Question

Which one is the correct order of acidity ?

<!--td {border: 1px solid #cccccc;}br {mso-data-placement:same-cell;}--> NEET 2017

• A. $CH\equiv CH>C{H}_2=C{H}_2>C{H}_3-C\equiv CH>C{H}_3-C{H}_3$
• B. $CH\equiv CH>C{H}_3-C\equiv CH>C{H}_2=C{H}_2>C{H}_3-C{H}_3$
• C. $C{H}_3-C{H}_3>C{H}_2=C{H}_2>C{H}_3-C=CH>CH\equiv CH$
• D. $C{H}_2=C{H}_2>C{H}_3-CH=C{H}_2>C{H}_3-C\equiv CH>CH\equiv CH$

Answer 1

The correct option is B $CH\equiv CH>C{H}_3-C\equiv CH>C{H}_2=C{H}_2>C{H}_3-C{H}_3$

Alkynes having $sp$ - hybridised carbon has maximum $s$ - character and hence maximum electronegativity so that it attracts the electron pair of the bond $C-H$ to a greater extent and makes the removal of proton easier than the alkenes having $s{p}^2$ hybridised carbon.
$s{p}^3$ hybridised carbon is less electronegative than $s{p}^2$.

Hence, the correct order is
$\underset{\text{2- acidic hydrogen}}{CH\equiv CH}>\underset{\text{1- acidic hydrogen}}{C{H}_3-C\equiv CH}>C{H}_2=C{H}_2>C{H}_3-C{H}_3$

Thus, option (b) is correct.