[mana]

hi all
I have read in solubility chapter of shriner (isolation of organic compounds) that ortho substituted benzoic acid is more acidic than benzoic acid because of steric strain which causes carboxyl group goes up (out of the plate of molecule) so the anion CO2- is more stable than CO2H.
I can't understand why
is it because of smaller size of CO2- than CO2H?

[Babcock_Hall]

Draw out the structure of the conjugate base, and another explanation may occur to you.

[orgopete]

This is interesting. I was generally aware of this, but I never had what I thought was a good explanation. Today, the combination of twisting, resonance, and electron withdrawing came together in a different way.

Generally, we know that pi-electrons can participate to extend conjugation, aromaticity, stabilize carbocations, and reduce the reactivity of the carbonyl group when attached to a benzene ring. Benzoyl chloride and benzaldehyde are less reactive than acetyl chloride and acetaldehyde. Participation by pi-electrons might have been predicted to reduce acidity. However, we also know that sp and sp2-carbons are electron withdrawing. Ethylene and acetylene are more acidic than ethane.

Benzoic acid can combine these properties together, but which should dominate? Should benzoic acid be a weaker acid due to resonance donation or a stronger acid due to sp2-electron withdrawal? Herein the ortho substitution may offer a hint. If the carboxyl group is twisted out of plane to the benzene ring, this may alter the resonance donation without changing the electron withdrawing properties. If so, this could explain why ortho-substitution increases acidity.

[Babcock_Hall]

@orgopete, When a hydroxyl group is found at the ortho position, the pK_a is 2.98.  When a hydroxyl group is found at the para position, the pK_a is 4.58.  A methoxy group (CH₃O-) does not produce a corresponding reduction of the value of pK_a in the ortho position.  The _ortho-substituted acid has a pK_a of 4.09, and the para-substituted acid has a pK_a of 4.47.  IMO a good explanation must account for these observations.

@OP, Draw out the ortho-substituted molecule in its conjugate base form, and think about weak forces.

[orgopete]

Oh I concede that I cannot explain all acidities and this increases if we include solvent effects. The acidities measured in water normalize conditions to a degree. Additional substituents add electron withdrawing, vinologous effects, and local environments in ways that are difficult to generalize on. I tried to generalize on a generalization. Your mileage may vary.

An often used explanation that I do not find satisfactory is to use the stability of the conjugate base to explain the acidity of an acid. For example, if we use phthalimide, we could draw resonance structures for the anion in which we place the negative charge on the oxygen atoms. This implies those electrons are somehow participating in the resonance structure. However, I argue the bonding N-H electrons are actually orthogonal to the pi-electrons and thus are unable to participate in the resonance structure. I do not foresee this as a problem. Just as N, O, and F are more electron withdrawing due to a greater nuclear charge, the result is an increase in acidity. Similarly, an sp or sp2-atom, especially if attached to a N or O, will be more electron withdrawing, e.g. HCN (I don't need a resonance structure for the anion), HNCO. Therefore one should expect any of these arrangements should increase the acidity, e.g. phthalimide, pyrrole, cyclopentadiene, acetone, acetoacetate, etc.

I acknowledge that the conjugate base argument may be easier for students to grasp and for that reason may be preferred. However, that does not mean that I must agree with it.