Which of the below compounds has lowest P^ks value? 

CONCEPT:

pK_a and Acidity

• The pK_a value of a compound is the negative logarithm of its acid dissociation constant (K_a). It is a measure of the strength of an acid.
• A lower pK_a value corresponds to a stronger acid, meaning the compound more readily donates a proton (H⁺).
• In hydrocarbons, acidity is influenced by the hybridization of the carbon atom bonded to the acidic hydrogen.
• The order of acidity for common hybridizations is: sp (triple bond) > sp² (double bond) > sp³ (single bond).

EXPLANATION:

• Acidity is directly related to the stability of the conjugate base formed after losing an H⁺. Stability is influenced by the s-character of the hybrid orbitals. The more s-character, the more stable the conjugate base, and the stronger the acid.
• In acetylene (HC≡CH), the carbon is sp hybridized (50% s-character), making its conjugate base (HC≡C^-) highly stable. This gives acetylene the lowest pK_a value among the given compounds.
• In ethylene (H₂C = CH₂) and propene (H₃C - CH = CH₂), the carbons are sp² hybridized (33% s-character), so their conjugate bases are less stable compared to acetylene.
• In ethane (H₃C - CH₃), the carbons are sp³ hybridized (25% s-character), making it the least acidic with the highest pK_a.

Conclusion: Acetylene (HC≡CH) has the lowest pK_a value among the given compounds due to its sp hybridization, which provides the highest s-character and the most stable conjugate base.