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Propylene Oxide

C3H6O organic
Molar Mass
58.079 g/mol
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Properties

StateLiquid (colorless, volatile with sweet ethereal odor)
ColorColorless
SolubilitySoluble in water (590 g/L at 20°C); miscible with most organic solvents
Melting Point-112°C
Boiling Point34°C

About Propylene Oxide

Propylene oxide is the methyl-substituted three-membered epoxide (C3H6O, 58.079 g/mol) that ranks as one of the largest-volume organic chemicals in the world — global production crossed 11 million tonnes annually around 2020, almost all of it consumed within hours of synthesis as a feedstock for polyether polyols. The strained oxirane ring carries about 27 kcal/mol of ring strain that drives ring-opening with essentially any nucleophile under mild conditions. The methyl substituent introduces both chirality (PO is the textbook chiral epoxide for asymmetric synthesis exercises) and regiochemistry: under acidic catalysis the nucleophile prefers the more substituted carbon (Markovnikov-like, because protonation builds carbocation character on the tertiary side), while under basic catalysis the nucleophile prefers the less hindered terminal carbon (anti-Markovnikov, because steric access dominates SN2-like attack). About 65% of propylene oxide goes to polyether polyols for polyurethane foams; the rest splits among propylene glycol (food, pharma, antifreeze), glycol ethers, and propylene carbonate. The two dominant industrial routes are the chlorohydrin process (older, generates calcium chloride brine waste) and the increasingly common HPPO process (hydrogen peroxide plus propylene over a TS-1 titanosilicate catalyst, water as the only by-product).

Where you'll encounter it

If you've ever sat on a memory-foam mattress, leaned against an automotive headrest, or insulated an attic with rigid polyurethane board, the polyols that built that foam started as propylene oxide ring-opened by a glycerol or sucrose initiator over a KOH catalyst. In a flavor-and-fragrance lab, ring-opening of propylene oxide with carboxylic acids gives propylene glycol esters used as humectants and flavor carriers in chewing gum and beverages. In aseptic-packaging plants for liquid foods like UHT milk and aseptic juice, propylene oxide gas was historically used as a sterilant for spice and nut shipments — though that use is now restricted in the EU because of mutagenicity concerns. And in any organic chemistry teaching lab, the regiochemistry of HBr and NaOMe ring-opening of propylene oxide is the standard worked exercise that locks in the difference between acid-catalyzed and base-catalyzed epoxide opening.

Common Uses

  • Polyether polyol production by KOH-catalyzed ring-opening with glycerol or sucrose initiators
  • Propylene glycol manufacture via hydrolysis for food, pharmaceutical, and antifreeze applications
  • Propylene carbonate synthesis with CO2 over zinc-bromide catalysts for lithium-ion battery electrolytes
  • Glycol ether solvent production (propylene glycol monomethyl ether, PGMEA) for semiconductor photoresist
  • Spice and nut sterilant under restricted protocols where ethylene oxide is unsuitable
  • Teaching reagent for acid- versus base-catalyzed epoxide ring-opening regiochemistry

Safety Information

GHS classification: Extremely flammable liquid Category 1 (H224, flash point -37 °C), Carcinogen Category 1B (H350), Mutagen Category 1B (H340), Acute toxicity oral and inhalation Category 3 (H301, H331), Eye irritation Category 2 (H319), Skin irritation Category 2 (H315). OSHA PEL is 100 ppm 8-hour TWA; ACGIH TLV is 2 ppm TWA based on the carcinogenicity classification. IARC Group 2B (possibly carcinogenic to humans) since 1994 with strong animal evidence and limited human data. Vapor explosion limits 2.3-36% in air make it one of the widest flammable ranges of any common solvent. Industrial handling means closed-system reactors, continuous gas-phase monitoring at the 1 ppm level, nitrogen blanketing on storage tanks, and emergency depressurization to scrubbers — not bench-scale chemistry territory.

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.

Constituent Elements

C — Carbon H — Hydrogen O — Oxygen

Frequently Asked Questions

What is the molar mass of propylene oxide?
58.079 g/mol, from 3 × 12.011 (C) + 6 × 1.008 (H) + 15.999 (O). At 0.83 g/mL density and 34 °C boiling point, a 1 mol charge to a small bench reactor is only 70 mL of liquid at room temperature — easy to overdose. The low boiling point also means any leak vaporizes immediately, which is why bench-scale reactions are usually run in sealed pressure tubes or sparged with the epoxide as a gas rather than handled as a free liquid.
How does the methyl group affect ring-opening regioselectivity?
Acid catalysis protonates the epoxide oxygen, weakening both C-O bonds but loading more positive charge onto the more-substituted carbon — the methyl group stabilizes the developing carbocation character. Nucleophile attack therefore goes to the substituted carbon (Markovnikov regiochemistry). Base catalysis runs an SN2-like mechanism on the unprotonated epoxide where steric access is everything, so the nucleophile attacks the unsubstituted terminal carbon (anti-Markovnikov). The two regiochemistries give different products from the same reagents — HBr opens to 2-bromo-1-propanol under acid, and to 1-bromo-2-propanol under base — which is why this is the canonical teaching example in undergraduate organic.
What are polyurethane foams?
Polyurethane foams come from a two-component reactive injection: a polyol stream (long-chain triols and diols built by ring-opening polymerization of propylene oxide onto a glycerol or sucrose initiator) and an isocyanate stream (typically MDI for rigid foams or TDI for flexible foams). The OH groups on the polyol and the N=C=O groups on the isocyanate condense into urethane linkages, while a small amount of water reacts with isocyanate to release CO2 — that gas is the blowing agent that builds the cellular foam structure. Flexible foams with high-MW soft segments end up in mattresses and car seats; rigid foams with short, highly cross-linked polyols end up as building insulation, foam-board, and refrigerator wall fill.