[thetada]

Just did a really good exam question with my students which asked which is the stronger acid between ethanoic and ethandioic acid and why? (The question stated that the dioic acid is "initially monoprotic".)

What the students struggled to understand from the mark scheme is the apparently contradictory point that in ethandioic acid the -OH bond would be more polar and would hence dissociate more readily.

As the second -COOH group is electron withdrawing (in contrast to the methyl group in ethanoic acid) they felt that the oxygen in the dissociating -OH bond would be less negative and hence that the -OH bond would be less polar.

I rationalised it as follows: 1) the electron withdrawing activities of the second -COOH group would stabilise the negative charge of the resulting anion and 2) the bonding pair of electrons in the dissociating -OH bond would also be drawn away from the dissociating hydrogen atom, hence increasing the polarity of the bond.

Frankly I felt pretty unsure of the explanation which is why I'm keen to get a second opinion.

Thanks

[Dan]

For acid/base equilibria I always told my students to focus on the relative stability of the charged species - to me this has always been the most intuitive approach.

For AH A^- + H⁺   more stable A^- means stronger AH acid.

So for Acetic acid vs Oxalic acid I would present a similar argument to yours, compare acetate and hydrogenoxalate and conclude that the -ve charge is more stable in the latter due to 1) induction from the -CO₂H and 2) intramolecular H-bonding to the anion.

For B⁺H  B + H⁺   more stable B⁺H means stronger base B.

e.g. guanidinium vs ammonium

[thetada]

Fantastic, thanks for the confirmation.