Question

Which one of the following is more acidic?

• A. Butane
• B. 1-Butene
• C. 1-Butyne
• D. 2-Butyne

Answer 1

The correct option is C

1-Butyne

Explanation for correct option

(C) $\mathbf{1}\mathbf{-}\mathbf{Butyne}$

Acidic character

• Acidic character increase with the increase of $s$ character.
• This indicates that a compound with $sp$ hybridization will be more acidic than a compound with other hybridization.
• Generally, the acidity order is as follows: Alkynes > Alkenes > Alkanes

$\mathbf{1}\mathbf{-}\mathbf{Butyne}$

• The percentage of $s$ character of hybridized carbon in $1-\mathrm{butyne}$ and $2-\mathrm{butyne}$ is more than the Butane which is $s{p}^3$ and $1-\mathrm{Butene}$ which is $s{p}^2$ hybridized.
• The $sp$ hybridized carbon in $1-\mathrm{butyne}$ only receives electrons from the side of neighboring carbon.
• While the $sp$ hybridized carbon in $2-\mathrm{butyne}$ receives electrons from both sides through the inductive effect.
• Therefore, $2-\mathrm{butyne}$ will have a lower electronegativity.
• Consequently, $1-\mathrm{butyne}$ will have a larger tendency to donate the hydrogen ion.
• The order of acidic strength is as follows:

$\mathrm{Butane}<1-\mathrm{Butene}<2-\mathrm{Butyne}<1-\mathrm{Butyne}$

Explanation for incorrect options

• The $1-\mathrm{butyne}$ is more acidic.
• Hence, all other options (A), (B), and (D) are incorrect.

Therefore, the correct option is (C) $\mathbf{1}\mathbf{-}\mathbf{Butyne}$.