Which of the following bases can remove a proton from a...

Which of the following bases can remove a proton from a terminal alkyne in a reaction that favors products? $$\mathrm{CH}_{3} \mathrm{O}^{-} \quad \mathrm{NH}_{3} \quad \mathrm{CH}_{3} \overline{\mathrm{C}} \mathrm{H}_{2} \quad \mathrm{H}_{2} \mathrm{C}=\overline{\mathrm{C}} \mathrm{H} \quad \mathrm{F}^{-}$$

Which of the following bases can remove a proton from a terminal alkyne in a reaction that favors products?
$$\mathrm{CH}_{3} \mathrm{O}^{-} \quad \mathrm{NH}_{3} \quad \mathrm{CH}_{3} \overline{\mathrm{C}} \mathrm{H}_{2} \quad \mathrm{H}_{2} \mathrm{C}=\overline{\mathrm{C}} \mathrm{H} \quad \mathrm{F}^{-}$$

Key Concepts

pKa and Conjugate Acid Relationship

The concept of pKa is a quantitative measure of acidity, and in acid–base reactions, a base will deprotonate an acid effectively if the pKa of its conjugate acid is higher than that of the acid being deprotonated. This principle helps predict the direction of equilibrium; when a terminal alkyne (with a pKa of about 25) is deprotonated, the base’s conjugate acid must be significantly weaker (i.e., have a higher pKa) in order for the reaction to be thermodynamically favored.

Terminal Alkyne Acidity

Terminal alkynes are notable for having relatively acidic C–H bonds compared to typical sp3?hybridized C–H bonds, with pKa values around 25. This enhanced acidity arises from the sp-hybridization of the carbon atom bonded to the hydrogen, which increases the s-character and thus the electronegativity, stabilizing the negative charge upon deprotonation.

Equilibrium in Acid–Base Reactions

In acid–base chemistry, the equilibrium position is largely determined by the relative acidities of the reactants and products. A reaction that removes a proton from a terminal alkyne will only favor product formation if the base used is strong enough such that its conjugate acid is much weaker than the alkyne. This pKa comparison is central to designing reactions, particularly in synthetic organic chemistry, where generating acetylide ions from terminal alkynes is a common step.

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