Question

give reason for increasing order of acidic strength
propanoic acid<ethanoic acid<2-chloroethanoic acid<2-fluoroethanoic acid<dichloroethanoic acid.

Answer 1

bcz of hyperconjugation 3°is more stable than 2and so on

Answer 2

Answer:

The acidic strength depends on the stability of the resulting carboxylate ion.

The presence of electron donating groups enhance the negative charge on carboxylate ion and thus decrease the stability of acids.Thus presence of electron withdrawing groups which withdraws the electrons from negatively charged oxygen of carboxylate results in more stability.

Moreover if electron donating or electron withdrawing group is attached closer to the carbon containing the -COOH group, the effect is more prominant. As the distance of the group increases from the -COOH group, the effect becomes less prominent.

$RCOOH\rightarrow RCOO^-+H^+$

Thus the order is : propanoic acid<ethanoic acid<2-chloroethanoic acid<2-fluoroethanoic acid<dichloroethanoic acid.

Propanoic acid contains $-CH_3CH_2$ which is more electron donating than  $-CH_3$ in ethanoic acid. Flourine is more electronegative than chlorine and thus 2-chloroethanoic acid is less acidic than 2-fluoroethanoic acid. Dichloroethanoic acid contains two chlorine groups which is more electron withdrawing than single fluorine and thus dichloroethanoic acid is more acidic than 2-fluoroethanoic acid.