Acidity of substituted benzoic acids

[CrimpJiggler]

Heres meta methoxy benzoic acid:

and here's para methoxy benzoic acid:

m-methoxybenzoic acid is a stronger acid than its para substituted analogue. I'm a bit confused about why this is. From what I've read, its because at the meta position, only the methoxy groups inductive withdrawing effects apply whereas at the para position, it also acts as a mesomeric (resonance) donator. Why does the methoxy groups resonance effects only work when its at the para position? Why doesn't it act as a resonance donator when its at the meta position?

 

[Ygggdrasil]

Draw out the resonance structures for the deprotonated form of both acids. For the para isomer, you will see that you will have more resonance structures because the negative charge can move to the methoxy group. For the meta isomer, however, you cannot draw a resonance structure that moves the negative charge to the methoxy group.

 

[CrimpJiggler]

**** my organic chem is a bit rusty, I don't know how to draw this resonance structure. Heres my attempt for the meta isomer:
[PLAIN]http://img28.imageshack.us/img28/2921/resonance.png
what am I doing wrong there?

 

[Ygggdrasil]

First, I made a mistake in my post. None of the resonance structures should move the negative charge to the methoxy group. Rather, for the para-isomer, there are resonance structures that give a slight negative charge to certain positions of the ring, and this slight negative charge will destabilize the carboxylate.

As for your resonance structures, the first two are ok, but the thrid is wrong as it involves the movement of a proton. Below are some resonance structures for methoxybenzene.

When you draw the resonance structures for the para-isomer you should find a resonance structure that would disfavor dissociation of the proton from the carboxyl group. This resonance structure you would not be able to see with the meta-isomer.

 

[LudusRex]

I can explain the differences in acidity between meta and para substituted benzoic acids in terms of their molecular structure and electronic effects. The acidity of a molecule is determined by the stability of its conjugate base, which is influenced by the distribution of electrons within the molecule.

In the case of substituted benzoic acids, the methoxy group (-OCH3) acts as an electron-withdrawing group due to its electronegativity. This results in a decrease in electron density on the benzene ring, making the carboxylic acid group more acidic. However, the position of the methoxy group on the benzene ring can affect its electronic effects.

At the meta position, the methoxy group is located at a distance from the carboxylic acid group, so its inductive withdrawing effect is the dominant factor in determining the acidity. This effect pulls electron density away from the carboxylic acid group, making it more stable and therefore more acidic.

On the other hand, at the para position, the methoxy group is located directly next to the carboxylic acid group. This allows for a resonance effect, where the lone pair of electrons on the oxygen atom can delocalize into the benzene ring, increasing electron density on the carboxylic acid group. This resonance effect can counteract the inductive withdrawing effect of the methoxy group, resulting in a weaker overall acidity compared to the meta substituted analogue.

In summary, the difference in acidity between meta and para substituted benzoic acids can be attributed to the different electronic effects of the methoxy group at each position. The meta position only experiences the inductive withdrawing effect, while the para position also experiences the resonance effect, leading to a weaker acidity.