Difficulty: Intermediate | Prerequisites: Alkene/alkyne structure, cis-trans isomerism, Mechanism III (concerted addition), basic understanding of catalysis.
Reduction reactions add hydrogen across double and triple bonds. For alkenes, catalytic hydrogenation is the workhorse: H₂ gas and a metal catalyst deliver two hydrogens from the same face (syn addition, Mechanism III). For alkynes, the picture is richer because you can choose to stop at the alkene stage or go all the way to the alkane, and you can choose the geometry of the resulting alkene (cis or trans).
This is one of the few places in the course where you have genuine synthetic control over stereochemistry: poisoned catalysts give cis alkenes, dissolving metal conditions give trans alkenes. The lecture also covers hydroboration of alkynes, which introduces keto-enol tautomerism.
H₂ with Pd, Pt, or Ni reduces alkenes to alkanes (syn addition). The same conditions reduce alkynes all the way to alkanes. To stop at a cis alkene, use a poisoned catalyst (Pd-CaCO₃ or Pd-BaSO₄ with quinoline or PbO). To get a trans alkene from an alkyne, use dissolving metal reduction (Na or Li in liquid NH₃). Hydroboration of alkynes with BH₃ followed by oxidative workup gives ketones via keto-enol tautomerism.
Catalytic hydrogenation
The addition of H₂ across a double or triple bond using a metal catalyst. Both hydrogens are delivered from the metal surface to the same face of the pi bond. Think of it as the alkene lying flat on the metal and getting two hydrogens glued on from below.
Pd/C (palladium on carbon)
A heterogeneous catalyst: palladium metal deposited on activated carbon. Used with H₂ gas for hydrogenation of alkenes and alkynes.
PtO₂ (Adams' catalyst)
Another common hydrogenation catalyst, often written as PtO₂ but reduced in situ to platinum metal.
Poisoned catalyst (Lindlar-type catalyst)
A deliberately deactivated hydrogenation catalyst that is reactive enough to reduce an alkyne to an alkene but too sluggish to continue to the alkane. Allows partial reduction and stops at the cis alkene.
Pd-CaCO₃ / Pd-BaSO₄
The metal support for poisoned catalysts. Combined with quinoline or PbO to reduce activity.
Quinoline
A nitrogen-containing aromatic compound added as a "poison" to the palladium surface, reducing its catalytic activity so the reaction stops at the alkene stage.
PbO (lead oxide)
An alternative catalyst poison to quinoline, serving the same purpose.
Dissolving metal reduction
Reduction of alkynes to trans alkenes using an alkali metal (Na⁰ or Li⁰) dissolved in liquid ammonia (NH₃). The metal donates electrons one at a time, and the geometry of the resulting alkene is trans (anti addition).
Na/NH₃ or Li/NH₃
The standard dissolving metal conditions. Na⁰ and Li⁰ are the reducing agents; liquid NH₃ is the solvent. Important: NaNH₂ is a strong base and is a completely different reagent.
Cis alkene
An alkene with both larger substituents on the same side of the double bond. Formed by syn addition of H₂ to an alkyne (poisoned catalyst).
Trans alkene
An alkene with the larger substituents on opposite sides of the double bond. Formed by anti addition (dissolving metal reduction of an alkyne).
Keto-enol tautomerism
The equilibrium interconversion between a ketone (keto form) and its corresponding enol form (a vinyl alcohol). The keto form is almost always more stable. Relevant here because hydroboration of an alkyne produces an enol that tautomerises to a ketone.
Enol
A vinyl alcohol: a C=C double bond with an OH group attached to one of the carbons. Unstable relative to the keto tautomer.
Reagents: H₂ gas with a metal catalyst (Pd, Pd/C, PtO₂, Ni, or Pt).
The alkene adsorbs to the metal surface. Both hydrogens are delivered from the surface to the same face of the double bond.
Stereoselectivity: always syn addition (Mechanism III).
Regioselectivity: not applicable (both atoms added are H).
The reaction is not selective for more or less substituted positions.
The same catalysts (Pd/C, PtO₂, Ni) with H₂ will reduce an alkyne all the way through the alkene stage to the fully saturated alkane.
Two equivalents of H₂ are consumed.
Both additions are syn, but since the final product is an alkane with four single bonds to each carbon, the stereochemistry of the intermediate alkene does not matter in the product.
To stop the reduction at the alkene stage, you must make the catalyst less active by adding a "poison."
Reagents: H₂ with Pd-CaCO₃ or Pd-BaSO₄, plus quinoline or PbO.
The poisoned catalyst is active enough to reduce the alkyne to an alkene but too sluggish to reduce the alkene further.
Stereoselectivity: syn addition gives the cis (Z) alkene. Both hydrogens are delivered from the same face.
Example: 2-butyne + H₂/Pd-CaCO₃/quinoline gives cis-2-butene.
Reagents: Na⁰ or Li⁰ in liquid NH₃ (l).
The metal donates single electrons to the triple bond in a stepwise radical/anion mechanism.
Stereoselectivity: anti addition gives the trans (E) alkene.
This is the only way covered in this course to convert an alkyne directly to a trans alkene.
Important distinction: Na/NH₃ (dissolving metal, reducing agent) is completely different from NaNH₂ (sodium amide, a strong base used to deprotonate terminal alkynes).
BH₃ adds across a triple bond in a syn, anti-Markovnikov fashion (same selectivity as with alkenes).
The initial product after oxidative workup (H₂O₂/NaOH) is a vinyl alcohol (an enol).
The enol spontaneously tautomerises to the more stable ketone (keto-enol tautomerism).
Net result: an internal alkyne is converted to a ketone; a terminal alkyne gives an aldehyde (anti-Markovnikov placement of the OH, then tautomerism).
With 2-butyne (a symmetrical internal alkyne): BH₃ then H₂O₂/NaOH gives 2-butanone (methyl ethyl ketone).
Substrate | Reagents | Product | Stereoselectivity | Mechanism |
|---|---|---|---|---|
Alkene | H₂, Pd/C or PtO₂ or Ni | Alkane | syn | III |
Alkyne | H₂, Pd/C or PtO₂ or Ni | Alkane (full reduction) | syn | III |
Alkyne | H₂, Pd-CaCO₃ or Pd-BaSO₄ + quinoline/PbO | Cis (Z) alkene | syn | III |
Alkyne | Na or Li in NH₃ (l) | Trans (E) alkene | anti | radical/anion |
Alkyne | (1) BH₃, (2) H₂O₂/NaOH | Enol, then ketone (tautomerism) | syn, anti-Markovnikov | III |
Students often confuse Na/NH₃ (dissolving metal reduction, gives trans alkene) with NaNH₂ (sodium amide, a strong base that deprotonates terminal alkynes). These are entirely different reagents with entirely different purposes.
Assuming that poisoned catalysts can reduce alkenes. They cannot: the whole point of the poison is that the catalyst is too weak for alkene reduction but strong enough for alkyne reduction.
Thinking that dissolving metal reduction uses a metal catalyst like Pd. It does not. Na⁰ and Li⁰ are consumed as reducing agents (electron donors), not catalysts. The solvent is liquid ammonia, not a metal surface.
Forgetting keto-enol tautomerism when predicting hydroboration products of alkynes. The immediate product is an enol, but it tautomerises to the ketone. The final answer on an exam is the ketone, not the enol.
⚠️ The cis vs. trans alkene from alkyne reduction is a classic exam question. Poisoned catalyst (H₂, Pd-CaCO₃, quinoline) = cis. Dissolving metal (Na/NH₃) = trans. You will be asked to choose one.
⚠️ Hydrogenation is always syn. If the question asks for stereochemistry of the product, both new C-H bonds are on the same face.
⚠️ Hydroboration of alkynes: the product you write is the ketone (after tautomerism), not the enol. But you should be able to show the enol intermediate and the tautomerism step.
⚠️ Know the difference between Na/NH₃ and NaNH₂ cold. It appears on nearly every exam covering this material.
True or false: H₂/Pd reduces an alkyne to a trans alkene. (False, it reduces all the way to the alkane. You need a poisoned catalyst to stop at the cis alkene.)
Fill in the blank: To convert 2-butyne to trans-2-butene, use ______. (Na (or Li) in liquid NH₃.)
True or false: Quinoline is a catalyst for hydrogenation. (False, it is a catalyst poison that reduces the activity of Pd.)
Fill in the blank: Hydroboration of a terminal alkyne followed by H₂O₂/NaOH gives an ______ as the final product. (Aldehyde.)
True or false: Catalytic hydrogenation of an alkene gives anti addition. (False, it gives syn addition.)
Q: What reagents convert 3-hexyne into cis-3-hexene?
A: H₂ with Pd-CaCO₃ (or Pd-BaSO₄) and quinoline (or PbO). The poisoned catalyst reduces the triple bond to a cis double bond and stops.
Q: What reagents convert 3-hexyne into trans-3-hexene?
A: Na (or Li) in liquid NH₃. Dissolving metal reduction gives the trans alkene.
Q: Methylcyclohexene is treated with H₂/Pd. Draw the product and indicate stereochemistry.
A: Methylcyclohexane. Both hydrogens add syn to the same face of the ring. The product is a mixture of cis and trans isomers only if the ring already has another substituent to define a stereochemical relationship; otherwise it is simply the saturated ring.
Q: 1-Butyne is treated with (1) BH₃, (2) H₂O₂/NaOH. What is the final product?
A: Butanal (butyraldehyde). BH₃ adds anti-Markovnikov across the triple bond. Oxidative workup gives the enol at C-1, which tautomerises to the aldehyde.
Q: 2-Butyne is treated with (1) BH₃, (2) H₂O₂/NaOH. What is the final product?
A: 2-Butanone (methyl ethyl ketone). The symmetrical internal alkyne gives a ketone after tautomerism.
Catalytic hydrogenation connects to thermodynamics and kinetics: the reaction is thermodynamically favourable but kinetically slow without a catalyst, which is why the metal surface is required. Heat of hydrogenation data (covered earlier) uses this reaction to compare alkene stabilities.
Keto-enol tautomerism from hydroboration of alkynes recurs extensively in carbonyl chemistry, aldol reactions, and enolate chemistry later in the course.
The cis/trans selectivity in alkyne reduction is the direct complement of the syn/anti selectivity in alkene addition reactions (covered in the companion mechanisms notes). Together, they represent the full toolkit for controlling stereochemistry in synthesis.
Related Terms / Search Tags
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