The pKa values of acetylene, ethylene, and ethane are 25, 44, and 50, respectively. This hierarchy of pKa values directly correlates with their acidity levels, with a lower pKa indicating a stronger acid. Consequently, acetylene, with the lowest pKa of 25, is the strongest acid of the three.
To understand why acetylene is more acidic than ethylene and ethane, we need to look at the structure and hybridization of these compounds. Acetylene (C2H2) features a carbon-carbon triple bond, where each carbon is sp-hybridized. This sp hybridization results in a greater s-character (50%) compared to ethylene (C2H4) and ethane (C2H6), which are sp² (33% s-character) and sp³ (25% s-character) hybridized, respectively.
The higher the s-character of the orbital that contains the negative charge of the conjugate base (which is formed when the acid donates a proton), the more tightly that charge is held by the nucleus. Thus, the conjugate base of acetylene (C2H)⁻ is more stable than those of ethylene (C2H4)⁻ and ethane (C2H5)⁻. This stability explains the greater acidity of acetylene because a more stable conjugate base corresponds to a stronger acid.
In summary, the order of acidity among these hydrocarbons is influenced by the hybridization state of the carbon atoms, with acetylene being the strongest acid due to its sp hybridization, leading to a more stable conjugate base.