Propanediol
Propanediol is an organic compound with the formula C₃H₈O₂ that exists as several isomers, the most commercially significant being 1,2-propanediol (propylene glycol) and 1,3-propanediol.[1] 1,3-Propanediol (1,3-PDO), with the chemical formula C₃H₈O₂, is a straight-chain organic diol consisting of a three-carbon backbone with hydroxyl groups attached to the first and third carbons.[2] It appears as a clear, colorless, and odorless viscous liquid that is fully miscible with water, alcohols, ethers, formamide, and chloroform, but insoluble in hydrocarbons like benzene.[3] This compound has a molecular weight of 76.09 g/mol, a boiling point of 214 °C, a melting point of -27 °C, and a density of 1.05 g/cm³ at 25 °C, making it suitable for applications requiring thermal stability and solubility.[4][2] Commercially, 1,3-propanediol is produced through both chemical and biological routes, with the latter gaining prominence for its renewability. Chemical synthesis traditionally involves the hydration of acrolein or hydroformylation of ethylene oxide, derived from petrochemical feedstocks.[5] In contrast, biological production utilizes microbial fermentation of glycerol—a byproduct of biodiesel—or glucose, employing engineered bacteria such as Escherichia coli or Klebsiella pneumoniae to achieve high yields under mild conditions, often exceeding 100 g/L in optimized processes.[6][7] This bio-based approach reduces reliance on fossil fuels and supports sustainable manufacturing, with global production capacity reaching tens of thousands of tons annually as of 2024.[8] The compound's versatility stems from its bifunctional hydroxyl groups, enabling it to serve as a key building block in polymer chemistry. It is primarily used as a monomer in the synthesis of polytrimethylene terephthalate (PTT), a polyester valued for its elasticity in carpets, textiles, and engineering plastics, as well as in polyurethanes, adhesives, and coatings.[5] Additional applications include antifreeze and coolant formulations due to its low toxicity and high boiling point, and as a humectant, solvent, and penetration enhancer in cosmetics, pharmaceuticals, and personal care products, where it provides moisturizing effects without irritation.[3][9] Unlike its isomer 1,2-propanediol (propylene glycol), 1,3-propanediol offers superior performance in certain polymer applications and is generally recognized as safe for broader industrial and consumer uses.[10]Nomenclature
IUPAC and common names
Propanediols constitute a group of organic compounds with the general molecular formula C3H8O2, characterized by two hydroxyl groups attached to a propane backbone.[2] In IUPAC nomenclature, these diols are systematically named as derivatives of propane, with the suffix "-diol" appended to indicate the presence of two hydroxy groups, and locants specifying their positions on the carbon chain; for example, the compound with hydroxyl groups on carbons 1 and 2 is designated propane-1,2-diol.[11] This convention follows the broader IUPAC recommendations for polyols, where the parent hydrocarbon chain is numbered to give the lowest possible locants to the functional groups.[11] Common names for propanediols often reflect historical or industrial usage. The 1,2-isomer is widely known as propylene glycol, a term derived from its relation to propylene oxide in synthesis.[12] The 1,3-isomer is commonly referred to as trimethylene glycol or, in industrial contexts, 1,3-PDO, where "PDO" serves as a standard abbreviation for propanediols, particularly the 1,3 variant.[13] Geminal propanediols, featuring both hydroxyl groups on the same carbon, are named propane-1,1-diol and propane-2,2-diol.[1][14] The suffix "glycol" in common names traces its origin to ethylene glycol (ethane-1,2-diol), coined by French chemist Charles-Adolphe Wurtz in 1856 to describe its sweet taste and properties intermediate between glycerol and ethanol; this nomenclature has since extended to vicinal diols like propylene glycol.[15][16]Isomer classification
Propanediol encompasses four constitutional isomers of the molecular formula C₃H₈O₂, distinguished by the positions of the two hydroxyl groups on the propane backbone. These isomers are 1,2-propanediol (CH₃CH(OH)CH₂OH), 1,3-propanediol (HOCH₂CH₂CH₂OH), 1,1-propanediol (CH₃CH₂CH(OH)₂), and 2,2-propanediol ((CH₃)₂C(OH)₂).[17] The isomers are classified into two main categories based on the relative positions of the hydroxyl groups: non-geminal diols and geminal diols. Non-geminal diols include 1,2-propanediol and 1,3-propanediol, where the hydroxyl groups are attached to different carbon atoms; 1,2-propanediol is specifically a vicinal diol with hydroxyls on adjacent carbons, while 1,3-propanediol has them separated by one carbon.[17][18] In contrast, geminal diols, such as 1,1-propanediol and 2,2-propanediol, feature both hydroxyl groups on the same carbon atom and are generally unstable, serving as transient hydrates of the corresponding carbonyl compounds (propanal for 1,1-propanediol and acetone for 2,2-propanediol).[17]/Aldehydes_and_Ketones/Reactivity_of_Aldehydes_and_Ketones/Addition_of_Water_to_form_Hydrates_(Gem-Diols)) Among these, 1,2-propanediol exhibits stereochemistry due to a chiral center at the carbon bearing one hydroxyl group (carbon 2), resulting in two enantiomers: (R)-1,2-propanediol and (S)-1,2-propanediol.[17] The other isomers lack chirality in their standard forms. Only the non-geminal diols, 1,2-propanediol and 1,3-propanediol, hold industrial significance as versatile chemical intermediates and solvents, whereas the geminal diols are not commercially produced due to their instability.[19][20][5]1,2-Propanediol
Physical properties
1,2-Propanediol, also known as propylene glycol, is a colorless, odorless, and viscous liquid at room temperature. It is hygroscopic, meaning it readily absorbs moisture from the air.[12][21] The compound has a low melting point of -59 °C and a high boiling point of 188 °C at standard pressure, indicating stability across a wide temperature range. Its density is approximately 1.036 g/mL at 25 °C, and the refractive index is 1.432 at 20 °C. 1,2-Propanediol exhibits low vapor pressure, contributing to its low volatility.[12] It is fully miscible with water, ethanol, acetone, and chloroform, reflecting its polar nature due to the hydroxyl groups. Solubility decreases in non-polar solvents such as ether and benzene. The dynamic viscosity is around 0.042 Pa·s at 25 °C, underscoring its syrupy consistency.[12][22][21]| Property | Value | Conditions | Source |
|---|---|---|---|
| Molecular formula | C₃H₈O₂ | - | PubChem |
| Molecular weight | 76.09 g/mol | - | PubChem |
| Appearance | Colorless viscous liquid | Room temperature | Sigma-Aldrich |
| Melting point | -59 °C | - | PubChem |
| Boiling point | 188 °C | 101.3 kPa | PubChem |
| Density | 1.036 g/mL | 25 °C | Sigma-Aldrich |
| Refractive index | 1.432 | 20 °C | PubChem |
| Solubility in water | Miscible | - | PubChem |
Chemical properties
1,2-Propanediol, with the molecular formula C₃H₈O₂ and systematic name propane-1,2-diol, features a three-carbon chain bearing hydroxyl groups on adjacent carbons: a primary alcohol at C1 and a secondary alcohol at C2 (CH₃CH(OH)CH₂OH). This vicinal diol structure imparts characteristic reactivity typical of polyols, including hydrogen bonding capabilities that contribute to its hygroscopic nature. The compound is chemically stable under standard ambient conditions (room temperature and pressure), showing no significant decomposition when stored properly, though it is incompatible with strong oxidizing agents, acid chlorides, acid anhydrides, and reducing agents, which can lead to exothermic reactions or degradation.[12][23][24] The hydroxyl groups exhibit weak acidity, with a predicted pKa of approximately 14.5, consistent with aliphatic alcohols, indicating low tendency to deprotonate under neutral conditions but enabling reactions under basic catalysis. In terms of reactivity, 1,2-propanediol undergoes esterification with carboxylic acids or their derivatives to form mono- or diesters, often requiring acid catalysts like heteropolyacids for selectivity toward the primary hydroxyl group. For instance, esterification with propanoic acid yields propyl propanoate derivatives, useful in surfactant synthesis. Oxidative reactions are prominent due to the diol functionality; mild oxidation with supported gold-platinum catalysts selectively converts it to lactic acid under alkaline conditions, while stronger oxidants like periodate cleave the C2-C3 bond to produce formaldehyde and acetaldehyde.[24][25][26] Further reactivity includes dehydration to form propylene oxide or allyl alcohol under acidic conditions, and catalytic hydrogenolysis in the presence of metals like ruthenium to yield propanol or propane. In homogeneous oxidation systems, such as with silver(II) or palladium catalysts, it produces hydroxyacetone or methylglyoxal via selective dehydrogenation of the secondary alcohol. These reactions highlight 1,2-propanediol's versatility as a building block in organic synthesis, though its stability limits unintended reactivity in most industrial applications.[27][28][29]Production
The primary industrial production of 1,2-propanediol, also known as propylene glycol, occurs through the hydrolysis of propylene oxide, a petrochemical intermediate derived from propylene. Propylene oxide is synthesized via several established processes, including the chlorohydrin method, which involves reacting propylene with hypochlorous acid to form chlorohydrin intermediates followed by dehydrochlorination, and the hydroperoxide routes such as the styrene monomer process (SMPO), cumene hydroperoxide process, and tert-butyl hydroperoxide process. These methods collectively account for the majority of global propylene oxide capacity, with the chlorohydrin process historically dominant due to its simplicity, though hydroperoxide routes have gained prevalence for their higher efficiency and lower environmental impact.[30] In the hydrolysis step, propylene oxide reacts with water under acidic or basic conditions, typically in a non-catalytic liquid-phase process using excess water (10-20:1 molar ratio) at temperatures of 100-150°C and pressures up to 10 bar to achieve high yields (over 90%) while minimizing byproducts like dipropylene glycol. The reaction proceeds via ring-opening of the epoxide, yielding 1,2-propanediol as the main product, followed by distillation to purify the diol from water and impurities. This integrated route from propylene to 1,2-propanediol has been the cornerstone of industrial production since the mid-20th century, with global production volume estimated at approximately 4.4 million tons annually as of 2025. In 2024, Dow expanded its propylene glycol production capacity in Thailand to 250,000 tons per year.[31][20][32][33] Emerging bio-based production methods offer sustainable alternatives, particularly through the hydrogenolysis of glycerol, a byproduct of biodiesel production, using copper-based catalysts (e.g., Cu/ZnO/Al₂O₃) under hydrogen pressure (20-50 bar) at 200-250°C to selectively cleave the C-O bond and yield 1,2-propanediol with selectivities up to 95%. Companies like Archer Daniels Midland and BASF have commercialized this route, leveraging the abundance of glycerol to reduce reliance on fossil feedstocks and lower greenhouse gas emissions by up to 60% compared to petrochemical methods. Other pathways, such as the hydrogenolysis of lactic acid or sorbitol derived from biomass fermentation, are under development but remain at pilot scale.[30][20][34]Uses
1,2-Propanediol, commonly known as propylene glycol, serves as a versatile compound in numerous industries due to its properties as a humectant, solvent, and antifreeze agent.[12] It is recognized as safe for various applications, including direct food additives by regulatory bodies.[35] In the food and beverage sector, 1,2-propanediol functions as an anticaking agent, antioxidant, dough strengthener, emulsifier, flavor agent, humectant, solvent, stabilizer, thickener, and texturizer.[36] It absorbs extra water to maintain moisture in products, acts as a solvent for food colors and flavors, and is used in items such as alcoholic beverages (up to 5%), confections (up to 24%), frozen dairy (up to 2.5%), and seasonings (up to 97%), with a general limit of 2% in other foods.[36][35] Additionally, it preserves texture in packaged foods, drinks, and dairy as a preservative, emulsifier, and stabilizer.[37] In pharmaceuticals, 1,2-propanediol is employed as a solvent in oral, injectable, and topical formulations, such as in drugs like diazepam and ceftriaxone, where it enhances drug permeation and stability.[37] It maintains moisture in medicines, serves as an excipient and carrier, and stabilizes metal nanoparticles for diagnostic assays.[35][38] In veterinary medicine, it functions similarly as a solvent and humectant.[35] For cosmetics and personal care, 1,2-propanediol acts as a moisturizer, skin conditioning agent, viscosity reducer, solvent, and fragrance ingredient, helping to extend shelf life in lotions, creams, and e-cigarette liquids, often comprising a significant portion (50-70% or more) of e-liquid formulations.[37][12] It absorbs water to preserve moisture in these products and is used in hand sanitizers for its humectant properties.[35][37] Industrially, 1,2-propanediol is a key component in non-toxic antifreeze and coolant formulations for automotive, aircraft, HVAC systems, and geothermal applications, often at concentrations of 25-33 wt.%.[37] It serves as a base for deicing solutions on runways and aircraft, and in paints, coatings, adhesives, lubricants, polishes, and waxes.[35][37] The compound is also used to produce polyester resins and other polymers, as well as in brake and hydraulic fluids.[35] In textiles, it aids in processing as a solvent and softener.[38] Other applications include generating artificial smoke or fog for firefighting training and theatrical productions, and as a humectant or preservative in cleansing creams.[35][12]Safety and environmental impact
1,2-Propanediol, commonly known as propylene glycol, is classified by the U.S. Food and Drug Administration (FDA) as generally recognized as safe (GRAS) for use as a direct food additive in various applications, including as a humectant, solvent, and preservative. The World Health Organization (WHO) has established an acceptable daily intake of 25 mg/kg body weight for humans, reflecting its low acute toxicity profile, with oral LD50 values in rats ranging from 8 to 46 g/kg.[39] In humans, it is rapidly metabolized via alcohol dehydrogenase to lactic acid and then to glucose, with a half-life of approximately 4 hours, and primarily excreted in urine, minimizing accumulation risks under normal exposure conditions.[39] Despite its safety in typical uses, propylene glycol can cause mild irritation upon direct contact with skin, eyes, or mucous membranes, particularly with repeated or prolonged exposure, though systemic absorption through intact skin is limited.[39] Inhalation of mists or vapors at occupational levels above 51 ppm may lead to nasal irritation and hemorrhaging in animal models, prompting the Agency for Toxic Substances and Disease Registry (ATSDR) to derive an intermediate inhalation minimal risk level (MRL) of 0.009 ppm.[39] Vulnerable populations, such as infants, those with renal impairment, or individuals receiving high-dose intravenous administrations (e.g., in medications), may experience metabolic acidosis or elevated osmolal gaps at doses exceeding 9,000 mg/kg, though such cases are rare and reversible.[39] No evidence of carcinogenicity, mutagenicity, or reproductive toxicity has been observed in available studies, and it is not classified as a hazardous substance under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).[39] Environmentally, 1,2-propanediol is highly biodegradable, with aerobic half-lives in water and soil ranging from 1 to 7 days, and it undergoes rapid photochemical oxidation in air within 24 to 50 hours, preventing long-term persistence.[39] Its log Kow of -0.92 indicates low bioaccumulation potential in aquatic organisms, and ecotoxicity studies show minimal hazard to fish, invertebrates, and algae, with 96-hour LC50 values for rainbow trout exceeding 40,000 mg/L.[39] Releases from industrial processes, de-icing fluids, or consumer products contribute to environmental loading, but its ready biodegradability under both aerobic and anaerobic conditions supports low ecological risk, as confirmed by the European Chemicals Agency (ECHA), which reports no classified environmental hazards under REACH.[40] Propylene glycol is not persistent, bioaccumulative, or toxic (PBT) according to regulatory assessments, though large-scale spills, such as in airport runoff, warrant monitoring due to potential short-term oxygen depletion in receiving waters.[39]1,3-Propanediol
Physical properties
1,3-Propanediol is a colorless, odorless, and viscous liquid at room temperature. It is hygroscopic and miscible with water.[2] The compound has a melting point of -27 °C and a boiling point of 214 °C at standard pressure, indicating stability across a wide temperature range. Its density is 1.053 g/mL at 25 °C, and the refractive index is 1.440 at 20 °C. 1,3-Propanediol exhibits low vapor pressure (0.8 mmHg at 20 °C), contributing to its low volatility.[13][4] It is fully miscible with water, alcohols, and ethers, reflecting its polar nature due to the two primary hydroxyl groups. Solubility is limited in non-polar solvents such as hydrocarbons. The dynamic viscosity is 0.052 Pa·s at 20 °C, underscoring its syrupy consistency.[13][41]| Property | Value | Conditions | Source |
|---|---|---|---|
| Molecular formula | C₃H₈O₂ | - | PubChem |
| Molecular weight | 76.09 g/mol | - | PubChem |
| Appearance | Colorless viscous liquid | Room temperature | Sigma-Aldrich |
| Melting point | -27 °C | - | Sigma-Aldrich |
| Boiling point | 214 °C | 101.3 kPa | Sigma-Aldrich |
| Density | 1.053 g/mL | 25 °C | Sigma-Aldrich |
| Refractive index | 1.440 | 20 °C | Sigma-Aldrich |
| Solubility in water | Miscible | - | PubChem |