Orgo questions

[gangazi]

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For the carbanion question, which is more stable?
For carboxylic acid, which is more acidic? A little confused here because carbanion is stabilized by Ph so makes it more stable but it also makes COOH to be less stable than formic acid..

 

[tommyngu]

No one's responded to you, yet? Haha.
Oops sorry. Phenyl group is electron donating so the formic acid is stronger.
The first carbanion is more stable because the negative charge can be delocalized across the phenyl groups. More resonance = more stable.

 

[Donchacka]

I agree with ph3c- being more stable. And because of the same reason, I think, benzene ring stabilizes the negative charge once H+ leaves, therefore it makes the benzoic acid more acidic.

 

[tommyngu]

That's what I put at first about benzoic acid, but upon further inspection and if you actually write out the resonance structures for the conjugate base of benzoic acid, you're only getting carbocations around the benzene ring instead of spreading a negative charge over it.

As a quick exercise, compare the resonance forms for phenol and benzoic acid.

 

[Crithu]

I believe formic acid is more acidic since it has a lower pKa value than benzoic acid. I with the OP on the confusion over why this is so.

 

[tommyngu]

Ack, I think I'm thinking too much into this? At first I thought benzoic acid was more acidic due to more resonance, but I thought it's really due to delocalization of negative charge and looking at the resonance forms of say benzoic acid vs phenol, phenol would have more negative charge spread over the benzene ring and so would be more acidic. Looking at pKa values contradicts this, though.
Actually, I think I am looking at it wrong. I was looking at each resonance structure as it's own thing and when looking at charge delocalization, I was looking at the actual positive or negative charge as being the delocalized charge in each structure. But charge delocalization is more of the hybrid resonance structure, so if that hybrid structure can spread out the electrons across more atoms, it is considered more delocalized.
So I'd like to change my answer back to benzoic acid as being more acidic compared to formic acid because of the greater charge delocalization.

 

[Crithu]

Okay, I think I figured it out. Someone correct me if I'm wrong. The actual form of the the structure is a combination of all possible resonance (as shown above). The conjugate base of formic acid has 2 resonance forms. This means that the partial negative charge on one of the oxygen's is 1/2.

In benzoate (file attached) there are 5 possible resonance forms. In these forms, a particular oxygen has a negative charge on it 4 out of the 5 structures. This means it has 4/5 negative charge. (This is actually not exactly right. The 3 resonance forms with the positive charges are bad resonance forms and would not be as likely to occur as the other two, but they would still contribute to the structure. This means that the structure would be more closely related to the two structures with just a single negative charge. Anyways, the partial negative charge would probably be between 1/2 and 4/5, but the point is that it is more negative than the formate structure).

Therefore, the conjugate base of formic acid is more stable than that of benzoic acid, meaning that formic acid is the most acidic!

 

[tommyngu]

Upon further examination, I realized that I had some misunderstandings about donating and withdrawing groups as they apply to substituents around a benzene ring. Some of the groups listed under activating and deactivating groups only act as EDG or EWG when they're bound directly to a benzene ring (or other aromatic systems I guess).
For example, -OH is an EDG when bound to benzene, but elsewhere it would be considered an EWG.
-Ph would also be considered an EDG when bound to benzene, but elsewhere it would be considered an EWG such as in this problem with benzoic acid.
The -Ph on benzoic acid is electron withdrawing to the acid portion and so increases the acidity of benzoic acid compared with formic acid.
This behavior of groups on and off the benzene ring are due to resonance/mesomeric and inductive effects if anyone wants to look more into it.

 

[Crithu]

Upon further examination, I realized that I had some misunderstandings about donating and withdrawing groups as they apply to substituents around a benzene ring. Some of the groups listed under activating and deactivating groups only act as EDG or EWG when they're bound directly to a benzene ring (or other aromatic systems I guess).
For example, -OH is an EDG when bound to benzene, but elsewhere it would be considered an EWG.
-Ph would also be considered an EDG when bound to benzene, but elsewhere it would be considered an EWG such as in this problem with benzoic acid.
The -Ph on benzoic acid is electron withdrawing to the acid portion and so increases the acidity of benzoic acid compared with formic acid.
This behavior of groups on and off the benzene ring are due to resonance/mesomeric and inductive effects if anyone wants to look more into it.

I'm still saying that formic acid is more acidic than benzoic acid

 

[tommyngu]

Right, sh** what have I been doing. Brain was not working yesterday. Starvation is not good for the brain. I'll have to get back to this one.

Scratch what I said about the Ph being electron donating as a substituent when bound to benzene and electron withdrawing elsewhere. This is not a valid pattern.

The Ph group on benzoic acid is actually electron donating and so it reduces acidity. I'll look into the rest of what I replied above when I have time since a lot of it should still be true.