[furaniki18]

my textbook gives the following order for acidic strength:

m-aminobenzoic acid> p-aminobenzoic acid> o-aminobenzoic acid

i understand that -NH₂ will exert a -I  effect when present on meta position and thus will be more acidic than benzoic acid. also when present at para position it will exert +R effect and thus p-aminobenzoic acid will be less acidic than benzoic acid. but the problem is coming when i am trying to analyse when -NH₂ is present at ortho position...
shouldn't it be most acidic of the three? shouldn't due to ortho effect (steric factor) -COOH be pushed out of plane of the benzene ring such that resonance effect (-R) can no longer take place between the benzene ring and -COO^- ion and it will be more stable. thus making o-aminobenzoic acid most acidic?

[Dan]

There is indeed another factor at play....

Hint: Fumaric acid and Maleic acid

[furaniki18]

are you referring to intramolecular hydrogen bonding? in that case intramolecular hydrogen bonding between O^- of the carboxylate ion and H atom of -NH₂ group present at the ortho position would again make the conjugate base of o-aminobenzoic acid extra stable. which means that o-aminobenzoic acid will be more acidic...
then why is it the least acidic among the 3 compounds?

[Nescafe]

are you referring to intramolecular hydrogen bonding? in that case intramolecular hydrogen bonding between O^- of the carboxylate ion and H atom of -NH₂ group present at the ortho position would again make the conjugate base of o-aminobenzoic acid extra stable. which means that o-aminobenzoic acid will be more acidic...
then why is it the least acidic among the 3 compounds?

Which book is this from? I think the only thing the book wanted to point out is the factors that influence pKa which you must learn to account for when you have substitutions in different parts of the ring. These being, as you pointed them out yourself -I effect and the +R effect. The relative acidity of the acid in this case to be honest is a bit of a tough one to determine by theory alone. You are absolutely correct, for the ortho substituted case, after the carboxylic groups deprotonates a strong and favored intramolecular hydrogen bonding takes place between the -NH2 and O-; overall, this favors the deprotonated state of the hydrogen-bond-accepting group (acid moiety), lowering its pKa. However, at the end of the day, at least in this case the -I and +R effects seem slightly more important and a bigger (yet not by much) influence on the pka of the acid.

Here are the Pka values

2-Aminobenzoic acid    4.946    
3-Aminobenzoic acid    4.78    
4-Animobenzoic acid    4.874    

Hope that helps,

Nescafe.

[Dan]

are you referring to intramolecular hydrogen bonding? in that case intramolecular hydrogen bonding between O^- of the carboxylate ion and H atom of -NH₂ group present at the ortho position would again make the conjugate base of o-aminobenzoic acid extra stable. which means that o-aminobenzoic acid will be more acidic...
then why is it the least acidic among the 3 compounds?

Yes, I was going to argue that hydrogen bonding between the COO-H and the N stabilises the acid, but you make a good point about the conjugate base - both the acid and the conjugate base are stabilised by H bonding so it's perhaps not such an important factor.

It seems that Nescafe has correctly spotted a mistake in your book, here is a link to a reputable pKa table http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf

[furaniki18]

here is a link to a reputable pKa table http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf

thank you Dan..this table is definitely of great help