Chapter 17 — Case Study 1: Acetylene — From Welding Torches to Industrial Chemistry
"Acetylene is the simplest alkyne — and the most consequential. It powered welding for a century, fed the Reppe synthesis programs of the 1920s-1950s, and still anchors industrial chemistry of vinyl chloride, vinyl acetate, and acrylonitrile. Every triple bond in industry traces back to acetylene's chemistry." — paraphrase from an industrial chemistry text
This case study traces acetylene (HC≡CH) — the simplest alkyne — through its industrial history. Acetylene's chemistry is the source of many polymer monomers and remains central to specialty applications.
Production of acetylene
Three main routes industrially:
From calcium carbide (the historic route)
$$CaC_2 + 2\,H_2O \to C_2H_2 + Ca(OH)_2$$
Calcium carbide is made from limestone (CaCO₃) + coke (carbon) at very high temperature in an electric arc furnace: $$CaO + 3\,C \to CaC_2 + CO$$
Add water (or hydrochloric acid) to CaC₂ and acetylene gas evolves. Easy to produce on demand; gas can be stored in special tanks (acetylene cannot be compressed alone — it polymerizes; instead it's dissolved in acetone in porous-filled cylinders).
This was the dominant route in the early 20th century. Still used in some applications.
From methane (modern partial oxidation)
$$2\,CH_4 + \tfrac{3}{2}O_2 \to C_2H_2 + 3\,H_2O$$
Methane is partially oxidized at very high temperature (~1500 °C). Modern industrial production. Cheaper at large scale than CaC₂.
From thermal cracking of hydrocarbons
Steam cracking of higher hydrocarbons gives a mix that includes acetylene. Recovered as a byproduct.
Properties
- Combustion: with O₂ gives the hottest flame (~3300 °C) without exotic oxidizers.
- Density: lighter than air; flammable; explosive in some conditions.
- Solubility: dissolves well in acetone (the basis of acetylene cylinders).
Oxy-acetylene welding
The most famous use of acetylene is welding. The oxy-acetylene flame: $$2\,C_2H_2 + 5\,O_2 \to 4\,CO_2 + 2\,H_2O + heat$$
The flame temperature is high enough to melt steel (m.p. ~1500 °C). Used for: - Cutting thick metal plates. - Welding structural steel, plumbing. - Brazing with brass filler rods. - Heat treatment of metal parts.
Oxy-acetylene torches were ubiquitous in industrial fabrication from ~1900 through 1970s. Today, electric arc welding (TIG, MIG) is more common for most applications, but oxy-acetylene remains for specific tasks (cutting thick steel; brazing; field repairs).
Reppe chemistry: the 1928 industrial revolution
In 1928, Walter Reppe at BASF began studying acetylene's reactions under high pressure. Over decades, he and his team developed:
Vinyl chloride from acetylene + HCl
$$HC{\equiv}CH + HCl \to CH_2{=}CHCl$$
Vinyl chloride (CH₂=CHCl) is the monomer for PVC (polyvinyl chloride) — the third-most-produced plastic. PVC is used in: - Plumbing pipes. - Vinyl flooring. - Window frames. - Cables and wiring insulation. - Many other rigid plastic applications.
Mechanism: alkyne + HCl gives Markovnikov vinyl chloride. With Hg or Pd catalyst, the addition is selective.
Modern industrial PVC is made from ethylene + Cl₂ then HCl elimination — but the historic acetylene route was important for decades.
Vinyl acetate from acetylene + acetic acid
$$HC{\equiv}CH + CH_3COOH \to CH_2{=}CH{-}OAc$$
Vinyl acetate is the monomer for polyvinyl acetate (PVA), used in: - Wood glue (Elmer's glue). - Water-based paints. - Adhesives.
This reaction (Reppe, 1928) made vinyl acetate cheap and abundant.
Acrylonitrile from acetylene + HCN
$$HC{\equiv}CH + HCN \to CH_2{=}CH{-}CN$$
Acrylonitrile (CH₂=CHCN) is the monomer for polyacrylonitrile, used in: - Acrylic fibers (carpets, sweaters). - Carbon fiber precursor. - ABS plastic (acrylonitrile-butadiene-styrene; used in Lego blocks, electronic housings).
Modern industrial acrylonitrile is from propylene + NH₃ + O₂ (the SOHIO process) — but the original Reppe route used acetylene.
2-butyne-1,4-diol from acetylene + 2 HCHO
$$HC{\equiv}CH + 2\,HCHO \to HOCH_2{-}C{\equiv}C{-}CH_2OH$$
This intermediate is hydrogenated to 1,4-butanediol, used as a polyester precursor (and in spandex synthesis).
The Reppe legacy
Reppe's industrial program produced: - Vinyl monomers (chloride, acetate). - Acrylonitrile. - Diols (butanediol, hexanediol). - Aldehydes via hydroformylation.
These products were the chemical building blocks of mid-20th-century plastics and synthetic fibers. Walter Reppe (1892-1969) was awarded the Liebig Medal and other honors.
The shift away from acetylene
By the 1960s-1970s, ethylene (cheaper, safer to handle) had replaced acetylene as the workhorse alkene/alkyne feedstock for most commodity products. The reasons:
- Cost: ethylene is cheaper to produce from petroleum.
- Safety: acetylene is flammable and explosive; ethylene is safer.
- Process integration: ethylene-based processes integrate well with petrochemical refining.
Today, acetylene is used for: - Welding (still the major use). - Specialty syntheses: vinyl monomers in some markets. - Fine chemicals: pharmaceutical intermediates. - Carbon nanotubes: precursor for carbon nanostructures.
Modern alkyne chemistry
Beyond the industrial commodity uses: - Sonogashira coupling (Ch 37): aryl halide + terminal alkyne + Pd + Cu → aryl alkyne. Used in pharmaceutical synthesis. - Click chemistry (Sharpless 2022 Nobel): Cu-catalyzed alkyne + azide → 1,2,3-triazole. Used in bioconjugation. - Strain-promoted azide-alkyne cycloaddition (SPAAC): cyclooctyne + azide; no Cu needed. - Alkyne metathesis: similar to olefin metathesis; gives new alkynes.
These modern alkyne chemistries are central to: - Drug discovery. - Bioconjugation (labeling proteins, sugars). - Materials science.
Take-home
- Acetylene (HC≡CH) is the simplest and most consequential alkyne.
- Produced from CaC₂ + H₂O (historic) or from methane (modern).
- Oxy-acetylene welding uses the hot flame (~3300 °C) for cutting and welding.
- Reppe chemistry (1928+) developed acetylene-based industrial syntheses: vinyl chloride (→ PVC), vinyl acetate (→ wood glue), acrylonitrile (→ acrylic fibers, ABS).
- Modern industrial chemistry has shifted to ethylene-based feedstocks for most commodity products.
- Acetylene remains for welding, specialty syntheses, and modern alkyne chemistry (Sonogashira, click).
- The chemistry of Chapter 17 — alkyne addition, alkynide alkylation — underlies a major fraction of the plastic industry.
- Modern click chemistry (Sharpless 2022 Nobel) is a new chapter in alkyne chemistry, with applications in bioconjugation and drug discovery.