Which of the following acid has the highest acid strength? (A) Acetic acid (B) fluoroacetic acid (C) 4-Nitrobenzoic acid (D) 4-Methyl Benzoic acid is

Step-by-step solution

Acid strength is directly proportional to the stability of the conjugate base, which increases with the presence of electron-withdrawing groups (-I, -R effects). In fluoroacetic acid ( F-CH₂-COOH ), the highly electronegative fluorine atom exerts a strong -I effect from the adjacent -carbon, highly stabilizing the conjugate base. Its pK_a is approximately 2.59 . In 4-nitrobenzoic acid, the -NO₂ group exerts both -I and -R effects, but it is located farther away on the benzene ring. Its pK_a is approximately 3.41 .