Question

Which of the following is the correct order of acidity of a carboxylic acid?
i.$C{{l}_{3}}CCOOH>C{{l}_{2}}CHCOOH>ClC{{H}_{2}}COOH$
ii.$C{{H}_{3}}C{{H}_{2}}COOH>{{\left( C{{H}_{3}} \right)}_{2}}CHCOOH>{{\left( C{{H}_{3}} \right)}_{3}}CCOOH$
iii.${{F}_{2}}C{{H}_{2}}COOH>FC{{H}_{2}}COOH>ClC{{H}_{2}}COOH$
a.(i) and (ii)
b.(ii) and (iii)
c.(i) and (iii)
d.(i), (ii) and (iii)

Answer 1

Hint : We know that electron-donating substituents decrease the acidity of the benzoic acid because they decrease the stability of the carboxylate ion. Electron-withdrawing substituents increase the acidity of the benzoic acid because they increase the stability of the carboxylate ion.

Complete Step By Step Answer:
These are acids so they will have conjugate bases. A conjugate base of an acid is when the acid has been deprotonated. This comes from the Bronsted-Lowry concept of acid and bases which states: acids are proton donating species and bases are proton accepting species. From the looks of it above, all of them bear a negative charge on one of the oxygen atoms of the carboxylic functional group. This particular functional group can undergo the phenomenon of resonance and therefore both the double bonds are partial in nature.
But as it is the same for all the three of them we cannot decide their stability solely on this factor. Therefore we go to one of the fundamental properties for help, which is the “inductive effect”. The inductive effect is only seen in single bonds and the electronegativity of an atom is the only deciding factor. It says if a relatively more electronegative (E.N.) atom is present in a molecule then the electrons in the single bond tend to shift towards it. For this reason the E.N. the atom in question gains a partial negative charge and all the other atoms gain a partial positive charge. Let us look at this property through acetic acid
The strength of the acid increases with the negative inductive effect of the electronegative group attached to it ( $-I$ effect) and decreases with the presence of the positive inductive effect of the electrons releasing group (\[+I\]effect).
The more the number of electronegative atoms attached more the acidic character of that acid. Thus (i) is correct.
More the number of electron releasing group, less the acidic character of acid. Thus, (ii) is correct.
Since \[F\] is more electronegative than \[Cl,\]thus (iii) is also correct. Hence, All orders of acidity of carboxylic acid are correct.

Hence, the correct option is D.
Note :
Remember that the acid-weakening effect of the electron-donating group and acid-strengthening effect of the electron-withdrawing group is more pronounced at the para than at the meta position of the benzoic acid. This is due to the resonating structure of the benzoic acid. The positive charge is on the para position but not on the meta position.