Propylene
Properties
| State | Gas |
| Color | Colorless |
| Solubility | Slightly soluble in water; soluble in organic solvents |
| Melting Point | -185.2°C |
| Boiling Point | -47.6°C |
About Propylene
Propylene (propene, CH3CH=CH2, 42.08 g/mol) is the second-largest-volume petrochemical building block on the planet — global production sits around 130 million tonnes annually, second only to ethylene. Roughly two-thirds of that volume comes out of steam crackers as a co-product of ethylene cracking from naphtha or LPG feeds; the rest comes from FCC off-gas at oil refineries and from on-purpose routes (propane dehydrogenation, methanol-to-olefins) that grew sharply through the 2010s as the U.S. shale boom shifted ethylene crackers toward ethane feedstock and starved the propylene co-product stream. The molecule's defining feature is the asymmetric C=C double bond between C1 and C2, with a methyl group hanging off C3. That asymmetry created the Nobel-Prize-winning chemistry of stereoregular polymerization — Karl Ziegler and Giulio Natta won in 1963 for the catalysts that lock the methyl side groups into a single isotactic orientation, producing the high-crystallinity, high-melting polypropylene that is now the second most-produced plastic globally. Beyond polypropylene, propylene feeds the value chain to propylene oxide (polyurethane), acrylonitrile (acrylic fiber and ABS), cumene (the Hock route to phenol and acetone), 2-propanol, and acrolein.
Where you'll encounter it
If you've ever poured boiling water into a polypropylene yogurt cup without it deforming, sutured a wound with monofilament Prolene, or sterilized a centrifuge tube in an autoclave, you've used isotactic polypropylene downstream of propylene polymerization on a Ziegler-Natta or single-site metallocene catalyst. In a refinery, the propylene fraction off the FCC unit is what splitter columns separate from propane to feed the polypropylene plant next door — usually a Spheripol or Unipol process running at 65-75 °C and 30-40 bar. In an organic chemistry teaching lab, the addition of HBr to propylene under radical conditions versus polar conditions is the worked example for Markovnikov versus anti-Markovnikov regiochemistry — peroxide-initiated radical addition gives 1-bromopropane, while polar acid-catalyzed addition gives 2-bromopropane. And in a working ammonia-fiber plant, the SOHIO process oxyammonates propylene with NH3 and O2 over a bismuth-molybdate catalyst to make acrylonitrile, the monomer for acrylic carpet fiber and ABS plastic.
Common Uses
- Isotactic polypropylene production via Ziegler-Natta or metallocene catalysts for fibers, films, and rigid containers
- Propylene oxide manufacture via the chlorohydrin or HPPO route for polyurethane polyols
- Acrylonitrile production through SOHIO oxyammonation for acrylic fiber and ABS resin
- Cumene synthesis via Friedel-Crafts alkylation of benzene en route to phenol and acetone (Hock process)
- 2-Propanol manufacture via direct or indirect hydration for solvent and disinfectant markets
- Acrolein and acrylic acid feedstock for superabsorbent polymer production in diaper cores
Safety Information
GHS classification: Extremely flammable gas Category 1 (H220), Compressed gas (H280, may explode if heated). Simple asphyxiant — displaces oxygen in confined spaces without producing warning symptoms before unconsciousness. OSHA has no specific PEL but applies the simple asphyxiant guidance; ACGIH TLV is 500 ppm 8-hour TWA. Vapor explosion limits 2.0-11.1 vol% in air, autoignition temperature 455 °C. Stored as a liquefied gas under its own vapor pressure (about 9 bar at 20 °C), so cylinders need pressure-rated regulators and outdoor or ventilated storage. Industrial handling means continuous combustible-gas monitoring at the LEL/4 level and nitrogen-purged piping during start-up and shutdown. Not classified as carcinogenic or mutagenic.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.