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Propylene oxide
Propylene oxide
Propylene oxide
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Propylene oxide
Structural formula
Structural formula
Ball-and-stick model of the propylene oxide molecule
Ball-and-stick model of the propylene oxide molecule
Names
Preferred IUPAC name
(2R)-2-Methyloxirane
(2S)-2-Methyloxirane
Other names
Propylene oxide
Epoxypropane
Propylene epoxide
1,2-Propylene oxide
Methyl oxirane
1,2-Epoxypropane
Propene oxide
Methyl ethylene oxide
Methylethylene oxide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.000.800 Edit this at Wikidata
EC Number
  • 200-879-2
KEGG
UNII
  • InChI=1S/C3H6O/c1-3-2-4-3/h3H,2H2,1H3
    Key: GOOHAUXETOMSMM-UHFFFAOYSA-N
  • CC1CO1
Properties
C3H6O
Molar mass 58.080 g·mol−1
Appearance Colourless liquid
Odor benzene-like[1]
Density 0.859 g/cm3[2]
Melting point −111.9 °C (−169.4 °F; 161.2 K)[2]
Boiling point 35 °C (95 °F; 308 K)[2]
41% (20 °C)[1]
Vapor pressure 445 mmHg (20 °C)[1]
−4.25×10−5 cm3/mol[3]
1.3660[2]
Thermochemistry
120.4 J·(K·mol)−1
196.5 J·(K·mol)−1
−123.0 kJ·mol−1[4]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Extremely flammable[5][6]
GHS labelling:
GHS02: FlammableGHS08: Health hazardGHS07: Exclamation mark
Danger
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g. propaneInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazards (white): no code
3
4
2
Flash point −37 °C (−35 °F; 236 K)
747 °C (1,377 °F; 1,020 K)
Explosive limits 2.3–36%[1]
Lethal dose or concentration (LD, LC):
660 mg/kg (guinea pig, oral)
380 mg/kg (rat, oral)
440 mg/kg (mouse, oral)
1140 mg/kg (rat, oral)
690 mg/kg (guinea pig, oral)[7]
1740 ppm (mouse, 4 h)
4000 ppm (rat, 4 h)[7]
2005 ppm (dog, 4 h)
4000 ppm (guinea pig, 4 h)[7]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 100 ppm (240 mg/m3)[1]
REL (Recommended)
 Ca[1]
IDLH (Immediate danger)
 Ca [400 ppm][1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Propylene oxide is an epoxide with the molecular formula C3H6O. This colourless volatile liquid with an odour similar to ether, is produced on a large scale industrially. Its major application is its use for the production of polyether polyols for use in making polyurethane plastics. It is a chiral epoxide, although it is commonly used as a racemic mixture.

This compound is sometimes called 1,2-propylene oxide to distinguish it from its isomer 1,3-propylene oxide, better known as oxetane.

Production

[edit]

Industrial production of propylene oxide starts from propylene.[8] Two general approaches are employed, one involving chlorohydrin formation and the other involving oxidation.[9] In 2005, about half of the world production was through chlorohydrin technology and one half via oxidation routes. The latter approach is growing in importance.[10]

Chlorohydrin route

[edit]

The traditional route proceeds via the conversion of propylene to propylene chlorohydrin according to the following simplified scheme:

The mixture of 1-chloro-2-propanol and 2-chloro-1-propanol then undergoes internal cyclization. For example:

Lime (calcium hydroxide) is often used to absorb the HCl.

Oxidation of propylene

[edit]

The other general route to propylene oxide involves oxidation of propylene with an organic peroxide. The reaction follows this stoichiometry:

CH3CH=CH2 + RO2H → CH3CHCH2O + ROH

The process is practiced with four hydroperoxides:[10]

In principle, this process produces only water as a side product. In practice, some ring-opened derivatives of PO are generated.[12]

Propylene oxide is chiral building block that is commercially available in either enantiomeric form ((R)-(+) and (S)-(–)). The separated enantiomers can be obtained through a Co(III)-salen-catalyzed hydrolytic kinetic resolution of the racemic material.[13]

Reactions

[edit]

Like other epoxides, PO undergoes ring-opening reactions. With water, propylene glycol is produced. With alcohols, reactions, called hydroxylpropylation, analogous to ethoxylation occur. Grignard reagents add to propylene oxide to give secondary alcohols.

Some other reactions of propylene oxide include:[14]

Uses

[edit]

Between 60 and 70% of all propylene oxide is converted to polyether polyols by the process called alkoxylation.[15] These polyols are building blocks in the production of polyurethane plastics.[16] About 20% of propylene oxide is hydrolyzed into propylene glycol, via a process which is accelerated by acid or base catalysis. Other major products are polypropylene glycol, propylene glycol ethers, and propylene carbonate.

Niche uses

[edit]

Fumigant

[edit]

The United States Food and Drug Administration has approved the use of propylene oxide to pasteurize raw almonds beginning on September 1, 2007, in response to two incidents of contamination by Salmonella in commercial orchards, one incident occurring in Canada and one in the United States.[17][18] Pistachio nuts can also be subjected to propylene oxide to control Salmonella.

Microscopy

[edit]

Propylene oxide is commonly used in the preparation of biological samples for electron microscopy, to remove residual ethanol previously used for dehydration. In a typical procedure, the sample is first immersed in a mixture of equal volumes of ethanol and propylene oxide for 5 minutes, and then four times in pure oxide, 10 minutes each.[citation needed]

Munition

[edit]

Propylene oxide is sometimes used in thermobaric munitions as the fuel in fuel–air explosives. In addition to the explosive damage from the blast wave, unexploded propylene oxide can cause additional effects from direct toxicity.[19]

Safety

[edit]

Propylene oxide is both acutely toxic and carcinogenic. Acute exposure causes respiratory tract irritation, eventually leading to death.[20] Signs of toxicity after acute exposure include salivation, lacrimation, nasal discharge, gasping, lethargy and hypoactivity, weakness, and incoordination. Propylene oxide is also neurotoxic in rats, and presumably in humans.[21] Propylene oxide alkylates DNA and is considered a mutagen for both animals and humans.[22][23][24] Pregnant rats exposed to 500ppm of propylene oxide for less than 8 hours gave birth to litters with significant deformities and weight deficiencies. Similar exposure has also shown to reduce animal fertility.[25] As such, it is a known animal carcinogen[26] and potential human carcinogen, and is included into the List of IARC Group 2B carcinogens.[27]

Propylene oxide is an extremely flammable liquid, and its vapors can form explosive mixtures with air at concentrations as low as 2.3% (Lower Explosive Limit).[25] Propylene oxide vapor is twice as dense as air. When exposed to an open atmosphere, the vapor can accumulate in low-lying areas while spreading out over long distances and reach ignition source, causing flashback or an explosion.[25][28] When heated, propylene oxide can rapidly self-polymerize and decompose producing other toxic gases such as carbon monoxide and various free radicals.[29][25] Propylene oxide fires are especially dangerous and difficult for firefighters to extinguish. In a fire, sealed tanks of propylene oxide should be cooled with fire hoses to prevent explosion from self-polymerization.[25] When burning in open air however, water can transport propylene oxide outside of the fire zone which can reignite upon floating to the surface. Additional firefighting measures should be taken to prevent propylene oxide from washing out to nearby drains and sewers contaminating the surrounding environment.[30][25]

Natural occurrence

[edit]

In 2016 it was reported that propylene oxide was detected in Sagittarius B2, a cloud of gas in the Milky Way weighing three million solar masses. It is the first chiral molecule to be detected in space, albeit with no enantiomeric excess.[31]

References

[edit]

Cited sources

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Propylene oxide

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from Grokipedia
Propylene oxide, also known as 1,2-epoxypropane or methyloxirane, is an with the C₃H₆O and a molecular weight of 58.08 g/mol. It is a colorless, volatile with an ethereal , characterized by a three-membered ring structure featuring a attached to one of the carbon atoms. Physically, it has a of 34.2 °C, a of -112 °C, and a density of 0.859 g/cm³ at 20 °C, making it highly flammable with a of -37 °C. Propylene oxide is primarily produced industrially through three main processes: the chlorohydrin process, which involves the reaction of with followed by treatment with a base like calcium or ; the indirect oxidation (peroxidation) process using and an organic ; and the process, a more recent direct oxidation method. In the United States, annual production has historically been significant, with estimates exceeding 1.7 billion pounds in the early 1980s; as of 2023, it exceeds 3.5 billion pounds annually, though emissions have decreased over time due to improved controls. Propylene oxide does not occur naturally on and is produced almost entirely synthetically. As a key chemical intermediate, propylene oxide is predominantly used in the manufacture of polyether polyols (about 70% of usage), which serve as building blocks for foams, elastomers, and coatings; propylene glycols (20%), employed in , pharmaceuticals, and food additives; and (10%), utilized in solvents and cleaners. It also functions as a fumigant for foodstuffs like dried fruits and nuts, as well as for medical equipment sterilization, and finds minor applications in herbicides, lubricants, , and detergents. Globally, it supports diverse industries including construction, automotive, and . Propylene oxide exhibits notable toxicity, acting as an irritant to the eyes, , and , with potential for at high exposures. It is classified as a possible () based on sufficient evidence of tumors in experimental animals, such as forestomach and cancers in , though evidence in humans is inadequate. Environmentally, it persists moderately in air ( 3-20 days) and (11.6 days at 7), with low to aquatic life. Regulatory limits include an OSHA of 100 ppm and a TLV of 2 ppm, reflecting its hazardous air pollutant status under U.S. EPA guidelines.

Properties

Physical properties

Propylene oxide has the molecular formula C₃H₆O and a consisting of a three-membered chiral ring with a attached to one of the carbon atoms, specifically as 2-methyloxirane. It appears as a clear, colorless volatile with an ethereal . Key physical properties of propylene oxide are summarized in the following table:
PropertyValueConditions
34.2 °C (93.6 °F)-
−112.1 °C (−169.8 °F)-
0.830 g/cm³20 °C
−37 °C (−35 °F)-
59 kPa20 °C
Solubility in 410 g/L (41 wt%)20 °C
The is 1.366 at 20 °C, and the dynamic is 0.28 mPa·s at 25 °C. Thermodynamic properties include a of of 28.3 kJ/mol and a of approximately 2.14 J/g·K.

Chemical properties

Propylene oxide is an unsymmetrical characterized by a three-membered ring in which an oxygen atom is bonded to a and a methyl-substituted carbon, imparting significant due to the compressed bond angles of approximately 60°—far from the tetrahedral ideal of 109.5°—and a total of about 27 kcal/mol. The molecule features a chiral center at the substituted ring carbon, existing as (R)- and (S)-enantiomers, though commercial propylene oxide is produced as a . Pure enantiomers exhibit specific rotations of [α]D +13.5° for the (R)-form and -13.5° for the (S)-form when measured neat. As a cyclic , propylene oxide displays weak basicity, with the pKa of its protonated conjugate acid around -2.4, attributable to the on the strained ring oxygen that facilitates but resists . Characteristic spectroscopic features confirm the epoxide functionality: reveals a prominent absorption at approximately 1250 cm-1 due to the symmetric ring breathing mode. In 1H NMR, the methyl protons appear at δ 1.3 ppm, while the ring methylene and methine protons resonate between δ 2.5 and 2.8 ppm. The 13C NMR spectrum shows the methyl carbon at δ 19 ppm and the ring carbons at δ 45 ppm (methylene) and δ 51 ppm (methine). Propylene oxide maintains thermal stability up to 200°C under neutral conditions but is prone to exothermic when exposed to acidic or basic catalysts, driven by the relief of .

Production

Chlorohydrin route

The chlorohydrin route represents the traditional industrial method for synthesizing propylene oxide, developed in the early and widely adopted due to its reliance on readily available . In this process, propylene reacts with (generated in situ from and ) to form a of chlorohydrin isomers, primarily 1-chloro-2-propanol. This intermediate then undergoes dehydrochlorination using (lime) as the base, yielding propylene oxide along with as a byproduct. The overall process operates in aqueous phases, requiring multiple separation steps to isolate the product from salt-laden effluents. The key reactions are as follows: First, chlorohydrination: \ceC3H6+HOCl−>C3H7OCl\ce{C3H6 + HOCl -> C3H7OCl} Followed by saponification: \ceC3H7OCl+Ca(OH)2−>C3H6O+CaCl2+H2O\ce{C3H7OCl + Ca(OH)2 -> C3H6O + CaCl2 + H2O} These steps typically achieve overall yields of 90-95% based on propylene, though the process is energy-intensive owing to the need for chlorine production, distillation, and wastewater treatment. Economically, it co-produces approximately 2 tons of calcium chloride salt per ton of propylene oxide, posing significant disposal challenges in regions without integrated salt utilization. Historically, the chlorohydrin route dominated global production, accounting for about 45% of propylene oxide capacity in the mid-2000s, particularly in areas with low-cost chlorine access such as parts of Asia and North America. As of 2025, it accounts for approximately 33-46% of global capacity, though its share has declined with the rise of less wasteful alternatives; it remains viable where chlorine is inexpensive, representing around 35-45% of output in certain markets. Key implementations include Dow Chemical's facility in Freeport, Texas, which integrated chlor-alkali production for feedstock supply until its scheduled closure by the end of 2025. Post-2010 optimizations by major operators, such as enhanced brine recycling and reduced wastewater volumes, have improved environmental performance while maintaining competitiveness.

Copoxidation route

The copoxidation route, also known as the hydroperoxide process, involves the epoxidation of using organic s such as tert-butyl hydroperoxide or ethylbenzene hydroperoxide, catalyzed by soluble molybdenum compounds. In this method, reacts with the to form propylene oxide and the corresponding alcohol as a , typically tert-butanol or , respectively. The general reaction is represented as: C3H6+ROOH→C3H6O+ROH\mathrm{C_3H_6 + ROOH \rightarrow C_3H_6O + ROH} where R denotes tert-butyl or 1-phenylethyl groups. This process was introduced in 1969 through a collaboration between Halcon International and Chemical, marking a significant advancement in propylene oxide synthesis by integrating the production of valuable coproducts. As of 2025, copoxidation routes account for approximately 40% of global propylene oxide capacity, combining the propylene oxide/styrene monomer (PO/SM), propylene oxide/ (PO/TBA), and propylene oxide/ (PO/Cumene) variants. Compared to the chlorohydrin route, the copoxidation process offers higher , minimizing waste by avoiding salt byproducts and generating marketable alcohols used as solvents, fuels, or chemical intermediates. Major industrial implementations include LyondellBasell's PO/SM technology, which coproduces styrene, and Shell's PO/TBA process, which yields tert-butanol. Recent expansions in , such as Tianjin Petrochemical's 150,000-ton/year cumene hydroperoxide-based plant commissioned in late 2023, underscore ongoing growth in this route.

Hydrogen peroxide route

The hydrogen peroxide to propylene oxide (HPPO) process represents a direct epoxidation method for producing propylene oxide from and , catalyzed by titanium-silicalite-1 (TS-1), a zeolite-like material, yielding only as a . The reaction proceeds as follows: C3H6+H2O2→C3H6O+H2O\mathrm{C_3H_6 + H_2O_2 \rightarrow C_3H_6O + H_2O} This process operates in fixed-bed reactors under mild conditions (typically temperatures below 100 °C and pressures around 30 bar), with dissolved in as a to facilitate the reaction, achieving high propylene conversion and selectivity. The BASF/Dow process employs a tubular fixed-bed reactor with TS-1 zeolite catalyst, often in a two-step configuration (main reactor achieving approximately 90% hydrogen peroxide conversion and a finishing reactor to exceed 99% conversion). The Evonik process (licensed with thyssenkrupp Uhde) uses a shell-and-tube fixed-bed reactor with TS-1 catalyst to effectively manage the exothermic reaction. Developed jointly by Dow and , the HPPO process was first commercialized in with a 300,000 metric tons per year plant at 's facility in , , marking a significant advancement in sustainable propylene oxide production. As of 2025, HPPO accounts for approximately 33% of global propylene oxide capacity, reflecting its rapid adoption due to environmental advantages over traditional routes. Key advantages of the HPPO process include hydrogen peroxide selectivity exceeding 99%, minimizing waste generation and eliminating salt or organic coproducts common in other methods, which reduces by 70-80% and by 35% compared to the chlorohydrin route. It also integrates seamlessly with on-site hydrogen peroxide production via the , enhancing overall efficiency and reducing transportation needs. Recent developments underscore HPPO's growth, with Evonik and Uhde licensing technology for multiple plants in , including the world's first industrial-scale facility in , (2008, 100,000 tons per year), a 300,000 tons per year plant in , (2014), and a new complex in , (2023, 240,000 tons per year). While bio-based pilots are emerging in to further green the process, no large-scale integrations with HPPO have been commercialized as of 2025.

Reactions

Ring-opening reactions

Propylene oxide, as an unsymmetrical , exhibits characteristic ring-opening reactivity driven by the strain in its three-membered ring. Under basic conditions, nucleophiles attack the less substituted (terminal) carbon via an SN2-like mechanism, resulting in inversion of configuration at that site and formation of the anti addition product. This arises because the minimizes steric hindrance at the primary carbon. In acidic conditions, of the oxygen enhances the electrophilicity of the epoxide, shifting the nucleophilic attack to the more substituted (secondary) carbon, which develops partial positive charge more readily, akin to an SN1 pathway with neighboring-group participation. These mechanistic differences allow selective control over product regiochemistry in synthetic transformations. A prominent example is the hydrolysis of propylene oxide to produce propylene glycol (1,2-propanediol), typically conducted under acidic conditions to favor attack at the secondary carbon. The reaction proceeds as follows: \ceC3H6O+H2O−>[H+]CH3CH(OH)CH2OH\ce{C3H6O + H2O ->[H+] CH3CH(OH)CH2OH} Industrially, this process uses excess water and achieves yields of over 90%, with the primary product being the 1,2-diol and minor higher glycols as byproducts. Alcoholysis involves the reaction of propylene oxide with alcohols in the presence of base, such as NaOH, yielding β-hydroxy ethers. For instance, with methanol, the nucleophilic attack occurs at the less substituted carbon under basic conditions, producing 1-methoxy-2-propanol as the major isomer. The general reaction is: \ceC3H6O+ROH−>[base]CH3CH(OH)CH2OR\ce{C3H6O + ROH ->[base] CH3CH(OH)CH2OR} This transformation is catalyzed efficiently by solid bases like MgO, with mechanisms involving surface-bound alkoxide intermediates. Aminolysis reactions with amines, including ammonia, lead to β-amino alcohols. Reaction with ammonia under controlled conditions yields isopropanolamine (1-amino-2-propanol) via nucleophilic ring opening at the terminal carbon. These processes are sequential, producing mono-, di-, and tri-substituted products depending on amine excess and conditions. Anionic polymerization of propylene oxide, initiated by (KOH), produces polyether polyols used in foams. The mechanism begins with of an initiator alcohol (e.g., or ) by OH⁻ to form an , which attacks the less substituted carbon of propylene oxide in an SN2 manner, opening the ring and generating a new chain end. Propagation occurs through repeated nucleophilic attacks by this on additional units, leading to chain growth with head-to-tail linkages and secondary hydroxyl end groups. Termination can involve or end-capping agents, yielding polymers with molecular weights controlled by monomer-to-initiator ratio. This process typically operates at 70–120 °C in bulk, with KOH providing high activity but potential for side reactions like .

Rearrangement and oxidation reactions

Propylene oxide can undergo thermal rearrangement in the gas phase at temperatures between 250 and 300°C when passed over alumina catalysts, yielding as the major product and as a minor product. This process exploits the inherent in the structure to facilitate skeletal without nucleophilic addition. The primary reaction pathway is depicted by the equation: \ceC3H6O−>[Al2O3][250−300∘C]CH2=CHCH2OH\ce{C3H6O ->[Al2O3][250-300^\circ C] CH2=CHCH2OH} Selectivity toward exceeds 60-80% under optimized conditions, with the minor formation arising from competing hydrogen migration pathways. Acid-catalyzed of propylene oxide proceeds via of the epoxide oxygen by protic acids (H⁺) or coordination with Lewis acids, leading to ring opening and subsequent 1,2-hydride shift to form either acetone or . Acetone is favored on stronger acid sites due to migration from the substituted carbon, while predominates on milder acidic surfaces through terminal carbon involvement. These transformations occur at lower temperatures (100-200°C) compared to thermal routes and are mediated by catalysts such as metal phosphates or zeolites, with quantum chemical calculations confirming single-transition-state mechanisms for each product. Hydrogenolysis of propylene oxide over catalysts, such as or supported Ni variants, effects reductive ring opening to yield primarily , with minor 2-propanol depending on reaction conditions like and (typically 100-200°C). The surface promotes selective cleavage at the less substituted C-O bond, favoring anti-Markovnikov alcohol formation, and this method offers high compared to platinum-based systems. Photochemical reactions of propylene oxide under UV (e.g., 185 nm) induce ring opening via radical intermediates, generating propanal and acetone as primary products through C-O bond homolysis and subsequent abstraction. However, radical yields remain low due to rapid recombination and competing channels, limiting practical applications. Reactive force field simulations confirm the radical-mediated pathways but highlight inefficient quantum yields under standard conditions. In laboratory settings, small-scale conversions of propylene oxide to aldehydes, such as propionaldehyde, are achieved via controlled isomerization over chromia-tungstia catalysts at mild temperatures, providing a route for synthetic intermediates without large-scale optimization. These niche transformations emphasize the epoxide's versatility for targeted organic synthesis.

Uses

Major industrial uses

Propylene oxide (PO) is predominantly utilized in the production of polyether polyols, which account for approximately 60-70% of global PO consumption. These polyols are synthesized through the ring-opening reaction of PO with water or glycols, serving as key building blocks for polyurethane products such as flexible and rigid foams used in furniture, bedding, insulation, and automotive components, as well as adhesives and sealants. Global consumption of polyether polyols reached over 9.8 million metric tons in 2023. A significant portion, about 20-30% of PO production, is directed toward propylene glycol (PG) via hydrolysis processes that yield high-purity products suitable for diverse applications. PG finds widespread use as an antifreeze in automotive coolants, a humectant and solvent in food additives, and a carrier in pharmaceuticals, with USP-grade variants produced under stringent purification to meet regulatory standards for medical and consumable uses. Annual global production of PG approximates 2 million tons as of recent estimates. Propylene glycol ethers, formed by the etherification of PO with alcohols, represent another major industrial outlet, serving as versatile solvents in formulations for paints, coatings, and industrial cleaners. Examples include monomethyl ether, valued for its low and effective solvency in water-based systems. The overall PO market was valued at approximately USD 22 billion in 2024, with demand expected to grow at a (CAGR) of around 5% through 2033, fueled by rising needs in for insulation materials and automotive for lightweight components.

Niche and specialized uses

Propylene oxide serves as an EPA-approved fumigant for sterilizing raw almonds and other nuts, with approval for post-harvest treatment established in the mid-2000s to reduce microbial contamination such as , yeasts, and molds. The process involves exposing nutmeats to propylene oxide vapor, which penetrates shells and kills and pathogens effectively due to its volatility, leaving residues below the EPA tolerance of 10 ppm for tree nuts and processed nutmeats, often dissipating to less than 1 ppm shortly after treatment. In electron microscopy, propylene oxide acts as a transitional during tissue preparation, facilitating after fixation and enabling infiltration to preserve cellular for high-resolution imaging. It is typically employed in graded concentrations, including 1-3% solutions in protocols to stabilize samples without distorting fine details, replacing more hazardous alternatives in some modern workflows. Propylene oxide is utilized as a fuel component in thermobaric weapons, such as , where it aerosolizes to create a high-pressure enhanced by its rapid properties. The U.S. has incorporated such systems since the , leveraging propylene oxide's ability to form vapor clouds in munitions for increased destructive radius. (Note: The 2023 Fuel journal article references historical military applications.) Beyond these, propylene oxide functions as a sterilant for medical devices, applied in gas-phase processes to eliminate microorganisms on heat-sensitive equipment like immunoadsorbents without residue buildup. It also serves as an intermediate in synthesizing nonionic , such as alcohol ethoxylates modified with propylene oxide segments to improve solubility and performance in formulations. Additionally, it acts as a additive to boost oxygen content and power output by up to 8%, though its use is restricted in many sanctioned events due to regulatory prohibitions on performance enhancers. In recent developments, bio-based propylene oxide derived from renewable feedstocks has entered pilot-scale production in the for applications in sustainable polymers, including polyols for foams, with market projections indicating growth from USD 0.8 billion in 2024 to USD 2.1 billion by 2034 driven by environmental demands.

Safety and health effects

Toxicity and carcinogenicity

Propylene oxide is an irritant to the eyes, skin, and upon acute exposure, causing symptoms such as redness, tearing, coughing, and in severe cases. In , the 4-hour LC50 in rats is approximately 4000 ppm, while the oral LD50 in rats ranges from 382 to 587 mg/kg body weight. Chronic exposure to propylene oxide leads to genotoxic effects through its action as a direct alkylating agent, forming adducts with DNA nucleophiles such as guanine, which can result in mutations. It is classified as a Group 2B carcinogen (possibly carcinogenic to humans) by the International Agency for Research on Cancer, based on sufficient evidence from animal studies but inadequate evidence in humans. In rodents, inhalation exposure at 500 ppm induced forestomach tumors, primarily squamous-cell carcinomas, while lower levels around 300 ppm were associated with increased mammary gland tumors in female rats; subcutaneous or oral administration also produced local tumors at the site of contact. Although no clear causal link to cancer has been established in humans, limited occupational studies suggest a potential association with leukemia, though confounding factors like co-exposure to other chemicals limit conclusions. Occupational exposure limits for propylene oxide include an OSHA of 100 ppm as an 8-hour time-weighted average; NIOSH classifies it as a potential occupational with no numerical REL, recommending reduction to the lowest feasible level; and an ACGIH TLV of 2 ppm as an 8-hour TWA, reflecting its carcinogenic potential. , propylene oxide is rapidly metabolized primarily to , a less toxic compound, via and conjugation, which mitigates some systemic effects. Regarding reproductive and developmental toxicity, animal studies in rats showed no adverse effects on fetuses at exposure levels up to 300 ppm, but increased fetal resorptions occurred at 500 ppm alongside maternal toxicity; assays indicate weak mutagenic potential, though no strong evidence of heritable genetic damage exists.

Flammability and exposure risks

Propylene oxide is classified as a Class IA flammable liquid due to its low flash point of -35°F (-37°C) and boiling point below 100°F (38°C), making it highly susceptible to ignition. Its autoignition temperature is 465°C (869°F), and it forms explosive vapor-air mixtures over a wide range of 2.3-36% by volume. The National Fire Protection Association (NFPA) rates it as 3 for health hazards, 4 for flammability, and 2 for instability, indicating severe fire risks and potential for violent polymerization under heat or contamination. Vapors of propylene oxide are heavier than air and can travel considerable distances to ignition sources, leading to flash fires or explosions; additionally, static electricity poses a significant ignition during transfer and handling operations. To mitigate these hazards, storage should occur in cool, well-ventilated areas under an inert blanket to prevent , using explosion-proof equipment and grounded containers to eliminate static discharge. Non-sparking tools and bonding/grounding procedures are essential during handling to avoid sparks. Primary exposure routes for propylene oxide are of vapors and dermal absorption through contact, as it is readily absorbed and can cause acute irritation or burns. (PPE) includes full-face respirators with organic vapor cartridges or for airborne exposures above permissible limits, along with chemical-resistant suits made of or Teflon, gloves, and to prevent and . A notable historical incident occurred on July 5, 1990, at the Chemical plant in , where a runaway reaction in a propylene oxide production unit led to an explosion that killed 17 workers and injured five others, highlighting the dangers of inadequate temperature control and contaminant management.

Environmental aspects

Emissions and persistence

Propylene oxide is released into the environment primarily through emissions during its industrial production and use, with significant contributions from the chlorohydrin process where it appears in streams alongside chlorinated byproducts. emissions, arising from leaks in equipment such as valves, pumps, and storage tanks, are estimated at 0.1–1 kg per metric ton of propylene oxide produced, based on industry emission factors for organic chemicals in petrochemical facilities. In the chlorohydrin route, which accounts for a substantial portion of global production, releases additional propylene oxide, contributing to aquatic discharges. As of 2024, global production was approximately 10.1 million metric tons, with projections for growth to 10.6 million in 2025. Annual environmental releases are estimated at 0.1–1 kg per metric ton produced, leading to totals in the thousands of metric tons globally, though actual amounts have decreased due to improved controls. In the atmosphere, propylene oxide exists predominantly in the vapor phase due to its volatility and undergoes degradation mainly through reaction with photochemically produced hydroxyl (OH) radicals, with an estimated half-life of 10–20 days under typical tropospheric conditions. Photolysis contributes to its breakdown, yielding products such as formaldehyde and methyl radicals, though this pathway is secondary to OH-initiated oxidation. The primary degradation mechanism involves ring-opening by OH, leading to oxygenated intermediates that further react to form compounds like acetaldehyde and peroxyacetyl nitrate (PAN). These processes limit long-range atmospheric transport, as the compound does not persist beyond regional scales. In aqueous environments such as water bodies and soil pore water, propylene oxide hydrolyzes rapidly to propylene glycol via acid- or base-catalyzed ring-opening, with a half-life of approximately 11.6 days at 7 and 25°C; this rate accelerates at more extreme values, dropping to about 6–12 days. The hydrolysis product, , is biodegradable and non-toxic at environmental concentrations. In soil, propylene oxide exhibits high mobility due to its low adsorption potential but degrades similarly through and microbial activity. Its octanol-water partition coefficient (log Kow) of 0.03 indicates negligible potential in aquatic organisms, as it preferentially remains in the water phase rather than partitioning into . Overall, propylene oxide demonstrates low environmental persistence owing to its reactivity and volatility, characterized by a Henry's law constant of approximately 7 × 10^{-5} atm·m³/mol, which facilitates rapid evaporation from water surfaces. It does not meet criteria for persistent, bioaccumulative, and toxic (PBT) substances, as its degradation half-lives are short and bioaccumulation factor is minimal. Recent shifts toward the hydrogen peroxide-based propylene oxide (HPPO) process have reduced emissions of chlorinated byproducts and wastewater volumes by 70–80% compared to traditional routes, improving local air and water quality. A October 2025 report faulted the EPA for underassessing risks in Cancer Alley, highlighting volatile organic compound concerns near petrochemical plants including those producing epoxides like propylene oxide, prompting enhanced monitoring efforts.

Regulations and sustainability

Propylene oxide is regulated under the U.S. Agency's Toxic Substances Control Act (TSCA) as an active subject to testing requirements and evaluations. It is also listed as a hazardous air (HAP) under the Clean Air Act, where emissions are controlled to limit volatile organic compounds (VOCs) from industrial sources, including wastewater and equipment leaks in production facilities. In 2025, the EPA received a to establish tolerances for residues of propylene oxide on various dried and spices, aiming to set permissible levels for its use as a fumigant in . The EPA has classified propylene oxide as a probable (Group B2), influencing its regulatory scrutiny for environmental and s. In the , emission controls for propylene oxide production emphasize best available techniques () to minimize waste gas releases, with typical post-treatment emission levels for propylene oxide maintained below 0.5 mg/m³ in modern plants. reference documents require integrated management of diffuse VOC emissions and to reduce overall environmental impact from chemical manufacturing. Under the Clean Air Act in the U.S., VOC emission standards apply to synthetic organic chemical manufacturing, including controls on storage, handling, and flaring to prevent releases during propylene oxide synthesis. Sustainability efforts in propylene oxide production are shifting toward low-carbon methods, such as the to propylene oxide (HPPO) process using green derived from renewable energy sources, which produces only as a and significantly cuts compared to traditional routes. Bio-based es utilizing bio-propylene from renewable feedstocks like or bio-ethanol are also emerging to lower the of the . The global market for low-carbon propylene oxide is projected to grow at a (CAGR) of 6.3% through 2034, driven by demand for eco-friendly alternatives in polyurethanes and glycols. Global initiatives include the EU Emissions Trading System (ETS), which mandates a 62% reduction in covered emissions by 2030 compared to 2005 levels, applying to propylene oxide facilities through carbon pricing on releases. Major producers like have pledged carbon neutrality by 2050, with interim targets for a 42% absolute reduction in Scope 1 and 2 emissions by 2030, supporting broader industry transitions via renewable energy integration. Challenges persist with legacy waste from the chlorohydrin process, which generates substantial calcium chloride solids and polluted , complicating remediation in older sites. Additionally, proposed 2025 EPA rollbacks on chemical risk assessment procedures and petrochemical emission standards are under review, potentially easing controls on VOCs and HAPs from production.

Occurrence

Extraterrestrial detection

Propylene oxide (CH₃CHCH₂O) was first detected in the interstellar medium in 2016 toward the Sagittarius B2 North (Sgr B2(N)) molecular cloud complex near the Galactic center, marking the initial identification of a chiral molecule in space. The detection was achieved through radio observations using the Green Bank Telescope (GBT) in West Virginia and the Parkes radio telescope in Australia, revealing absorption features from low-energy rotational transitions of the molecule. These transitions, observed primarily in the 12–14 GHz frequency range (e.g., the 2_{1,1}–2_{0,2} line at approximately 12.8 GHz), indicated a cold molecular environment with an excitation temperature of about 5 K. The derived column density was approximately 1 × 10^{13} cm^{-2}, consistent with a low-abundance species in an extended, quiescent envelope surrounding the region's massive protostellar clusters. Spectral analysis confirmed the presence of both enantiomers—(R)-propylene oxide and (S)-propylene oxide—in equal proportions, forming a with no detectable enantiomeric excess. This observation was supported by laboratory spectra of the individual enantiomers, which showed indistinguishable rotational signatures under the telescope's resolution, precluding differentiation of any potential . Proposed formation pathways for interstellar propylene oxide include gas-phase ion-molecule reactions, such as the addition of hydroxyl (OH) radicals to propene (C₃H₆), though kinetic barriers and low efficiencies in cold environments limit this route's viability. More favorable mechanisms involve ice-mantle chemistry on dust grains, where suprathermal atomic oxygen atoms react with adsorbed propene in interstellar ices, potentially driven by cosmic ray-induced or UV photolysis. These surface reactions yield racemic propylene oxide, aligning with the observed equal distribution and offering insights into prebiotic emergence. The detection holds significant implications for , as propylene oxide represents a complex organic capable of forming in non-terrestrial conditions, bridging simple interstellar chemistry to potential of life's building blocks. However, the lack of enantiomeric excess challenges hypotheses of cosmic origins for biomolecular , suggesting that any preferential in life's molecules may arise later, perhaps on planetary surfaces or via amplification mechanisms. Follow-up observations in the , including targeted searches in cold dark clouds like TMC-1, have yielded negative results for propylene oxide, consistent with its preferential association with warmer, more chemically active regions like Sgr B2(N). Astrochemical models predict fractional abundances of propylene oxide relative to H₂ on the order of 10^{-11} in such environments, aiding in the interpretation of non-detections and guiding future surveys for chiral species.

Terrestrial traces

Propylene oxide is not known to occur as a and has no known biogenic sources on . It forms transiently in the atmosphere through the oxidation of hydrocarbons during processes, such as in exhaust and other burning activities. Trace detections of propylene oxide have been reported in urban air at low concentrations attributable to emissions from exhaust and industrial activities. In tobacco smoke, concentrations range from 0.65 to 0.93 µg per in reference cigarettes analyzed by gas chromatography-mass spectrometry. In food, minor residues of propylene oxide occur in some spices, processed nuts, and cocoa powder following treatments, though it is not endogenous to these products. It degrades rapidly to in moist environments. In the United States, regulatory tolerances for residues of the fumigant propylene oxide (measured as propylene oxide) are set at 300 ppm for nutmeats (except ) and dried spices (group 19), and 200 ppm for cocoa powder (cacao bean). Environmentally, propylene oxide has been detected in groundwater near industrial facilities at concentrations below 1 µg/L, where it degrades quickly via with a of approximately 12 days at neutral pH. No significant natural reservoirs exist, and it is absent in pristine ecosystems, such as remote forests and oceans, underscoring its anthropogenic origin.

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