Question

Acetylenic hydrogens are acidic because

• A. sigma electron density of C-H bond in acetylene is nearer to carbon which has 50% s-character
• B. acetylene has only one hydrogen on each carbon
• C. acetylene contains least number of hydrogens among the possible hydrocarbons having two carbons
• D. acetylene belong to the class of alkynes with molecular formula $C_n{H}_{2n-2}$

Answer 1

Answer: (A)

Acidic character of alkynes can be explained on the basis of $sp$ -hybridisation state of the carbon atom in alkynes.
We know that an electron in $s$ orbital is more tightly held than in a $p$ orbital because $s$-electrons are more closer to the nucleus.
In $sp$-hybridisation, $s$-character is more $(50\%)$ as compared to $s{p}^2-(33\%)$ or $s{p}^3-(25\%)$ hybrid orbitals.
Due to very large $s$-character, the electrons in $sp$-hybrid orbitals are held tightly by the nucleus and are quite electronegative.
Consequently, the electron pair of $H-C\equiv$ bond gets displaced more towards the carbon atom and help in the release of $H^{+}$ions.
For a examples ethyne react with sodium metal and release the $H^{+}$ions.
$CH\equiv CH+Na⟶HC\equiv C^{-}N{a}^{+}$