?
From my understanding, since benzene is aromatic, it should be neither withdrawing nor donating, (maybe a little donating due to the electron cloud, but not enough to talk about) so it should depend on the group that's on it? activating/deactivating groups?

I don't get what benzenes role is if you look at it as a functional group rather than an aromatic compound with a withdrawing group on it. Does it donate or withdraw from the acidic -COOH group?

2. Also
(Cl)3C-COOH vs (Me)3C-COOH

Which one is more acidic?

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[justadream]

@GoldenTruth

If you're really into that electron donating/withdrawing thing, I guess in benzoic acid, the benzene ring is electron withdrawing (although I don't think people refer to it as that).

Benzene rings stabilize the negative charge formed after the COOH donates its H to become COO-. The electron can be delocalized through the benzene ring, thereby stabilizing it. Stability is GOOD and makes benzoic acid readily give up its H. This is why benzoic acid is a stronger acid than a normal COOH.

For your second question, the one with the Chlorine is more acidic. The Cl's are electron withdrawing (inductive effect) and stabilize the negative charge after the COOH donates its H to become COO-.

 

[texan2414]

@GoldenTruth

If you're really into that electron donating/withdrawing thing, I guess in benzoic acid, the benzene ring is electron withdrawing (although I don't think people refer to it as that).

Benzene rings stabilize the negative charge formed after the COOH donates its H to become COO-. The electron can be delocalized through the benzene ring, thereby stabilizing it. Stability is GOOD and makes benzoic acid readily give up its H. This is why benzoic acid is a stronger acid than a normal COOH.

For your second question, the one with the Chlorine is more acidic. The Cl's are electron withdrawing (inductive effect) and stabilize the negative charge after the COOH donates its H to become COO-.

Can you double check this?
I tried to resonate the electron on the benzene ring and ended up creating more charges - which destabilizes the system.

But I do agree that electron density from the benzene ring can be donated to the carbonyl carbon through conjugation.

 

[justadream]

Can you double check this?
I tried to resonate the electron on the benzene ring and ended up creating more charges - which destabilizes the system.

But I do agree that electron density from the benzene ring can be donated to the carbonyl carbon through conjugation.

Hmm actually I'm not so sure anymore.

 

[Teleologist]

Can you double check this?
I tried to resonate the electron on the benzene ring and ended up creating more charges - which destabilizes the system.

But I do agree that electron density from the benzene ring can be donated to the carbonyl carbon through conjugation.

The ring does not participate in resonance withdrawal. I really doubt that there is hyperconjugation here either.

 

[Teleologist]

Okay I am getting seriously pissed off and confused about the donating/withdrawing affects and effects.

Reading my notes and destroyer and some other stuff it looks like donating and withdrawing effects contradict their effect on acidity. I won't show exact examples of contradictions because I lost them, but I'll go over again and let authors know.

So let's use carboxylic acid as example, please tell me if I am thinking correctly here.

Q1: What is benzene ring?

Simple carboxylic acid with a methyl group has pKa of almost 5
But, carboxylic acid with a benzene ring, Benzoic Acid, has a pKa of like 4

SO, is benzene ring donating or withdrawing?
From my understanding, since benzene is aromatic, it should be neither withdrawing nor donating, (maybe a little donating due to the electron cloud, but not enough to talk about) so it should depend on the group that's on it? activating/deactivating groups?

I don't get what benzenes role is if you look at it as a functional group rather than an aromatic compound with a withdrawing group on it. Does it donate or withdraw from the acidic -COOH group?

2. Also
(Cl)3C-COOH vs (Me)3C-COOH

Which one is more acidic?

Stop fixating on the damn benzene ring. Is a methyl group electron donating or withdrawing with respect to carboxylic acids?

 

[justadream]

Stop fixating on the damn benzene ring. Is a methyl group electron donating or withdrawing with respect to carboxylic acids?

Electron-donating.

So does that mean that benzoic acids are worse acids than phenol then?

 

[Teleologist]

[QUOTE="justadream, post: 15680314]

So does that mean that benzoic acids are worse acids than phenol then?[/QUOTE]

On what basis do you make your statement? I'm confused, because neither has a methyl group.

 

[justadream]

@Teleologist

lol whoops I just realized it has no basis.

But can you explain the relative acidity of phenol and benzoic acid (and the reason why one is more acidic than the other)?

 

[Teleologist]

@Teleologist

lol whoops I just realized it has no basis.

But can you explain the relative acidity of phenol and benzoic acid (and the reason why one is more acidic than the other)?

Resonance. Phenoxide delocalizes charge to carbon atoms rather than oxygen.

 

[justadream]

Resonance. Phenoxide delocalizes charge to carbon atoms rather than oxygen.

So phenoxide is less acidic, correct?

Actually now that I think about it, aren't carboxylic acids much more acidic than alcohols anyways? So it should be obvious that benzoic acid is more acidic that an alcohol like phenol.

 

[Teleologist]

So phenoxide is less acidic, correct?

Actually now that I think about it, aren't carboxylic acids much more acidic than alcohols anyways? So it should be obvious that benzoic acid is more acidic that an alcohol like phenol.

Be careful. Tri nitro substituted alcohols can easily be more acidic than an unsubstituted carboxylic acid. pKa for such an alcohol can be in the 2s if I remember correctly.

 

[justadream]

Be careful. Tri nitro substituted alcohols can easily be more acidic than an unsubstituted carboxylic acid. pKa for such an alcohol can be in the 2s if I remember correctly.

Lol okay. Hopefully I don't see a tri nitro substituted alcohol on the MCAT

 

[texan2414]

I would say phenol is undoubtedly more acidic because the negative charge can be delocalized on the benzene ring. However, for benzoic acid, it cannot.

 

[GoldenTuth]

My god I confused you all. I slipped thinking about withdrawing and donating effects, while forgot to think about charge delocalization. Basically first post was correct @justadream and @texan2414 confused you.

Texan you can't draw the charge delocalization because the system is conjugated, it's all big electron cloud all atoms are SP2, making any if them something else would require too much energy.

As per phenol vs. benzoic acid, (besides obvious difference in pKa) looking at the anatomy, again benzoic acid is benzyl-allylic? Might not be the correct term but that basically means it's a super stable beast, and as just explained, due to the delocalized charge. Phenoxy is stabilized but without extra conjugation.

 

[Teleologist]

Texan you can't draw the charge delocalization because the system is conjugated, it's all big electron cloud all atoms are SP2, making any if them something else would require too much energy.[\quote]

What does this mean?

As per phenol vs. benzoic acid, (besides obvious difference in pKa) looking at the anatomy, again benzoic acid is benzyl-allylic? Might not be the correct term but that basically means it's a super stable beast, and as just explained, due to the delocalized charge. Phenoxy is stabilized but without extra conjugation.

There are allylic positions in benzoic acid?

 

[kraskadva]

My god I confused you all. I slipped thinking about withdrawing and donating effects, while forgot to think about charge delocalization. Basically first post was correct @justadream and @texan2414 confused you.

Texan you can't draw the charge delocalization because the system is conjugated, it's all big electron cloud all atoms are SP2, making any if them something else would require too much energy.

As per phenol vs. benzoic acid, (besides obvious difference in pKa) looking at the anatomy, again benzoic acid is benzyl-allylic? Might not be the correct term but that basically means it's a super stable beast, and as just explained, due to the delocalized charge. Phenoxy is stabilized but without extra conjugation.

yes, you figured it out

and the term you're looking for is benzylic.

 

[texan2414]

yes, you figured it out

and the term you're looking for is benzylic.

I still don't understand.
Benzoic acid is stabilized by hyperconjugation NOT electron delocalization. You cannot delocalize the negative charge of the carbonyl somehow to the benzene ring.

I understand that by the hyperconjugation argument, benzoic acid might be stronger than phenol but not due to electron delocalization of benzoic acid.
Also, I forgot that resonating the charge from phenoxide onto the benzene ring takes away aromaticity, thus the enol tautomer is more stable. That certainly makes the argument that hyperconjugation wins. My only confusion was benzoic acid's relative superior strength due to hyperconj. or elec delocalization.

 

[kraskadva]

A benzene ring is electron withdrawing.
Phenol is stable because the charge can be delocalized around the ring. (and the aromaticity doesn't go away, though yes, it is happier on the oxygen)
Benzoic acid has both the benzene ring (e- withdrawing) and the carboxylic group. Carboxyls can stabilize a charge all on their own because resonance delocalizes the charge between the two oxygens. When the benzene ring is behind it stabilizing that by withdrawing electrons, the -COOH is even happier to give up the proton.
So you're right, the charge doesn't delocalize around the ring in benzoic acid, but it is super stable, thus that proton is more acidic (i.e. has a lower pKa)

 

[texan2414]

Thanks for the clarification.

 

[Teleologist]

A benzene ring is electron withdrawing.
Phenol is stable because the charge can be delocalized around the ring. (and the aromaticity doesn't go away, though yes, it is happier on the oxygen)
Benzoic acid has both the benzene ring (e- withdrawing) and the carboxylic group. Carboxyls can stabilize a charge all on their own because resonance delocalizes the charge between the two oxygens. When the benzene ring is behind it stabilizing that by withdrawing electrons, the -COOH is even happier to give up the proton.
So you're right, the charge doesn't delocalize around the ring in benzoic acid, but it is super stable, thus that proton is more acidic (i.e. has a lower pKa)

The ring is electron withdrawing but do you mean resonance or inductive withdrawal?

The ring also has little to do with why benzoic acid is stronger than phenol.

 

[mehc012]

Electron donating vs withdrawing is an important concept to understand in general, but if you're thinking MCAT prep specifically, aromatics aren't really covered (I mean, they even removed alkene topics) so don't stress too much. As long as you understand what electron withdrawing/donating means, and specifically how it can affect acidity/basicity, you should be good.

 

[GoldenTuth]

I'm DAT

 

[mehc012]

I'm DAT

Ah. You can see how I was confused, given the title of the forum

Not saying you shouldn't post in here - I imagine there is a lot of material overlap and you should post wherever you will get responses...just explaining where I was coming from!