Consider the most acidic protons in the following molecules: (A) Acetic acid (B) Acetylacetone (pentane-2,4-dione) (C) Phenol (D) Propyne What is the correct decreasing order of acidity for these protons?

Step-by-step solution

The acidity of a proton depends on the stability of its conjugate base. (A) Acetic acid ( ext CH ₃ ext COOH ) forms an acetate ion, which is highly stabilized by equivalent resonance structures. It is the most acidic among the given compounds ( ext p K_a 4.7 ). (B) Acetylacetone ( ext CH ₃ ext COCH ₂ ext COCH ₃ ) contains an active methylene group. Its conjugate base is stabilized by resonance with two carbonyl groups, making it quite acidic ( ext p K_a 9.0 ). (C) Phenol ( ext C ₆ ext H ₅ ext OH ) forms a phenoxide