Textbook Problem 6.C.21

[Ryan Khiev 1L]

I have a question about this problem:
Suggest an explanation for the different strengths of (a) acetic acid (CH3COOH) and trichloroacetic acid (CCl3COOH); (b) acetic acid (CH3COOH) and formic acid (CH2O2).

I got the answer for part a (trichloroacetic acid) because it was mentioned in the textbook that Cl- is more electronegative than H, so it would pull more electrons and thus be the stronger acid. Does polarity of the molecule affect how strong the acid is? I was a bit confused about part b, which compared acetic acid with formic acid. The textbook says that formic acid is slightly stronger. Is this another case where polarity determines the strength of the acid? Or is there another explanation?

Thank you in advance!

[jennaj3k]

I saw an explanation for another problem saying that the polarity of the O-H bond is really important, since with a strong electronegative central atom attached to O-H , you have electron density pulled towards the central atom and the O atom, so the H+ comes off really easily and the acid is therefore stronger. Hope that helps a bit.

[Phoebe Ko 3E]

I believe you're correct that polarity plays a role in determining the acid strength. In the first case, trichloroacetic acid is stronger than acetic acid because the three Cl atoms pull the overall electron density towards themselves, making the H+ atom less attached to the central atom. Since the H+ atom is now easier to be donated, trichloroacetic acid would result in a stronger acid. In the second case, the CH3 compound in acetic acid has a lone pair, making it an electron donating compound. As a result, it pushes the electron density to COOH and increases the difficulty for the H+ atom to be donated. Therefore, formic acid would be stronger than acetic acid. I hope this was helpful!