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 .