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 $C_nH_{2n-2}$

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 $s p$-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 $s p$-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.
$CH \equiv CH + Na \longrightarrow HC \equiv C ^{-} Na ^{+}$