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 $C_nH_{2n-n}$

Answer 1

Answer: (A)

sigma electron density of C-H bond in acetylene is nearer to carbon which has 50% s-character

Answer 2

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 $sp^2-(33 \%)$ or $sp^3-(25 \%)$ 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. \hspace15mm CH \equiv CH +Na \longrightarrow HC \equiv C^- Na^+