Read Terminal-Alkyne Acidity Before Predicting Its Reactions
A terminal alkyne's C-H is unusually acidic (sp carbon, high s-character holds the bonding electrons closer to carbon) — this single fact drives both how terminal alkynes are distinguished from alkenes and how they're alkylated.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Terminal alkyne acidity as a diagnostic and synthetic handle
Ammoniacal silver nitrate (and ammoniacal cuprous chloride) precipitates terminal alkynes specifically (forming a silver or copper acetylide) but not internal alkynes or alkenes — this is the standard reagent to distinguish propyne from propene, or ethyne from ethene, since only the terminal C-H is acidic enough to be deprotonated and replaced by the metal.
Alkylating a terminal alkyne via its acetylide anion
NaNH2 deprotonates a terminal alkyne to its acetylide anion, which then acts as a nucleophile in an SN2 reaction with a primary alkyl halide (like EtBr) — this two-step sequence (deprotonate, then alkylate) is the standard way to extend an alkyne chain, and only works cleanly with primary halides (secondary/tertiary halides favour elimination over substitution with this strong, bulky base).
Controlled hydration and hydroboration give different carbonyl products
Markovnikov hydration (HgSO4/H2SO4) of a terminal alkyne gives a methyl ketone (via the more stable, more substituted vinyl cation intermediate then tautomerisation); anti-Markovnikov hydroboration-oxidation of the same terminal alkyne gives an aldehyde instead — same starting alkyne, genuinely different carbonyl product, depending on which regiochemistry the addition method follows.
Reducing an alkyne to an alkene: syn vs anti addition dictates stereochemistry
H2/Lindlar's catalyst (or H2/Pd poisoned) adds syn, giving the CIS alkene; Na/liquid NH3 (dissolving-metal reduction) adds anti, giving the TRANS alkene — for a symmetric internal alkyne, only the anti (Na/NH3) reduction gives a trans alkene with zero net dipole moment by symmetry, while the cis (Lindlar) product from the same alkyne retains a net dipole.
OlympIQ exam lens
- ✓Ammoniacal AgNO3/CuCl precipitates ONLY terminal alkynes — the standard test to distinguish them from alkenes or internal alkynes.
- ✓NaNH2 deprotonates a terminal alkyne; the resulting acetylide anion alkylates cleanly only with primary alkyl halides (SN2).
- ✓Markovnikov hydration gives a methyl ketone; hydroboration-oxidation of the same terminal alkyne gives an aldehyde instead.
- ✓Lindlar's catalyst (H2) gives syn addition (cis alkene); Na/liquid NH3 gives anti addition (trans alkene) — from the same starting alkyne.
Don't assume hydrogenating an alkyne always gives the same alkene stereochemistry — Lindlar's catalyst and Na/liquid NH3 give OPPOSITE stereochemical outcomes (cis vs trans) from the identical starting alkyne, which matters directly for predicting a product's net dipole moment or optical activity.
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