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 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, $C_n{H}_{2n-2}.$

Answer 1

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

The formation of C - H bond in acetylene involves sp-hybridised carbon atom. Since s-electrons are closer to the nucleus than p electrons, sp hybrid orbitals are more electronegative through its smaller(in length) and thicker lobes which increases the time spent by electrons near nucleus than in $s{p}^2$ and $s{p}^3$. Thus, the electrons present in a bond having more s-character will be more closer to the nucleus. In alkynes ($\equiv C-H$) bond, s- character is 50%, the electrons constituting this bond are more strongly bonded by the carbon nucleus. Thus, acetylenic C-atom becomes more electronegative in comparison to $s{p}^2,s{p}^3$ and hence the hydrogen atom present on carbon atom ($\equiv C-H$) can be easily removed.