Which is a stonrgest acid....butanoic acid2chlorobutanoic acid3chlorobutanoic acid4 chlorobutanoic acid

2 chloro butanoic acid is strongest.As the -I effect increases, the acidic strength increases. Since the substituent is same(Cl) for both the compound, the strength of -I effect depends on the distance between the substituent group and the main group that react. Greater the distance, the weaker the effect. In 2-chlorobutanoic acid substituent chloro is closer to the C-O-H(no. of sigma bond is less from C-O-H to Cl) bond and -I effect (polarization of bond) is higher as compared to 3-chlorobutanoic acid. Therefore H remove easily as proton in 2-chlorobutanoic acid and hence it is stronger acid.