Question

Which is the weakest acid?

Cl-CH₂CO₂H

Br-CH₂CO₂H

F-CH₂CO₂H

F-CH₂CH₂CO₂H

Answer 1

The strength of halogen substituted acids depend on the electronegativity of the halogen and how far away it is from the carboxylic functional group. The farther away the halogen substitution is the weaker is the acid. Because of the electron withdrawing nature of halogens, they reduce the electron density on the oxygen atoms of the carboxylic functional group. Hence, the proton is more easily detachable if their is less electron density on the O atoms to hold it.

Now, lets compare and see which of the given acid is the weakest:

Hence, the correct choice is .

Alternative explanation is that the conjugate anion formed after the removal of proton is easily stabilised if an electronegative atom is closer to distribute the electron density of the -ve ion and stabilise the molecule.