Consider the following hydrocarbons: (I) 1,3-Cyclopentadiene (II) Cyclopropene (III) Cyclopentane (IV) 1,4-Pentadiene The correct decreasing order of their pK_a values (considering the most acidic sp^3 C-H bond) is:

Step-by-step solution

The acidic strength of a hydrocarbon depends on the stability of its conjugate base (carbanion). A more stable conjugate base implies a stronger acid and a lower pK_a value. Deprotonation of the most acidic sp^3 C-H bond yields the following carbanions: (I) 1,3-Cyclopentadiene gives the cyclopentadienyl anion, which is aromatic ( 6 electrons) and highly stable. (II) Cyclopropene gives the cyclopropenyl anion, which is anti-aromatic ( 4 electrons) and highly unstable. (III) Cyclopentane gives the cyclopentyl anion,