The strength of the acid is expressed in terms of the dissociation constant( Ka), also called acidity constant. A stronger acid has higher
Ka but lesser pKavalue( pKa ==-log Ka).
The electron releasing substituents(+ 1 effect) decrease the acidic strength of the carboxylic acids by destabilising the carboxylate ion.
The electron withdrawing substituents(-1 effect) such as halogen atoms( X), nitro( NO2) group increase the acidic strength by decreasing the magnitude of the negative charge on the carboxylate anion and thus stabilising it. The release of H + ion becomes easy.
Acidic strength order
This is because-1 effect decreases in the order: F > C1 > Br > I.
This is because – I effect decreases with distance.