Alkyne Acetylide Alkylation

Alkyne Reactions: Acetylide Formation & Alkylation (NaNH₂; R–X)

Alkyne Reactions: Acetylide Formation & Alkylation (NaNH₂; then R–X)

Terminal alkynes (pKₐ ≈ 25) are acidic enough that sodium amide (NaNH₂; conjugate acid NH₃, pKₐ ≈ 38) deprotonates them completely. The resulting alkynyl (acetylide) anion is a potent, linear nucleophile/base that performs SN2 on methyl, primary, allylic, or benzylic halides/tosylates, forging new C–C σ bonds. With acetylene itself, two rounds of deprotonation/alkylation deliver unsymmetrical internal alkynes. Selectivity is all about the electrophile: hindered (2°/3°) halides eject via E2, while vinyl/aryl halides are inert to SN2.


Key Emphasis (Learning Pivots)

  • pKₐ logic drives deprotonation. NH₂⁻ (conjugate acid NH₃, pKₐ ≈ 38) comfortably removes the sp–C–H (pKₐ ≈ 25) of terminal alkynes, giving a metalated acetylide.
  • SN2 only works on unhindered halides. Methyl, primary, allylic, and benzylic halides/tosylates undergo clean substitution; secondary/tertiary electrophiles give E2 instead.
  • Chain extension: Acetylene can be alkylated twice (R¹ then R²) to furnish unsymmetrical internal alkynes.
  • Protic incompatibilities: Any –OH, –NH, –CO₂H, or water instantly quenches the acetylide.
  • Vinyl/aryl halides are SN2-inert. Use cross-coupling (e.g., Sonogashira) if you must functionalize those carbons.

Quick Summary

  • Stage 1 (base): Terminal alkyne + NaNH₂ (liquid NH₃, THF, DMSO) → acetylide + NH₃.
  • Stage 2 (alkylation): Acetylide + R–X (I, Br, OTs, OMs; methyl/primary/allyl/benzylic) → SN2 substitution with inversion at the electrophilic carbon.
  • Optional Stage: Repeat Stage 1 + Stage 2 when starting from acetylene to install a second R group.
  • Outcome: C–C σ-bond construction; stereoinversion at the electrophilic carbon (if chiral).
  • Pitfalls: Secondary/tertiary halides → E2 alkenes; vinyl/aryl halides → no SN2; propargyl halides may give SN2′ (allene) side-products.

Mechanism — Three Steps (with optional repetition)

NaNH2 deprotonates the terminal alkyne
**Step 1 — Base-mediated deprotonation.** NH₂⁻ removes the sp–C–H, electrons return to carbon, and the acetylide (R–C≡C⁻ Na⁺) plus NH₃ are formed.
Acetylide performs SN2 on R–X
**Step 2 — SN2 alkylation.** The linear acetylide attacks backside at the electrophilic carbon; C–X breaks to X⁻, forging a new C–C bond with inversion at that carbon.
Optional second deprotonation/alkylation
**Step 3 — Optional repeat.** Starting from HC≡CH, repeat Steps 1–2 with a second R–X to reach an internal alkyne (R¹–C≡C–R²) so both R placeholders carry through.

This optional frame now renders the R placeholder directly (not the earlier asterisk), reinforcing that every addition hands the same R over to the next bond.


Mechanistic Checklist (Exam Focus)

  • Terminal alkyne required (must possess ≡C–H).
  • SN2 scope limited to methyl/primary/allyl/benzylic; secondary/tertiary halides undergo E2.
  • Inversion occurs at the electrophilic carbon (if stereogenic).
  • Protic functionalities or water quench the acetylide instantly.
  • Vinyl/aryl halides require cross-coupling (no backside approach).
  • Propargyl electrophiles can give SN2′ (allenes) — mention when relevant.

Worked Examples

Example A — 1-Heptyne + NaNH₂ ; 1-bromopropane. Chain extension by three carbons via clean SN2.
Reactant: 1-heptyne
Reactant
Reagents: NaNH2, then R–X
Reagents
Product: internal alkyne
Product
Example B — Acetylene double alkylation. MeI installs the first methyl; a second NaNH₂/alkylation with 1-bromopropane furnishes 2-pentyne.
Reactant: acetylene
Reactant
Reagents
Reagents
Product: 2-pentyne
Product
Example C — Phenylacetylene + benzyl bromide. Benzylic halides are superb SN2 partners, giving a diaryl-substituted alkyne.
Reactant: phenylacetylene
Reactant
Reagents
Reagents
Product: diaryl alkyne
Product

Scope & Limitations

  • Best electrophiles: Methyl, primary (esp. allylic/benzylic) halides/tosylates.
  • Borderline: Neopentyl-type primary halides (sterically slowed); propargyl halides (possible SN2′ → allenes).
  • Poor: Secondary halides (E2 competes), tertiary halides (E2 only), vinyl/aryl halides (no SN2).
  • Functional groups: Protect or avoid protic sites that quench acetylide.
  • Solvent: Liquid NH₃ for classic protocols; THF/DMSO/DMF for convenience; all must be anhydrous/inert.

Practical Tips & Pitfalls

  • Generate the acetylide fully (monitor gas evolution in NaNH₂/NH₃ or NaH/THF protocols) before adding R–X.
  • Add alkyl halide slowly to maintain excess acetylide and minimize elimination.
  • Prefer I > Br ≫ Cl; tosylates/mesylates are acceptable.
  • Keep temperatures low for deprotonation; warm gently for SN2 as needed.
  • If E2 dominates, switch to a less hindered electrophile or use milder bases (e.g., NaH) with polar aprotic solvent.

Exam-Style Summary

Terminal alkyne + NaNH₂ → acetylide; acetylide + methyl/primary/allyl/benzylic R–X → SN2 coupling (inversion). Secondary/tertiary halides give E2; vinyl/aryl halides do not undergo SN2. Acetylene can be dialkylated by repeating the sequence.


Interactive Toolbox

Reagent: NaNH₂ + R–X

Hit the nanh2_rx.png reagent button to launch the Mechanism Solver and replay the acetylide formation + alkylation frames (including the optional repeat from acetylene).

  • Mechanism Solver — Animate NaNH₂ deprotonation → SN2 addition (and the optional second alkylation) to reinforce the closed-shell arrow practice.
  • Reaction Solver — Compare substitution vs elimination for methyl/primary versus secondary/tertiary halides + terminal alkynes.
  • IUPAC Namer — Caption both the starting alkyne and the alkynes produced in the worked examples (no SMILES shown to readers).

FAQ

  • Why does NaNH₂ work while NaOH doesn’t? NH₂⁻ has a much stronger conjugate acid, so the equilibrium strongly favors acetylide formation; HO⁻ (pKₐ ≈ 15.7) cannot deprotonate the ≡C–H effectively.
  • Can I use secondary halides? Not productively—expect β-elimination to alkenes; pick a less hindered halide.
  • Do vinyl/aryl halides participate? No, backside approach is blocked; use Pd-catalyzed couplings instead.
  • How do I avoid SN2′ with propargyl halides? Choose electrophiles that lack adjacent π-systems or be prepared to draw both propargyl and allenyl products.
  • Is a workup required? Typically NH₄Cl(aq)/H₂O to neutralize excess base and dissolve inorganic salts before isolation.