Arrange the following compounds in decreasing order of their acidic strength (increasing pK _ a values): (I) (II) (III) (IV)

Step-by-step solution

The given compounds are: (I) Formic acid ( HCOOH ) (II) Oxalic acid ( HOOC - COOH ) (III) Malonic acid ( HOOC - CH ₂- COOH ) (IV) Acetic acid ( CH ₃ COOH ) Acidic strength depends on the stability of the conjugate base formed after the removal of an H ^+ ion. Electron-withdrawing groups ( -I effect) stabilize the carboxylate anion by dispersing the negative charge, thereby increasing acidity. Conversely, electron-donating groups ( +I effect) destabilize the anion, decreasing acidity. In oxalic acid (II), the strong