Dihalide To Alkyne Excess Nanh2 Water

Alkyl Halide Reactions: Dihalide → Alkyne using excess NaNH₂, then H₂O

Dihalide → Alkyne via Double Dehydrohalogenation (excess NaNH₂, then H₂O) | OrgoSolver

Alkyne Reactions: Dihalide → Alkyne using excess NaNH₂, then H₂O


Excess sodium amide (NaNH₂) in liquid ammonia converts vicinal (1,2‑) or geminal (1,1‑) dihalides into alkynes via two sequential β-eliminations. The first E2 furnishes a vinyl halide; the second, a vinylic E2, requires the same powerful base to forge the C≡C. When the product is terminal, NaNH₂ immediately deprotonates the C≡CH to a sodium acetylide that must be protonated during the H₂O (or NH₄Cl) workup. Plan on at least two equivalents of base for internal alkynes and three equivalents when a terminal alkyne is the goal.

Quick Summary


  • Class: Two-step elimination sequence — first E2 (alkyl dihalide → vinyl halide), then vinylic E2 (vinyl halide → alkyne).
  • Reagents/conditions: Excess NaNH₂ (typically NH₃(l), −78 to −33 °C up to 0 °C), followed by an H₂O or NH₄Cl workup.
  • Vicinal vs geminal: Both react; the carbon framework dictates whether the resulting C≡C is internal or terminal.
  • Equivalents: ≥2 equiv for internal alkynes; ≥3 equiv when a terminal alkyne is formed (two eliminations + one to deprotonate C≡CH).
  • Stereochemical notes: Anti-periplanar β-H/C–X alignment governs the first E2; the vinylic E2 demands coplanar anti H/X on the alkene.
  • Pitfalls: Insufficient base, forgetting the final protonation of terminal acetylides, or attempting the reaction on carbons lacking β-H atoms.

Mechanism (3 Frames)


Step 1: NaNH2 abstracts the beta hydrogen anti to the leaving group, forming a vinyl halide.
**Step 1 — First E2:** NaNH₂ abstracts the β-hydrogen anti to C–X as the halide leaves, generating the vinyl halide intermediate.
Step 2: Second equivalent of NaNH2 removes the vinylic hydrogen and expels the final halide to create the alkyne.
**Step 2 — Vinylic E2:** A second equivalent of base removes the vinylic β-H while the remaining halide departs, forging the C≡C bond.
Step 3: Terminal alkyne shown with reminder to protonate the acetylide during workup.
**Step 3 — Product:** The alkyne is formed; terminal products remain as Na⁺ RC≡C⁻ until quenched with H₂O/NH₄Cl to reveal RC≡CH.

Worked Examples


Vicinal → Terminal Alkyne

Reactant: 1,2-dibromobutane Reagent: excess NaNH2, then H2O Product: 1-butyne

1,2-dibromobutane + excess NaNH₂ (NH₃(l)), then H₂O → 1-butyne (terminal). The final protonation step is mandatory.

Vicinal → Internal Alkyne

Reactant: 2,3-dibromopentane Reagent: excess NaNH2, then H2O Product: 2-pentyne

2,3-dibromopentane + excess NaNH₂ (NH₃(l)) → 2-pentyne (internal); no additional workup is required beyond quenching the reaction.

Geminal → Terminal Alkyne

Reactant: 1,1-dibromopentane Reagent: excess NaNH2, then H2O Product: 1-pentyne

1,1-dibromopentane → 1-pentyne. Two equivalents of base build the C≡C; the third equivalent is needed to protonate RC≡C⁻ at workup.

Scope & Limitations


  • Well-suited substrates: Vicinal or geminal dihalides with accessible β-H atoms; Br/I leaving groups give the cleanest results.
  • Sluggish cases: Chlorides, heavily β-branched systems, or substrates that struggle to adopt anti geometry — expect slower eliminations or mixtures.
  • Not viable: Dihalides lacking β-H (e.g., neopentyl analogues) and attempts to form highly strained small-ring alkynes.
  • Functional-group sensitivity: NaNH₂ is a very strong base; acidic N–H, O–H, or S–H groups must be protected, and carbonyl α-protons may be deprotonated competitively.
  • Terminal products: Remember the acetylide stage — water or NH₄Cl is required to reveal the neutral C≡CH.

Practical Tips & Pitfalls


  • Charge the reaction with the full base stoichiometry up front: ≥2 equiv (internal) or ≥3 equiv (terminal outcome).
  • Keep the NaNH₂/NH₃(l) mixture cold during additions; warm slightly only if the vinyl halide intermediate lingers.
  • Agitate vigorously — NaNH₂ suspensions settle quickly.
  • Maintain anhydrous conditions until the deliberate quench; NaNH₂ reacts violently with water and air.
  • Vent ammonia safely and quench terminal acetylides with H₂O or NH₄Cl to avoid carrying the salt forward.

Exam-Style Summary


Excess NaNH₂ drives two eliminations on vicinal or geminal dihalides: alkyl dihalide → vinyl halide (E2) → alkyne (vinylic E2). Terminal alkynes are trapped as sodium acetylides and require aqueous workup to reveal RC≡CH. Anti-periplanar geometry is mandatory for the first elimination, Br/I > Cl for leaving-group ability, and ≥3 equiv base is the rule of thumb whenever the product is terminal.

Key reminders:

  • Show both eliminations explicitly — the intermediate vinyl halide is real.
  • Track the base equivalents and include the water (or NH₄Cl) quench step in terminal examples.
  • Highlight the anti requirement and the “no-rearrangements” nature of the mechanism (closed-shell E2 sequence).

Interactive Toolbox


  • Mechanism Solver — Step through the E2 → vinylic E2 sequence and toggle the terminal acetylide/workup panels.
  • Reaction Solver — Evaluate different dihalides to see the final product.
  • IUPAC Namer — Confirm nomenclature for the alkyne products you generate.