[Ksharindam]

Why o-methoxybenzoic acid is more acidic than benzoic acid and p-methoxybenzoic acid.please explain.

[camptzak]

electronegative oxygen atom adjacent to carboxylic acid group

[408]

more res forms, more stability.
OMePhCOOH has an additional res form not available in PhCOOH

[Babcock_Hall]

The ortho- and the para-isomers both have resonance forms; therefore, the difference between them must lie elsewhere.

[408]

charge separation within the res form

[Dan]

You are arguing that a +M effect will stabilise an anion?

If we only consider resonance, a mesomerically donating group at the 2- or 4-position will destabilise the carboxylate (conjugate base), so we would expect acidity to decrease if we introduced a methoxy group at the 2- or 4-positions of benzoic acid.

The increased acidity of 2-anisic acid relative to benzoic acid is not due to resonance effects - it is the opposite of what the resonance structures predict. Resonance is not the only electronic effect in operation.

[AWK]

Take into account also the intermolecular hydrogen bond.

[orgopete]

Take into account also the intermolecular hydrogen bond.

The poster asked why the anisate anion is more stable than benzoate. Please explain the intermolecular hydrogen bond effect.

[souro10]

The three compounds whose acidity is to be compared are benzoic acid, ortho-methoxybenzoic acid and para-methoxybenzoid acid.

Recall ortho effect. Any substituent/group ( whether EDG or EWG ) if present Ortho to the COOH of benzoic acid, with increase the acidity. Hence ortho-methoxybenzoic acid is more acidic than benzoic acid.

Now, forget ortho-effect for the time being.

Now let us come to electronic effects, in order to argue about the greater acidity of para methoxybenzoic acid than benzoic acid.

The basic idea is, if a substituent somehow removes electrons (or electron density) from the negative oxygen of the conjugate base/ the anion, the equilibrium is favored in the direction of acid dissociation.

Similarly acidity is expected to be decreased if a substituent somehow pushes electrons towards the negatively charged oxygen.

Resonance effect says that both ortho and para substituted methoxybenzoic acid should be less acidic than benzoic acid itself ( try drawing the resonance forms and try to appreciate that increase in electron density at the 1 position will cause electron-electron repulsion, destabilizing the anion and shifting the equilibrium towards left ).

Thus para methoxybenzoic acid is a weaker acid than benzoic acid.

The ortho methoxybenzoic acid, should however have been weaker than benzoic acid too. But due to the operation of ortho effect, it is stronger.

Hope this helps.

[Dan]

Recall ortho effect. Any substituent/group ( whether EDG or EWG ) if present Ortho to the COOH of benzoic acid, with increase the acidity. Hence ortho-methoxybenzoic acid is more acidic than benzoic acid.

This is not an explanation of why 2-anisic acid is stronger than benzoic, you have just given it a name.

It's a sweeping generatisation that does not teach anyone anyhing, and is not always true. The second pKa of phthalic acid is ~5.3, so a carboxylate substituent does not fall in the category of "any substituent/group". Anthanilic acid (pKa ~4.8 ) is also weaker than benzoic, and I expect many N-alkyl derivatives of anthranilic acid to be weaker acids than benzoic acid.

[souro10]

I thought Ortho Effect is well known, and hence refrained from explaining it in my words because there are professors and books that can explain it much, much better.

But yes, I was wrong. Well then, if its not the explanation, then fact is, a methoxy substituent at ortho position does not cause any Steric Inhibition of Resonance effect. Yes, I read from the wrong book, I guess.

I think there are many effects operating, but maybe there is one particular effect which is very strong, and wins all over the others.

I will think and reply.

[souro10]

I would just like to quote a section from morisson and boyd text book:

"Ortho substituted acids do not fit into the pattern set by their meta and para isomers and by aliphatic acids. Nearly all ortho substituents exert an effect of the same kind- acid-strengthening- whether they are electron-withdrawing or electron-releasing, and the effect is unusually large. This Ortho-effect undoubtedly has to do with the nearness of the groups involved , but is more than just steric hindrance arising from their bulk. "

The only exception listed is the anthranilic acid, and maybe it is due to strong hydrogen bonding or some other factor.

[Dan]

I know the "ortho-effect" is a well documented observation, my point is that saying:

Recall ortho effect. Any substituent/group ( whether EDG or EWG ) if present Ortho to the COOH of benzoic acid, with increase the acidity. Hence ortho-methoxybenzoic acid is more acidic than benzoic acid.

...does not explain anything. It's like saying "2-anisic is more acidic than benzoic acid because that is usually what happens when you have a substituent at the 2-position" - it's just a general observation.

The "ortho-effect" is an umbrella term for the seemingly anomalous properties of 2-substituted benzoic acids, but can be due to a number of different interacting factors depending on the molecule in question. Perhaps what I'm saying here is that the question is: What is the origin of the ortho effect in 2-anisic acid?

You have to be careful with these "rules of thumb", the Markovnikov rule is another example - they can be a good device for remembering trends, but they explain nothing.

[souro10]

@Dan, I agree with you completely.

My point is/was, had it been something as simple as Markonikov's rule, one could have fit the entire thing about stability of carbocations,  unexpected products in a single forum post.

But because "Ortho-effect" is not well understood, or even well documented- as a high school student there wasn't anything more I could contribute- because there are several research papers on this topic, and in some cases results are conflicting.

I admit this isn't a proper explanation, but perhaps even the rule of thumb might have helped someone, in case the poster did not know about Ortho-effect. Ortho effect is an observation; I do not know the explanation or the origin, but I felt mentioning the observation may drive the poster to dig on the topic.