Question

Mark the stronger acid in the pair.

Answer 1

Acid strength depends on the stability of the anions formed - on how much it is possible to delocalise the negative charge. The less the charge is delocalised, the less stable the ion, and the weaker the acid. The acids can be strengthened by pulling charge away from the $-CO{O}^{-}$ end.It can be done by attaching electronegative atoms like chlorine to the chain. Fluorine is the most electronegative and it would be expected to be most successful at pulling charge away from the $-CO{O}^{-}$ end and so strengthening the acid. In $CC{l}_{3}C{O}_{2}H$ three $Cl$ are pulling the negative charge thus making it more acidic than $C{H}_{2}FC{O}_{2}H$. Similarily in $C{H}_{2}ClC{O}_{2}H$, the $Cl$ is more electron negative than $I$ and in $C{H}_{3}C{H}_{2}CHFC{O}_{2}H$ the $F$ is effective at withdrawing charge when it is next-door to the $-CO{O}^{-}$ group, and much less so as it gets even one carbon further away.