Question

Explain why $\mathbf{HC}\mathbf{\equiv }\mathbf{CH}$is more acidic than ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{3}}$, even though the C-H bond in $\mathbf{HC}\mathbf{\equiv }\mathbf{CH}$has a higher bond dissociation energy than the C-H bond in ${\mathbf{CH}}_{\mathbf{3}}{\mathbf{CH}}_{\mathbf{3}}$.

Short answer

Answer

This implies that the alkynes are more acidic than the alkanes due to the greater stability of the conjugate base of alkyne.

Step-by-step solution

Step-by-Step SolutionStep 1: Bond dissociation energy

The strength of a chemical bond can be determined by the bond dissociation energy. It is the energy required to break a chemical bond homolytically and results in the formation of free radicals in the gaseous phase.

Acidity of hydrocarbons

Among the hydrocarbons, alkanes are considered to be neutral molecules. Alkanes are the least acidic among the alkenes and alkynes. This is because of the almost similar electronegativity of carbon and hydrogen atoms; the loss of protons becomes very difficult.

On the other hand, the akenes and alkynes are acidic due to the increased s character in their hybridized orbitals. As a result, the loss of protons becomes easier.

Alkynes are more acidic than alkanes

The alkynes are more acidic than alkanes. The acidic nature of alkynes can be explained as follows:

• The bond dissociation energy of the alkynes is more than the alkanes due to the greater percentage of s character. In alkynes, the carbon atom is sp hybridized, and the hybrid orbitals have about 50 percent s character.
• In the case of alkane molecules, the carbon atom is ${\mathrm{sp}}^3$, and the hybrid orbitals have 25 percent s character. As a result of the high s character, the carbon-hydrogen bond becomes stronger and shorter, and the bond dissociation energy increases.
• The alkynes are more acidic than alkanes due to the high stability of their conjugate base. The conjugate base of an alkane ( ${\mathrm{CH}}_3{{\mathrm{CH}}_2}^{-}$) results from the ${\mathrm{sp}}^3$hybrid carbon atom.

• On the other hand, the conjugate base of an alkyne ($CH\equiv C^{-}$) results from the sp hybrid orbital of carbon atoms which have 50 percent s character. The higher character results in the stabilization of the conjugate base of the alkyne molecule.

This implies that the alkynes are more acidic than the alkanes due to the greater stability of the conjugate base of alkyne.