Why acetylene is more acidic than ethylene?

Answer 1

The acidity of a substance is measured by its pKa. A given acid will give up its proton to the base of an acid with a higher pKa value. The base of a given acid with deprotonate an acid with a lower pKa value.

The pKa of water is 15.74
The pKa of ammonia is 34
The pKa of acetylene is 25

What this means is that acetylene will not act as an acid in water and there will be essentially no dissociation of the hydrogen ion (the concentration of HCC- is essentially zero). This means that acetylene can dissociate in ammonia, but only to a small extent. Acetylene is considered a weak acid in ammonia.

See the Web Link for a very good explanation of this (with a direct comparison of water, ammonia, and acetylene). It is a pdf file.

Answer 2

Although the acidity is very, very weak, primary alkynes, unlike alkanes and alkenes, are weakly acidic? Why are primary alkynes weakly acidic?

The C-H bonding electrons in acetylene are contained in an sphybrid orbital. In the conjugate base of acetylene, acetylide carbanion, this sphybrid orbital holds an unshared electron pair. The sphybrid orbital holding this unshared electron pair has greater scharacter compared to the sp2 or sp3 orbitals holding an electron pair in analogous vinylic or aliphatic carbanions. This allows the unshared electron pair to be held at lower energy, which allows acetylene to have measurable, though still very weak, acidity.

With more scharacter, the acetylide carbanion holds the negative charge closer in to the carbon nucleus, a decrease in electrostatic potential energy (and by extention enthalpy and free energy), which makes the equilibrium favorable enough to be of practical laboratory benefit. Anions of acetylene and terminal alkynes are useful nucleophiles.

Answer 3

Acetylene gas in water is more acidic than alcohol.