Difficulty: Intermediate | Prerequisites: Alkene reactions (Chapters 6–7), acid-base chemistry
Big Picture: Alkynes are the triple-bond functional group, and they share much of the same reaction logic as alkenes but with extra versatility. A triple bond can react once (stopping at an alkene) or twice (going all the way to an alkane or dihalide). Alkynes also have a unique feature: terminal alkynes are weakly acidic, which opens up carbon-carbon bond-forming reactions that alkenes cannot do. This chapter connects alkene chemistry to the broader synthesis toolkit.
Alkynes undergo addition reactions similar to alkenes, but the triple bond can react once (giving an alkene or vinyl halide) or twice (giving an alkane, geminal/vicinal dihalide, or ketone). Terminal alkynes are acidic enough to be deprotonated by strong bases like NaNH2, forming acetylide anions that act as nucleophiles in carbon-carbon bond-forming reactions.
Alkyne
A hydrocarbon containing a carbon-carbon triple bond (one sigma bond + two pi bonds). General formula: CnH(2n-2). Think of it as an alkene with an extra degree of unsaturation.
Terminal alkyne
An alkyne where the triple bond is at the end of the carbon chain, with a hydrogen directly on the sp carbon. This C-H bond is weakly acidic (pKa ~25) because the sp orbital has 50% s-character, stabilising the conjugate base.
Internal alkyne
An alkyne where the triple bond is between two carbon substituents. No acidic proton, so it cannot form acetylide anions.
Acetylide anion
The conjugate base of a terminal alkyne, formed by deprotonation with a strong base (NaNH2/NH3). A powerful carbon nucleophile used in SN2 reactions to form new C-C bonds. In simple terms, this is how you extend a carbon chain using a triple bond.
Lindlar catalyst (Lindlar's catalyst)
A "poisoned" palladium catalyst (Pd on CaCO3, treated with lead acetate and quinoline) that reduces alkynes to cis-alkenes only. The catalyst is deactivated enough that it stops at the double bond and does not reduce further to the alkane.
Dissolving metal reduction
Reduction of an alkyne to a trans-alkene using Na or Li in liquid NH3. The mechanism goes through radical anion intermediates, and the geometry favours the trans product.
Geminal dihalide
A compound with two halogen atoms on the same carbon. Formed from alkynes by addition of 2 equivalents of HX (Markovnikov addition twice).
Vicinal dihalide
A compound with halogen atoms on adjacent carbons. From alkynes: one equivalent of X2 gives a vicinal dihaloalkene; two equivalents give a vicinal tetrahaloalkane.
Keto-enol tautomerism
The equilibrium between a ketone (keto form) and its enol form. Relevant here because Markovnikov hydration of an internal alkyne initially produces an enol, which tautomerises to the more stable ketone.
Catalytic hydrogenation with Lindlar catalyst (H2/Lindlar cat.) (F)
Reduces the triple bond to a cis (Z) double bond
Stops at the alkene stage because the poisoned catalyst cannot reduce the double bond further
Use when you need a cis-alkene from an alkyne
Dissolving metal reduction (Na/NH3) (G)
Reduces the triple bond to a trans (E) double bond
Radical anion mechanism, not catalytic hydrogenation
Use when you need a trans-alkene from an alkyne
Full catalytic hydrogenation (H2/Pd, Pt, or Ni) (E)
Reduces the triple bond all the way to a single bond (alkane)
Two equivalents of H2 are consumed
Not selective; cannot stop at the alkene stage with a standard catalyst
Acid-catalysed hydration (H2SO4/HgSO4) (A)
Markovnikov addition of water across the triple bond
Initially forms an enol, which tautomerises to the ketone
Internal alkynes give ketones
Terminal alkynes give methyl ketones (the OH adds to the internal carbon per Markovnikov's rule)
Hydroboration-oxidation of alkynes (1. BH3, 2. H2O2/NaOH)
Anti-Markovnikov addition of water
Terminal alkynes give aldehydes (OH ends up on the terminal carbon)
Use (sia)2BH (disiamylborane) for terminal alkynes to prevent double hydroboration (C)
Addition of HX (1 or 2 equivalents) (A)
1 eq HX: gives a vinyl halide (Markovnikov, halogen on the more substituted carbon)
2 eq HX: gives a geminal dihalide (both halogens on the same carbon, Markovnikov)
Addition of X2 (1 or 2 equivalents) (B)
1 eq X2: gives a vicinal dihaloalkene (trans addition)
2 eq X2: gives a vicinal tetrahaloalkane (all four halogens on adjacent carbons)
Deprotonation of terminal alkynes (NaNH2/NH3) (E)
Terminal alkyne pKa ~25; NaNH2 is strong enough (NH3 pKa ~38) to fully deprotonate
Produces the acetylide anion (RC≡C-), a carbon nucleophile
Acetylide SN2 reactions
The acetylide anion reacts with primary (and some methyl) haloalkanes via SN2
This forms a new C-C bond, extending the carbon chain
Does not work with secondary or tertiary haloalkanes (elimination dominates)
This is one of the few carbon-carbon bond-forming reactions in early organic chemistry
NaNH2/NH3 on vicinal dihaloalkanes
Double elimination (two equivalents of base) converts vicinal dihalides back to alkynes
This is the reverse direction on the roadmap: a synthesis route to alkynes from dihalides
Students often confuse Lindlar reduction (gives cis-alkene) with dissolving metal reduction (gives trans-alkene). Memory aid: Lindlar = L = liquefy to a "low" cis arrangement; Na/NH3 = trans.
A common mistake is thinking that hydroboration-oxidation of a terminal alkyne gives a ketone. It gives an aldehyde (anti-Markovnikov placement of OH on the terminal carbon, followed by tautomerism).
Students forget that acetylide SN2 reactions only work with primary or methyl haloalkanes. Trying this with a secondary or tertiary substrate gives elimination products, not substitution.
Some students believe that adding 1 eq of H2 to an alkyne always stops at the alkene. This is only controlled with Lindlar catalyst. A standard Pd or Pt catalyst will reduce all the way to the alkane.
⚠️ "Convert alkyne X to cis-alkene Y" or "to trans-alkene Y" is a classic exam question. The answer is always Lindlar (cis) or Na/NH3 (trans).
⚠️ Synthesis problems frequently require acetylide alkylation to build carbon skeletons. If the target has more carbons than the starting material, think acetylide.
⚠️ Distinguishing between aldehyde and ketone products from alkyne hydration: terminal alkyne + anti-Markovnikov hydration = aldehyde. Terminal alkyne + Markovnikov hydration = methyl ketone. Internal alkyne + either = ketone.
⚠️ Double elimination to form alkynes from vicinal dihalides is often tested as part of multi-step synthesis questions.
True or False: Lindlar catalyst reduces an alkyne to a trans-alkene.
Fill in the blank: Treating a terminal alkyne with NaNH2 in NH3 produces a(n) ______ anion.
True or False: H2SO4/HgSO4 hydration of 1-hexyne gives hexanal.
Fill in the blank: Adding 2 equivalents of HBr to a terminal alkyne gives a ______ dihalide.
True or False: Acetylide anions can react with tertiary haloalkanes via SN2.
Answers: 1. False (cis-alkene). 2. Acetylide. 3. False (it gives 2-hexanone, a methyl ketone, via Markovnikov addition). 4. Geminal. 5. False (only primary and methyl substrates work; tertiary gives elimination).
Q: What reagents convert 1-pentyne to pentanal?
A: 1. (sia)2BH (disiamylborane), 2. H2O2/NaOH. This is anti-Markovnikov hydration, placing the oxygen on the terminal carbon to give an aldehyde after tautomerism.
Q: What reagents convert 1-pentyne to 2-pentanone?
A: H2SO4/HgSO4 (acid-catalysed hydration). Markovnikov addition places the OH on the internal carbon, giving an enol that tautomerises to the methyl ketone.
Q: Show how to convert 1-bromobutane to 1-hexyne.
A: First, prepare an acetylide: treat propyne (or acetylene, depending on what is available) with NaNH2/NH3 to form the acetylide anion. Then react the acetylide with 1-bromobutane via SN2 to form the new C-C bond, giving 1-hexyne.
Q: What product do you get from treating 2-butyne with one equivalent of Br2?
A: (E)-2,3-dibromo-2-butene (a vicinal dihaloalkene, trans addition across the triple bond).
Q: How would you convert 3-hexyne to cis-3-hexene?
A: Treat 3-hexyne with H2 in the presence of Lindlar catalyst. This gives syn addition of H2, producing the cis (Z) alkene.
Alkyne reactions build directly on alkene chemistry: the same reagents often appear (HX, X2, H2/cat, BH3, H2SO4/H2O), and the regiochemistry rules (Markovnikov, anti-Markovnikov) carry over. If you are solid on alkene reactions, alkynes are a manageable extension.
Acetylide alkylation connects to nucleophilic substitution (Chapter 8-9). The acetylide anion is doing an SN2 reaction on the haloalkane, and the same substrate restrictions apply (primary/methyl only).
Keto-enol tautomerism from alkyne hydration is foundational for carbonyl chemistry in later chapters (Chapters 17-22), where enols and enolates drive aldol reactions and alpha substitutions.
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