Question

Explain the terms inductive and Electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids?
$\text{C}{{\text{l}}_{\text{3}}}\text{CCOOH}\,\,>\,\,\text{C}{{\text{l}}_{\text{2}}}\text{CHCOOH}\,\,>\,\,\text{ClC}{{\text{H}}_{\text{2}}}\text{COOH}$

Answer 1

Acidic strength of a compound depends on the stability of conjugate base. Electron withdrawing group has negative inductive effect while an electron donating group has positive inductive effect.

Complete answer:
Inductive effect – partial displacement of a shared pair of electrons toward the more electronegative atom in a molecule is called inductive effect. It is a permanent effect and it develops polarity in a covalent bond. It is a distance dependent effect which is effective up to 3rd or 4th carbon atom in the hydrocarbon chain. For example bond polarity in ethyl chloride which develops partial negative charge in chlorine atom and partial positive charge in ethyl group.
Molecule or atom having high electronegativity like halogen atoms$\text{(-X)}$, hydroxyl group$\text{(-OH)}$, nitro group$\text{(-N}{{\text{O}}_{\text{2}}}\text{)}$ has negative inductive effect$\text{(-I}\,\text{effect)}$and molecule or atom having less electronegativity like alkyl group$\text{(-C}{{\text{H}}_{3}}\text{, -C}{{\text{H}}_{2}}\text{C}{{\text{H}}_{3}}\text{)}$ and alkoxy group$\text{(-OC}{{\text{H}}_{3}}\text{)}$has positive inductive effect$\text{(}+\text{I}\,\text{effect)}$.
Electromeric effect – it is a temporary effect which is only applicable in a multiple bond containing organic compounds in the presence of attacking reagents. This effect is easily considerable in the addition reaction of $\text{HCl}$in propene$(\text{C}{{\text{H}}_{\text{3}}}\text{-CH=C}{{\text{H}}_{\text{2}}})$. In this effect transfer of multiple bonds depends on the attacking reagent.
Since, chlorine$\text{(-Cl)}$atom has negative inductive effect so when it will attach with acetic acid it increases acidity of acetic acid by stabilising the conjugate base or negative charge on the oxygen atom of acetate ion. On increasing the substitution by halogen atom acid, acidity increases.
So, the correct order of acidity of the carboxylic acid will be $\text{C}{{\text{l}}_{\text{3}}}\text{CCOOH}\,\,>\,\,\text{C}{{\text{l}}_{\text{2}}}\text{CHCOOH}\,\,>\,\,\text{ClC}{{\text{H}}_{\text{2}}}\text{COOH}$ which is explained by the $\text{(-I}\,\text{effect)}$of the chlorine atoms. Greater the number of chlorine atoms the stronger is the acid.

Note:
Chlorine is a halogen which has negative inductive effect$\text{(-I}\,\text{effect)}$ and halogen atoms predominantly show inductive effect, so always describe acidity of halogen substituted acids by inductive effect.