Question

Which of the follwing acidity order is correct?

• A. 1-Alkyne > Alkene > Alkane
• B. Alkene > Alkane > 1-Alkyne
• C. Alkane > Alkene > 1-Alkyne
• D. None of these

Answer 1

The correct option is A 1-Alkyne > Alkene > Alkane

$R-C\equiv CH\to R-C\equiv C^{-}$
$R-CH=C{H}_2\to R-CH=C{H}^{-}$
$R-C{H}_2-C{H}_3\to R-C{H}_{2}C{H}_2^{-}$

Stability of conjugate base $\propto$ Acidity strength
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.

s-character $\propto$ Electronegativity $\propto$ Stability of conjugate base

Alkyne has $sp$ carbon which is more electronegative which will stabilise the conjugate base formed.
Alkene $s{p}^2$ has less s character than alkyne so its conjugate base is less stable than alkyne.
Alkane $s{p}^3$ has least s character. Thus its conjugate base is least stabiled.