[ay2k]

I have a query regarding acidity of benzoic acid...

Some pKa values follow...

Benzoic Acid -  4.2
Ethanoic Acid -  4.8
Chloroethanoic Acid - 2.9

Im happy about chloroethanoic acid...What i need to know is the explanation for pKa value for benzioc acid...that is why is benzoic acid stronger than ethanoic acid but much weaker than chloroethanoic acid?



An answer ample for A'Level purposes ir required...

[nj_bartel]

My guess would be that the chlorine pulls away a huge part of the electron density, due to it's electronegativity.  Benzene's aromaticity also takes a large part of the electron density and would explain benzoic acid being stronger than ethanoic acid.

[spirochete]

SP2 carbons are somewhat electronegative, SP carbons even more so.  I'm not sure of the exact electronegativity values, but there is probably a small inductive effect there. 

It would be a good exercise to try to figure what effect resonance has in this situation.  What effect would you expect it to have compared to ethanoic acid and why?

[macman104]

I would imagine that the chlorines pull on the electrons to permanently stabilize the system and to allow for the bond of the proton to be more polarized.  However, the benzene operates to stabilize the state AFTER the hydrogen has left.  I think that something along these lines can indicate why the inductive pull of the three chlorines is greater than the resonance stabilizing effect of the benzene ring.

[agrobert]

The comparison between ethanoic and chloroethanoic can be only explained by induction of the chlorine atom.  Further analysis of trends will explain why fluoroethanoic is more acidic than chloroethanoic.