Question

Arrange the following compounds in order of their increasing acid strength

• A. I $<$ II $<$ III $<$ IV
• B. IV $<$ III $<$ II $<$ I
• C. I $<$ IV $<$ II $<$ III
• D. II $<$ I $<$ III $<$ IV

Hint:

Using a simple frame or just bolding for the box
Key Points:
Acid strength increases with the stability of the conjugate base.
The -COOH group has an electron-withdrawing inductive effect (-I).
In dicarboxylic acids, the -I effect of one -COOH group stabilizes the conjugate base formed by the ionization of the other -COOH group.
The inductive effect decreases significantly with increasing distance (number of separating CH$_2$ groups).
Acidity Order: Oxalic>Malonic>Succinic>Adipic.

Answer 1

The Correct Option is C

The acid strength of dicarboxylic acids (considering the first ionization, pKa1) depends mainly on the stability of the conjugate base formed after losing the first proton. The primary factor influencing this stability is the inductive electron-withdrawing effect (-I effect) of the second carboxyl group (-COOH) on the first one.

• The -COOH group is electron-withdrawing. When one proton is lost, the resulting carboxylate ion (-COO^-) is stabilized by the -I effect of the remaining -COOH group.
• The strength of the inductive effect decreases rapidly with distance. Therefore, the closer the two -COOH groups are, the stronger the stabilization of the conjugate base, and the stronger the acid (lower pKa1).

Let's compare the distance between the -COOH groups in the given acids:

• III (Oxalic acid): The two -COOH groups are directly attached (separated by 0 carbons). This allows for the strongest -I effect. It is the strongest acid.
• II (Malonic acid): The two -COOH groups are separated by one CH₂ group. The -I effect is weaker than in oxalic acid but significant. It is the second strongest acid.
• IV (Succinic acid): The two -COOH groups are separated by two CH₂ groups. The -I effect is weaker than in malonic acid. It is the third strongest acid.
• I (Adipic acid): The two -COOH groups are separated by four CH₂ groups. The -I effect is the weakest among the four acids because the groups are farthest apart. It is the weakest acid.

Therefore, the order of decreasing acid strength is: III>II>IV>I.
The order of increasing acid strength (weakest to strongest) is: I<IV<II<III.
This corresponds to option (C).