The correct order of acid strength of the following carboxylic acids is
(A) I > III > II > IV (B) III > II > I > IV
(C) I > II > III > IV (D) II > I > IV > III
Answers
The correct options (C) due to following reasons:
- Electro-donating Inductive effect or +I effect is the effect in which within a molecule through the atoms chain there occurs unequal sharing of bonding electrons that results in permanent dipole in a bond.
- Here I molecule contain triple bond whereas II and III contain double and single bond.
- All the molecules form their conjugate bases by removing H^+ ion from them.
- Conjugate bases of all molecules is shown in the attachment below.
- As the acid strength is directly proportional to the stability of conjugate bases.
- As Both I and II molecules show -I effect or the electron withdrawing effect . It is the effect in which carbon carriers positive charge due to electron withdrawing substitute that attract electron pair.
- As Stability of conjugate base increases with the -I effect .
So, triple bond of I molecule show more -I effect as compared to double bond of II molecule.
- III molecule show +M effect ( resonance effect) because of presence of benzene ring and IV molecule show +I effect because of electron donating group ( methyl group).
- +M and +I effect decreases the stability of conjugate base and the strength of acid is seen less in aromatic compounds as compared to alliphatic compounds.
- So, The acidic strength in decreasing order is
I > II > III > IV
- The correct option is (C).
