Consider the following hydrocarbons: (I) 1,3-cyclopentadiene (II) 1,4-pentadiene (III) cyclopropene The correct decreasing order of their pK _ a values is:

Step-by-step solution

The acidic strength of a hydrocarbon depends on the stability of its conjugate base. A more stable conjugate base implies a stronger acid, which corresponds to a lower pK _ a value. Deprotonating each compound yields the corresponding carbanion: (I) 1,3-cyclopentadiene cyclopentadienyl anion. This anion is planar, fully conjugated, and has 6 electrons, making it aromatic and highly stable. (II) 1,4-pentadiene pentadienyl anion. This anion is resonance stabilized but non-aromatic, giving it moderate stability. (III)