Alkyne Reduction Sodium

Alkyne Reactions: Alkyne Reduction using Na and NH3

Alkyne Reactions: Dissolving‑Metal Reduction to trans‑Alkenes (Na / NH₃(l))

Sodium dissolved in liquid ammonia (typically around −78 °C) reduces alkynes to trans (E) alkenes. The sequence is driven by single-electron transfers: a solvated electron adds to the alkyne to give a radical anion, protonation produces a vinyl radical, a second electron yields a vinyl anion, and a final protonation furnishes the alkene. Because each hydrogen is delivered from the opposite face, internal alkynes become (E)-alkenes. Terminal alkynes stop at terminal alkenes. Remember the complementarity: H₂/Lindlar or H₂/Ni₂B delivers syn, cis (Z) alkenes; Na/NH₃(l) delivers anti, trans (E) alkenes. Unpoisoned Pd/C or Pt/C continues all the way to the alkane.

This guide walks through the dissolving-metal mechanism, highlights why Na/NH₃(l) stops at the alkene, and shows how to choose between trans- and cis-selective conditions. Worked examples and toolbox integrations reinforce the SET–H⁺–SET–H⁺ logic.

Quick Summary

Reagents/conditions: Na (or Li, K) in liquid NH₃ held at −78 °C to −33 °C. A small amount of t-BuOH or EtOH can accelerate protonation; NH₃ is the default proton source.
Outcome: Alkynes reduce to trans (E) alkenes; terminal alkynes give terminal alkenes (no E/Z assignment).
Mechanism: SET → H⁺ → SET → H⁺ through radical-anion and vinyl-anion intermediates.
Stereochemistry: Anti addition overall explains the E geometry for internal substrates.
Selectivity: Stops at the alkene; Na/NH₃(l) does not hydrogenate alkenes under standard conditions.
Contrast: H₂/Lindlar or H₂/Ni₂B gives cis (Z) alkenes (syn); Pd/C or Pt/C drives fully to the alkane.

Mechanism (5 Frames) (SET–H⁺–SET–H⁺ sequence in liquid ammonia)

Showcased substrate: but-2-yne → (E)-but-2-ene.

Step 1: Solvated electron adds to the alkyne — Step 1 — A solvated electron (Na in liquid NH₃) adds to the alkyne, forming a radical anion aligned for anti delivery.

The deep-blue solution indicates solvated electrons. One electron enters the alkyne π system, giving a radical anion that prefers a geometry setting up anti protonation.

Step 2: Protonation gives a vinyl radical — Step 2 — Liquid NH₃ (or a co-solvent ROH) protonates one vinylic carbon, producing a vinyl radical.

Ammonia acts as the proton donor. Delivering H⁺ to one face converts the radical anion into a vinyl radical while preserving anti orientation for the second addition.

Step 3: Second SET delivers a vinyl anion — Step 3 — A second solvated electron reduces the vinyl radical to a vinyl anion, locking in anti geometry.

The second electron locks in the geometry. The resulting vinyl anion now bears a lone pair that will receive the final proton from the opposite face.

Step 4: Second protonation completes the anti addition — Step 4 — Liquid NH₃ donates a second proton to the carbanion, completing the anti addition.

The carbanion collapses as its lone pair reaches toward an N–H bond. The conjugate base of ammonia leaves with a fresh lone pair, setting up the neutral alkene.

Step 5: Trans alkene diffuses away — Step 5 — The trans (E) alkene diffuses away; Na/NH₃(l) stops at the alkene without over-reduction.

With both anti hydrogen deliveries complete, the neutral alkene leaves the medium. Under dissolving-metal conditions the process halts here—no syn addition occurs to give the alkane.

Mechanistic Checklist (Exam Focus)

Product: trans (E) alkene for internal alkynes; terminal alkynes stop at terminal alkenes.
Pathway: SET–H⁺–SET–H⁺ through radical anion → vinyl radical → vinyl anion.
Anti additions explain the E geometry; no carbocations or rearrangements occur.
Reaction stops at the alkene; Na/NH₃(l) does not hydrogenate alkenes.
Complementary to H₂/Lindlar or H₂/Ni₂B (cis, syn addition).
Use Pd/C or Pt/C with H₂ for full alkane hydrogenation instead.

Worked Examples

Substrate: but-2-yne — Substrate — but-2-yne

Reagents: Na in liquid NH₃ — Na / NH₃(l)

Product: (E)-but-2-ene — Product — (E)-but-2-ene

Internal alkyne → trans internal alkene via anti addition.

Substrate: hex-3-yne — Substrate — hex-3-yne

Product: (E)-hex-3-ene — Product — (E)-hex-3-ene

Longer internal alkyne → trans alkene; alkyl chain length does not disturb anti selectivity.

Substrate: phenylacetylene — Substrate — phenylacetylene (ethynylbenzene)

Terminal alkyne → terminal alkene; no E/Z label is needed.

Substrate: 4-methylpent-2-yne — Substrate — 4-methylpent-2-yne

Product: (E)-4-methylpent-2-ene — Product — (E)-4-methylpent-2-ene

Branched internal alkyne → (E) alkene; anti addition tolerates steric bulk.

Multiple Unsaturations & Selectivity

Alkynes reduce to trans alkenes; isolated alkenes usually survive Na/NH₃(l).
Aromatic rings remain untouched under standard conditions. Adding excess ROH and higher equivalents approaches Birch reduction conditions—avoid if you want to preserve the arene.
With two alkynes, both can be partially reduced. Control the outcome with equivalents and time; reduce the least hindered first when equivalents are limited.

Practical Tips & Pitfalls

Temperature: Work around −78 °C to keep NH₃ liquid and reactions controlled.
Proton source: NH₃ supplies H⁺; add a small amount of t-BuOH or EtOH only if protonation is sluggish. Excess alcohol risks Birch-type arene reduction.
Quench: Destroy solvated electrons before warming and quench cautiously to avoid runaway reactions.
Safety: Liquid NH₃ demands vented, cold-trap setups. Sodium metal is pyrophoric—handle strictly under inert atmosphere.

Exam-Style Summary

Na/NH₃(l) converts alkynes to trans (E) alkenes via SET–H⁺–SET–H⁺ steps (radical anion → vinyl radical → vinyl anion). The two anti additions produce the E isomer. Choose H₂/Lindlar or H₂/Ni₂B for cis (Z) alkenes, or Pd/C–H₂ for full reduction to alkanes.

Interactive Toolbox

Mechanism Solver — follow the Na/NH₃ cycle step by step.
Reaction Solver — compare cis vs trans outcomes (Na/NH₃ vs Lindlar/Ni₂B).
IUPAC Namer — verify product names such as (E)-but-2-ene and (E)-hex-3-ene.

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