Hydrocarbons - Result Question 62
####64. Acetylenic hydrogens are acidic because [1989]
(a) Sigma electron density of $C-H$ bond in acetylene is nearer to carbon, which has $50 % s$-character
(b) Acetylene has only open hydrogen in each carbon
(c) Acetylene contains least number of hydrogens among the possible hydrocarbons having two carbons
(d) Acetylene belongs to the class of alkynes with molecular formula, $C_n H _{2 n-2}$.
Topic 4: Aromatic Hydrocarbons
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Solution:
- (a) The acidity of acetylene or 1-alkynes can be explained on the basis of molecular orbital concept according to which formation of $C-H$ bond in acetylene involves $s p$-hybridised carbon atom. $s$ electrons are closer to the nucleus than $p$ electrons, the electrons present in a bond having more $s$ character will be correspondingly more closer to the nucleus. Thus, owing to high $s$ character of the $C-H$ bond in alkynes $(s=50 %)$, the electrons constituting this bond are more strongly held by the carbon nucleus (i.e., the acetylenic carbon atom) or the $s p$ orbital acts as more electronegative species than the $s p^{2}$ and $s p^{3}$ with the result the hydrogen present on such a carbon atom $(\equiv C-H)$ can be easily removed as a proton.
