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 acidity of dicarboxylic acids (first ionization, pKa1) primarily relates to the stability of the conjugate base formed after the first proton is lost. The main factor affecting this stability is the electron-withdrawing inductive effect (-I effect) of the second carboxyl group (-COOH) on the first.\r\n\r\n

\r\n
• The -COOH group withdraws electrons. When a proton is lost, the resulting carboxylate ion (-COO^-) is stabilized by the -I effect of the remaining -COOH group.
\r\n
• The inductive effect weakens with distance. Therefore, closer -COOH groups lead to greater conjugate base stabilization and stronger acidity (lower pKa1).
\r\n

\r\nLet's compare the distances between -COOH groups in the following acids:\r\n

\r\n
• III (Oxalic acid): The two -COOH groups are directly attached (0 carbon separation). This maximizes the -I effect, making it the strongest acid.
\r\n
• II (Malonic acid): The -COOH groups are separated by one CH₂ group. The -I effect is weaker than in oxalic acid but still significant, making it the second strongest acid.
\r\n
• IV (Succinic acid): The -COOH groups are separated by two CH₂ groups. The -I effect is weaker than in malonic acid, making it the third strongest acid.
\r\n
• I (Adipic acid): The -COOH groups are separated by four CH₂ groups. The -I effect is weakest due to the greatest separation, making it the weakest acid.
\r\n

\r\nThus, the order of decreasing acid strength is: III>II>IV>I.\r\n
\r\nThe order of increasing acid strength (weakest to strongest) is: I<IV<II<III.\r\n
\r\nThis matches option (C).\r\n\r\n