Question 2.4: Predicting Acid Strengths from pKa Values Water has pKa = 15...
Predicting Acid Strengths from pK_{a} Values
Water has pK_{a} = 15.74, and acetylene has pK_{a} = 25. Which is the stronger acid? Does hydroxide ion react with acetylene?
{H-C} \equiv {C-H}+OH^{-} \xrightarrow[]{?} {H-C} \equiv C\overset{-}{:} + H_{2}O
Acetylene
Strategy
In comparing two acids, the one with the lower pK_{a} is stronger. Thus, water is a stronger acid than acetylene and gives up H^{+} more easily.
Because water is a stronger acid and gives up H^{+} more easily than acetylene does, the HO^{-} ion must have less affinity for H^{+} than the HC \equiv C:^{-} ion has. In other words, the anion of acetylene is a stronger base than hydroxide ion, and the reaction will not proceed as written.