Glycerol

Ball-and-stick Stick Space-filling

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

14 atoms, 13 single bonds.

IUPAC name: propane-1,2,3-triol
Molar mass: 92.094(11) g/mol
Atoms: 14
SMILES: C(C(CO)O)O
InChIKey: PEDCQBHIVMGVHV-UHFFFAOYSA-N
PubChem CID: 753

Made of

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

On the tongue, glycerol tastes sweet and warm, and the pure liquid is clear, colourless and thick enough to pour like syrup. Its IUPAC name, propane-1,2,3-triol, lays out the molecule: the three-carbon chain of propane with a hydroxyl group, –OH, on carbons 1, 2 and 3. ChEBI, a database of small biological molecules, files it as a triol and an alditol, a sugar alcohol. The molar mass is 92.09 g/mol.

Glycerol mixes with water and with ethanol in any proportion. In benzene, chloroform and fixed or volatile oils it does not dissolve at all, according to HSDB, the Hazardous Substances Data Bank. It is also hygroscopic, taking up moisture from the air. HSDB gives a melting point of 18.1 °C, yet Haz-Map notes that the liquid freezes at a much lower temperature than the solid melts.

Structure and bonding

The readout counts 14 atoms and 13 single bonds. With Labels on, three C symbols run along the middle, each holding one O. The end carbons carry two H apiece, the middle carbon one, and every O ends in an H. In Ball-and-stick each carbon has four neighbours, making the backbone a row of carbon tetrahedra, as Chem1 describes carbon chains. This computed model sets the C–C–C angle at 111.4° and the O–C–C angles between 108.0° and 109.8°.

Every bond in the model is a single bond, as in the real molecule. HSDB lists glycerol as neutral to litmus, with a pKa of 14.4.

Stick view thins the atoms to lines, making the three O–H tips easy to trace. Space-filling merges everything into one knobbly lump with those oxygens on its surface. Each O–H hydrogen can form a hydrogen bond with an oxygen on a neighbouring molecule. Chem1 links that crowd of hydroxyl groups to glycerine’s high viscosity.

Where it turns up

On HSDB’s list, medicines, cosmetics, tobacco, food and drink all contain glycerol, and so do paints, resins and paper. Among the jobs the US Food and Drug Administration (FDA) lists for it in food are humectant, which keeps a product moist, sweetener, solvent, thickener, stabiliser and texturiser. The Consumer Product Information Database counts 2,656 products containing glycerin, from personal care to car and pet products.

Haz-Map lists further uses: keeping fabrics pliable, preserving printed patterns on cotton and defrosting car windscreens. In oil and gas fields it lubricates drilling equipment down the hole, and pesticide formulations use it as a wetting agent.

The body makes glycerol itself, and it is a metabolite in yeast, Escherichia coli and algae as well as in mice and people. Fat in food releases more, as intestinal lipase breaks the ester bonds that hold fatty acids at carbons 1 and 3.

Why it matters

Natural glycerol is a by-product, left over when fats and oils are converted to fatty acids or their methyl esters; synthetic glycerol is made from propene. HSDB puts soap-lye crude at about 80 per cent glycerol, and crude from the hydrolysis of fats at 88 to 91 per cent. Reports to the Environmental Protection Agency (EPA) put US volume for 2023 at between 1 and 2.5 billion pounds.

Its three hydroxyl groups make it a polyhydric alcohol, and Chem1 explains that such alcohols react with acids carrying several acid groups, such as phthalic acid, to give polyesters. Those are the alkyd resins, sold from 1902 as Glyptal varnishes for paint.

In the body, glycerol kinase in the liver and kidneys converts it to glycerol-3-phosphate. Further steps lead to pyruvic acid, glucose and glycogen. Given rectally as a laxative, it draws water from the tissues into the stool and can irritate the bowel lining, which triggers emptying, by HSDB’s account. How much each of these, and the simple stretching of the rectum, contributes is not known.

Properties

Properties of glycerol Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 753.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₃H₈O₃
Molar mass	92.094(11) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	3
Hydrogen atoms	8
Oxygen atoms	3
Bonds	13
PubChem molecular weight	92.09 g/mol	g/mol	PubChem

Safety

Glycerol is essentially harmless in its everyday forms, and the FDA recognises it as safe in food. Under the Globally Harmonized System (GHS), 98.3% (5522 of 5617) of reports find that it meets no hazard criteria. A drop in the eye stings and burns without obvious injury, and strong heating decomposes it to toxic acrolein fumes. The PubChem Laboratory Chemical Safety Summary and the NIOSH card for glycerin mist, which lists US workplace limits, are the sheets to consult.

Questions

What shape is a glycerol molecule?

No single shape word fits, since the molecule has six centres, three carbons and three oxygens, rather than one central atom. In PubChem's computed model each carbon holds four neighbours, the arrangement Chem1 describes for carbon chains as tetrahedra linked end to end. The C–C–C angle in the model is 111.4°, the O–C–C angles run from 108.0° to 109.8°, and all 13 bonds are single.

Why is glycerol so thick?

HSDB puts the viscosity of the pure liquid at 954 centipoises at 25 °C, while a 70 per cent solution in water measures 17 centipoises at the same temperature. Glycerol carries three hydroxyl groups on a small frame. Chem1 ties the syrupy flow of glycerine directly to them: every –OH can join a hydrogen bond with a neighbour. Dispersion forces, present between all molecules, add to the pull.

What is the difference between glycerol and glycerin?

Glycerol, strictly, names only the pure compound, propane-1,2,3-triol. Glycerin, on HSDB's definitions, is the purified commercial product, which holds at least 95 per cent glycerol. Natural glycerol comes from fats and oils, split into fatty acids or their methyl esters, while synthetic glycerol is made from propene. It is purified by redistillation and ion exchange.

What does the body do with glycerol?

Glycerol is already made in the body, and more arrives when intestinal lipase splits dietary fats, after which the gut absorbs it readily. On HSDB's account the liver, and to a lesser degree the kidneys, attach a phosphate to it with the enzyme glycerol kinase. From there it feeds the pathways that build glucose and glycogen, or it joins fatty acids again as triglycerides.

Made of the same elements

C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single 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.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₂O Formaldehyde 4 atoms, 3 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.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.