THE CONCEPT
Why the Terminal Alkyne C-H Is Unusually Acidic
A terminal alkyne is one with the triple bond at the very end of the chain, meaning one of the triple-bond carbons carries a hydrogen directly (R-C≡C-H). That particular hydrogen turns out to be dramatically more acidic than any ordinary alkane or alkene C-H bond — with a pKa around 25, compared to roughly 50 for an alkane C-H and roughly 44 for an alkene C-H.
The reason comes down to hybridization. The carbon holding that hydrogen is sp-hybridized, and sp orbitals have more s-character (50% s-character) than sp² (33%) or sp³ (25%) orbitals. Electrons in an s-orbital sit, on average, closer to the positively charged nucleus than electrons in a p-orbital, which means an sp-hybridized carbon holds its bonding electrons more tightly and behaves as if it were more electronegative. That extra electronegativity is exactly what makes it easier to pull the attached hydrogen off as a proton, leaving behind a comparatively stabilized anion.
💡 Memory Trick
The hub's trick is to memorize the acidity fact as a package: terminal alkyne R-C≡C-H has a pKa of about 25, and is deprotonated by strong bases like NaNH₂ (sodium amide) or n-BuLi (n-butyllithium). Neither of those bases is exotic or fussy to remember — they're simply strong enough (their conjugate acids, ammonia and butane, have pKa's around 38 and 50) to reliably strip off a pKa-25 proton, which an ordinary base like hydroxide or an alkoxide could never manage.
THE ACETYLIDE ANION AS A BUILDING BLOCK
Using Deprotonation to Form New C-C Bonds
Once deprotonated, the resulting acetylide anion (R-C≡C⁻) is both a strong base and an excellent nucleophile, and chemists use it constantly as a carbon-carbon bond-forming tool. Acetylides react cleanly with primary alkyl halides in an SN2 reaction, extending the carbon chain by attaching a new alkyl group directly onto the alkyne carbon — this is one of the most reliable ways to build a longer, more complex alkyne from a simpler terminal one.
Acetylides are equally happy attacking the electrophilic carbon of an aldehyde or ketone, adding across the carbonyl to generate a new carbon-carbon bond and, after aqueous workup, a propargylic alcohol (an alcohol sitting next to a triple bond). Between alkyl halide substitution and carbonyl addition, the acetylide anion is one of the most versatile carbanion nucleophiles available for building up carbon skeletons in a synthesis.
🧪 Lab Application
You need to extend 1-hexyne into a longer alkyne by attaching an ethyl group at the terminal carbon.
1
Deprotonate the terminal alkyne first. Treat 1-hexyne with NaNH₂ (or n-BuLi) to remove the acidic terminal C-H and generate the corresponding acetylide anion.
2
Choose an appropriate electrophile. Since a new alkyl group needs to be attached, react the acetylide with a primary alkyl halide — bromoethane, in this case — rather than an aldehyde or ketone.
3
Let the acetylide perform an SN2 displacement. The acetylide anion's lone pair attacks the primary carbon of bromoethane, displacing bromide and forming a brand-new carbon-carbon bond directly onto what was the terminal alkyne carbon.
4
Confirm the extended product. The product is now a longer internal alkyne — the original terminal C-H is gone, replaced by a bond to the new ethyl group, and the triple bond itself is untouched throughout the whole sequence.
📌 Exam Application
Expect this concept tested two ways: a straightforward acidity/pKa ranking question comparing alkyne, alkene, and alkane C-H bonds, and a synthesis question where you need to recognize that building a new C-C bond onto an alkyne requires deprotonation with a strong base first, before any electrophile can be introduced.
⚠️ Most Common Terminal Alkyne Reactions Mistakes
The most common mistake is trying to use a weak base like NaOH or an alkoxide to deprotonate a terminal alkyne — these bases simply aren't strong enough to reach a pKa-25 proton in any useful amount. The second common trap is forgetting that this reactive acidic hydrogen only exists on a TERMINAL alkyne; an internal alkyne (triple bond in the middle of the chain, no C≡C-H) has no analogous acidic proton to remove.
✓ Quick Self-Test
1) Approximately what is the pKa of a terminal alkyne C-H? 2) Why is that hydrogen more acidic than an alkane or alkene C-H? 3) Name two bases strong enough to deprotonate a terminal alkyne. 4) What kind of electrophile does an acetylide anion react with to extend a carbon chain? 5) Why does an internal alkyne lack this special acidic C-H?
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Conformational Analysis of Cyclohexane
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