Acidity of Alkynes


Alkynes are one of the simplest hydrocarbons known to us. They have a general formula of CnH2n-2. Alkynes belong to the family of unsaturated hydrocarbons that is; they contain both sigma and pi bond linkages between carbon and hydrogen.

An alkyne molecule contains at least one triple bond between two carbon atoms. For example: ethyne (CH≡CH). Ethyne reacts with strong bases such as sodiummetal and sodamide (NaNH2)to form sodiumacetylide along with the liberation of dihydrogen gas. This reaction of alkynes with bases to liberate dihydrogen gas indicates the acidity of alkynes.

HC ≡ CH + Na → HC ≡ C Na+ + 1/2H2

Acidity of alkynes is due to their ability to lose hydrogen atom to form alkynideions. Thus, alkynes act as Bronsted-Lowry acids. The triple bonded carbon atom in alkynes is “sp” hybridized. Due to high percentage of “s” character (50%) in alkynes, the “sp” hybridized orbitals of carbon atom in alkynes exhibit high electronegativity. These attract the C-H bond of alkynes to a great extent. Thus, alkyne molecules can easily loose hydrogen atom forming alkynide ions. Hence, we can say that hydrogen atom attached to the triply bonded carbon atom is acidic in nature.

The acidity of alkynes is greater than the acidity of alkanes and alkenes as the carbon atom in alkanes and alkenes are “sp3” and “sp2” hybridized respectively. Hence, these molecules contain lesser percentage of “s” character in comparison to alkynes.Thus, the electronegativity of carbon atom in these cases is lesser than alkynes. Hence, alkanes and alkenes don‘t show the reactions with bases to liberate hydrogen gas. It can further be noted that only hydrogen atom attached to triply bonded carbon atom are acidic not the other hydrogen atoms in the alkyne chain. General trend in acidity is seen as:

HC≡CH > H2C=CH2> CH3–CH3