Why Terminal Alkynes Are Acidic

A representative example — structures drawn live.

How acidic is a terminal alkyne?

A terminal alkyne — an alkyne with the triple bond at the end of the chain, R–C≡C–H — has an acidic C–H with a pK_a around 25. That is nowhere near as acidic as a carboxylic acid (pK_a ~5) or even water (15.7), but among C–H bonds it is remarkably acidic. Compare:

• Terminal alkyne C–H (sp carbon): pK_a ≈ 25
• Alkene =C–H (sp² carbon): pK_a ≈ 44
• Alkane C–H (sp³ carbon): pK_a ≈ 50

An internal alkyne (R–C≡C–R) has no such acidic hydrogen, because its terminal-type C–H is gone. Only terminal alkynes carry this special reactivity.

The reason: hybridization and s character

When you remove the terminal proton, the electron pair left behind sits in an orbital on the alkyne carbon. That carbon is sp hybridized, and an sp orbital has 50% s character (versus 33% for sp² and 25% for sp³).

Electrons in an orbital with more s character are held closer to the positively charged nucleus and are lower in energy. So the negative charge of the resulting carbanion — the acetylide — is stabilized far better on an sp carbon than on an sp² or sp³ carbon. A more stable conjugate base means a stronger acid, which is why the terminal alkyne is the most acidic of the three C–H types. This is the same logic that makes acidity track s character: more s character in the orbital holding the lone pair → more stable anion → lower pK_a.

Choosing a base to deprotonate it

To fully deprotonate a terminal alkyne, you need a base whose conjugate acid has a pK_a higher than 25, so that the equilibrium favors the acetylide. The workhorse is sodium amide (NaNH₂): its conjugate acid is ammonia (NH₃, pK_a ≈ 38), so deprotonation is strongly favorable.

By contrast, hydroxide (NaOH) is too weak — water has a pK_a of 15.7, well below 25, so hydroxide only deprotonates a tiny fraction of the alkyne and cannot drive the reaction. Other strong bases such as n-butyllithium also work. The key idea: compare pK_a values and pick a base strong enough to reach the acetylide.

Acetylides as nucleophiles

The real payoff is that the acetylide anion (R–C≡C:^–) is both a strong base and an excellent carbon nucleophile. Because it carries a negative charge on carbon, it can attack electrophilic carbons and form new carbon–carbon bonds — one of the most valuable chain-building tools in the introductory course.

The classic reaction is alkylation of a primary alkyl halide by an S_N2 mechanism:

• Deprotonate the terminal alkyne with NaNH₂ to form the acetylide.
• Add a primary alkyl halide (R'–CH₂–X). The acetylide displaces the halide, giving an internal alkyne with a longer carbon chain.

Because the mechanism is S_N2, it works best with methyl and primary halides; secondary and tertiary halides mostly undergo E2 elimination instead, since the acetylide is also a strong base. Used this way, terminal alkynes become a reliable method for stitching together larger carbon skeletons.