Propylene glycol

Ball-and-stick Stick Space-filling

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13 atoms, 12 single bonds.

IUPAC name: propane-1,2-diol
Molar mass: 76.0950(96) g/mol
Atoms: 13
SMILES: CC(CO)O
InChIKey: DNIAPMSPPWPWGF-UHFFFAOYSA-N
PubChem CID: 1030

Made of

Structural formula of propylene glycol. Each unlabelled corner is a carbon atom, and the three hydrogens on carbon are not drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Flavoured foods, cosmetics and medicines are the everyday ways people come across propylene glycol, according to ATSDR, the US Agency for Toxic Substances and Disease Registry. The liquid itself is practically odourless and tasteless. The IUPAC name is propane-1,2-diol. In ChEBI (Chemical Entities of Biological Interest) it is a glycol: propane with an –OH in place of one hydrogen on carbon 1 and another on carbon 2.

Each molecule holds three carbon, eight hydrogen and two oxygen atoms, and a mole of the compound weighs 76.10 g. In the European Union’s description it is a clear, colourless and viscous liquid, and hygroscopic: it takes up water from damp air. It mixes with water in any proportion. It dissolves many essential oils yet does not mix with fixed oils, the fatty oils that do not evaporate, according to HSDB, the Hazardous Substances Data Bank.

Structure and bonding

The model’s 13 atoms are joined by twelve single bonds, as in the real molecule, with no ring or charge. With Labels on, three C symbols form a short chain. The end carbon and the middle carbon each carry an O, positions 1 and 2 of the name, and each O ends in an H. In Ball-and-stick every carbon has four neighbours set towards the corners of a tetrahedron, a four-faced pyramid. Chem1 gives carbon this tetrahedral coordination in most of its organic compounds.

In Space-filling the three methyl hydrogens round off one end and the CH₂OH oxygen caps the other, while the second oxygen juts sideways from the middle carbon.

Four different groups meet on the middle carbon: H, OH, CH₃ and CH₂OH. That makes it a chiral carbon, as explained for lactic acid, so propylene glycol exists as two mirror-image forms. The model shows one; the flat structural formula, drawn without wedges, shows neither. Making propylene glycol gives both in a 1:1 ratio (HSDB), so the liquid sold is an even mix. HSDB gives the pure forms specific rotations of +15° and −15°: they turn polarised light, which vibrates in one plane, equally but oppositely.

Where it turns up

Cosmetics use it as a humectant, which holds moisture in, and as a solvent, a skin-conditioning agent and a thinner, on HSDB’s list. In foods and medicines, ATSDR says, it soaks up extra water, and it dissolves food colourings and flavourings. Dry ice, solid carbon dioxide, usually contains an added binder such as propylene glycol or mineral oil, according to the EU’s food-additive description of carbon dioxide.

Theatres and fire-fighting trainers use it to make artificial smoke or fog, according to ATSDR. The liquid forms a vapour only when heated or shaken hard. Half of any that reaches the air breaks down within 24 to 50 hours, and in water or soil it is gone within several days to a week.

Why it matters

The chemical, food and pharmaceutical industries use it as antifreeze wherever a leak might reach food, according to ATSDR. Its one industrial source is the direct hydrolysis of propylene oxide, a reaction with water (HSDB). Some product picks up more propylene oxide, so dipropylene and tripropylene glycol form alongside and are separated by distillation. In 1983, HSDB records, unsaturated polyester resins, a family of plastics, took 47 per cent of its use. Foods, drugs and cosmetics took 11 per cent, pet foods 10, and brake and other functional fluids 8.

In the body, the enzyme alcohol dehydrogenase oxidises it to lactaldehyde and aldehyde dehydrogenase then to lactate, HSDB reports. The lactic and pyruvic acids formed then join the body’s ordinary metabolism. HSDB also describes a route through methylglyoxal and a possible third that attaches a phosphate group. It breaks down too fast to detect easily, and large amounts raise the body’s acid level, according to ATSDR. HSDB traces this to excess lactic acid after very large exposures, which can lead to the acid excess called metabolic acidosis.

Properties

Properties of propylene glycol Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1030.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₈O₂
Molar mass	76.0950(96) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	8
Oxygen atoms	2
Bonds	12
PubChem molecular weight	76.09 g/mol	g/mol	PubChem

Safety

In food, cosmetics and medicines, propylene glycol is generally considered safe, according to ATSDR. It meets no hazard criterion of the Globally Harmonized System (GHS) in 98% (6762 of 6899) of reports to the European Chemicals Agency. Under Japan’s classification, though, it carries the signal word Danger, for drowsiness or dizziness and organ damage. The liquid mildly irritates the eyes and airways and is combustible; the PubChem Laboratory Chemical Safety Summary holds these hazards with both classifications.

Questions

Is propylene glycol safe to eat?

Propylene glycol is affirmed by the US Food and Drug Administration (FDA) as generally recognised as safe (GRAS), and it is acceptable as a direct food additive and in flavourings, drugs and cosmetics, according to ATSDR. Under the Safer Choice programme of the US Environmental Protection Agency (EPA) it is expected to be of low concern on experimental and modelled data. Animal studies have not shown it to cause cancer, and the EPA has not classified it for carcinogenicity.

What is the difference between propylene glycol and ethylene glycol?

Propylene glycol, C₃H₈O₂, is propane-1,2-diol, with a chain of three carbons. Ethylene glycol, C₂H₆O₂, is ethane-1,2-diol, with two. Each carries its two –OH groups on neighbouring carbons, and the extra carbon in propylene glycol is a methyl group, CH₃, at the end of the chain. In the body the two are broken down equally fast, according to ATSDR, but propylene glycol leaves no harmful crystals behind.

Can propylene glycol irritate the skin?

Sometimes. Frequent skin contact with propylene glycol can irritate the skin, according to ATSDR. HSDB reports primary irritation in some people, possibly from dehydration of the skin. On HSDB's account it is not a sensitiser, a substance that primes the immune system to react on later contact. Haz-Map, a database of workplace hazards, notes that it may cause allergic contact dermatitis, an immune skin rash, in a small percentage of cases.

Why is propylene glycol used in food flavourings?

Propylene glycol carries flavours in preference to ethanol, HSDB reports, because it barely evaporates and so gives a more uniform flavour. Among its technical effects in food, the FDA names humectant, a substance that keeps food moist, and solvent or vehicle. In medicines, HSDB adds, it outperforms glycerin as a general solvent, dissolving a wide range of materials.

Made of the same elements

C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.