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Glycerol
Glycerol
Glycerol
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Glycerol
Glycerol
Glycerol
Ball-and-stick model of glycerol
Ball-and-stick model of glycerol
Space-filling model of glycerol
Space-filling model of glycerol
Sample of glycerine
Sample of glycerine
Names
Preferred IUPAC name
Propan-1,2,3-triol[1]
Other names
  • 1,2,3-Trioxypropane
  • 1,2,3-Trihydroxypropane
  • 1,2,3-Propanetriol
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.000.263 Edit this at Wikidata
E number E422 (thickeners, ...)
KEGG
UNII
  • InChI=1S/C3H8O3/c4-1-3(6)2-5/h3-6H,1-2H2 checkY
    Key: PEDCQBHIVMGVHV-UHFFFAOYSA-N checkY
  • InChI=1/C3H8O3/c4-1-3(6)2-5/h3-6H,1-2H2
    Key: PEDCQBHIVMGVHV-UHFFFAOYAF
  • OCC(O)CO
Properties
C3H8O3
Molar mass 92.094 g·mol−1
Appearance Colorless hygroscopic liquid
Odor Odorless
Density 1.261 g/cm3
Melting point 17.8 °C (64.0 °F; 290.9 K)
Boiling point 290 °C (554 °F; 563 K)[5]
miscible[2]
log P −2.32[3]
Vapor pressure 0.003 mmHg (0.40 Pa) at 50 °C[2]
−57.06×10−6 cm3/mol
1.4746
Viscosity 1.412 Pa·s (20 °C)[4]
Pharmacology
A06AG04 (WHO) A06AX01 (WHO), QA16QA03 (WHO)
Hazards
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
1
0
Flash point 160 °C (320 °F; 433 K) (closed cup)
176 °C (349 °F; 449 K) (open cup)
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 15 mg/m3 (total)
TWA 5 mg/m3 (resp)[2]
REL (Recommended)
 None established[2]
IDLH (Immediate danger)
 N.D.[2]
Safety data sheet (SDS) JT Baker ver. 2008 archive
Supplementary data page
Glycerol (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Glycerol (/ˈɡlɪsərɒl/)[6] is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pharmaceutical formulations. Because of its three hydroxyl groups, glycerol is miscible with water and is hygroscopic in nature.[7]

Modern use of the word glycerine (alternatively spelled glycerin) refers to commercial preparations of less than 100% purity, typically 95% glycerol.[8]

Structure

[edit]

Although achiral, glycerol is prochiral with respect to reactions of one of the two primary alcohols. Thus, in substituted derivatives, the stereospecific numbering labels the molecule with a sn- prefix before the stem name of the molecule.[9][10][11]

Production

[edit]

Natural sources

[edit]

Glycerol is generally obtained from plant and animal sources where it occurs in triglycerides, esters of glycerol with long-chain carboxylic acids. The hydrolysis, saponification, or transesterification of these triglycerides produces glycerol as well as the fatty acid derivative:

3 NaOH / H2O

Rightward reaction arrow

Δ

3 × soap

3 × 

Triglycerides can be saponified with sodium hydroxide to give glycerol and fatty sodium salt or soap.

Typical plant sources include soybeans or palm. Animal-derived tallow is another source. From 2000 to 2004, approximately 950,000 tons per year were produced in the United States and Europe; 350,000 tons of glycerol were produced in the U.S. alone.[12] Since around 2010, there is a large surplus of glycerol as a byproduct of biofuel, enforced for example by EU directive 2003/30/EC that required 5.75% of petroleum fuels to be replaced with biofuel sources across all member states.[7] Crude glycerol produced from triglycerides is of variable quality, with a selling price as low as US$0.02–0.05 per kilogram in 2011.[13] It can be purified in a rather expensive process by treatment with activated carbon to remove organic impurities, alkali to remove unreacted glycerol esters, and ion exchange to remove salts. High purity glycerol (greater than 99.5%) is obtained by multi-step distillation; a vacuum chamber is necessary due to its high boiling point (290 °C).[7]

Consequently, glycerol recycling is more of a challenge than its production, for instance by conversion to glycerol carbonate[14] or to synthetic precursors, such as acrolein and epichlorohydrin.[15]

Synthetic glycerol

[edit]

Although more expensive than production from plant or animal triglycerides, glycerol can be synthesized by various routes. During World War II, synthetic glycerol processes became a national defense priority because it is a precursor to nitroglycerine. Epichlorohydrin is the most important precursor. Chlorination of propylene gives allyl chloride, which is oxidized with hypochlorite to dichlorohydrin, which reacts with a strong base to give epichlorohydrin. Epichlorohydrin can be hydrolyzed to glycerol. Chlorine-free processes from propylene include the synthesis of glycerol from acrolein and propylene oxide.[7]

Applications

[edit]

Food industry

[edit]

In food and beverages, glycerol serves as a humectant, solvent, and sweetener, and may help preserve foods. It is also used as filler in commercially prepared low-fat foods (e.g., cookies), and as a thickening agent in liqueurs. Glycerol and water are used to preserve certain types of plant leaves.[16]

It is recommended as an additive when polyol sweeteners such as erythritol and xylitol are used, as its heating effect in the mouth will counteract these sweeteners' cooling effect.[17]

Medical

[edit]
A bottle of glycerin purchased at a pharmacy
Personal lubricants commonly contain glycerol
Glycerol is an ingredient in products such as hair gel
Glycerol suppositories used as laxatives

Glycerol is used in medical, pharmaceutical and personal care preparations, often as a means of improving smoothness, providing lubrication, and as a humectant.

Ichthyosis and xerosis have been relieved by the topical use of glycerin.[18][19] It is found in allergen immunotherapies, cough syrups, elixirs and expectorants, toothpaste, mouthwashes, skin care products, shaving cream, hair care products, soaps, and water-based personal lubricants. In solid dosage forms like tablets, glycerol is used as a tablet holding agent. For human consumption, glycerol is classified by the FDA among the sugar alcohols as a caloric macronutrient. Glycerol is also used in blood banking to preserve red blood cells prior to freezing.[20]

Taken rectally, glycerol functions as a laxative by irritating the anal mucosa and inducing a hyperosmotic effect,[21] expanding the colon by drawing water into it to induce peristalsis resulting in evacuation.[22] It may be administered undiluted either as a suppository or as a small-volume (2–10 ml) enema. Alternatively, it may be administered in a dilute solution, such as 5%, as a high-volume enema.[23]

Taken orally (often mixed with fruit juice to reduce its sweet taste), glycerol can cause a rapid, temporary decrease in the internal pressure of the eye. This can be useful for the initial emergency treatment of severely elevated eye pressure.[24]

In 2017, researchers showed that the probiotic Limosilactobacillus reuteri bacteria can be supplemented with glycerol to enhance its production of antimicrobial substances in the human gut. This was confirmed to be as effective as the antibiotic vancomycin at inhibiting Clostridioides difficile infection without having a significant effect on the overall microbial composition of the gut.[25]

Glycerol solutions have been used for the preservation and storage of tissue grafts at ambient conditions as an alternative to frozen storage.[26]

Glycerol has also been incorporated as a component of bio-ink formulations in the field of bioprinting.[27] The glycerol content acts to add viscosity to the bio-ink without adding large protein, saccharide, or glycoprotein molecules.

It is on the World Health Organization's List of Essential Medicines.[28]

Botanical extracts

[edit]

When utilized in tincture method extractions, specifically as a 10% solution, glycerol prevents tannins from precipitating in ethanol extracts of plants (tinctures). It is also used as an "alcohol-free" alternative to ethanol as a solvent in preparing herbal extractions. It is less extractive when utilized in a standard tincture methodology. Alcohol-based tinctures can also have the alcohol removed and replaced with glycerol for its preserving properties. Such products are not "alcohol-free" in a scientific or FDA regulatory sense, as glycerol contains three hydroxyl groups. Fluid extract manufacturers often extract herbs in hot water before adding glycerol to make glycerites.[29][30]

When used as a primary "true" alcohol-free botanical extraction solvent in non-tincture based methodologies, glycerol has been shown to possess a high degree of extractive versatility for botanicals including removal of numerous constituents and complex compounds, with an extractive power that can rival that of alcohol and water–alcohol solutions.[31] That glycerol possesses such high extractive power assumes it is utilized with dynamic (critical) methodologies as opposed to standard passive "tincturing" methodologies that are better suited to alcohol. Glycerol does not denature or render a botanical's constituents inert as alcohols (ethanol, methanol, and so on) do. Glycerol is a stable preserving agent for botanical extracts that, when utilized in proper concentrations in an extraction solvent base, does not allow inverting or reduction-oxidation of a finished extract's constituents, even over several years.[citation needed] Both glycerol and ethanol are viable preserving agents. Glycerol is bacteriostatic in its action, and ethanol is bactericidal in its action.[32][33][34]

Electronic cigarette liquid

[edit]
Glycerin is often used in electronic cigarettes to create the vapor

Glycerin, along with propylene glycol, is a common component of e-liquid, a solution used with electronic vaporizers (electronic cigarettes). This glycerol is heated with an atomizer (a heating coil often made of Kanthal wire), producing the aerosol that delivers nicotine to the user.[35]

Antifreeze

[edit]
Main article: Antifreeze

Like ethylene glycol and propylene glycol, glycerol is a non-ionic kosmotrope that forms strong hydrogen bonds with water molecules, competing with water-water hydrogen bonds. This interaction disrupts the formation of ice. The minimum freezing point temperature is about −38 °C (−36 °F) corresponding to 70% glycerol in water.

Glycerol was historically used as an anti-freeze for automotive applications before being replaced by ethylene glycol, which has a lower freezing point. While the minimum freezing point of a glycerol-water mixture is higher than an ethylene glycol-water mixture, glycerol is not toxic and is being re-examined for use in automotive applications.[36][37]

In the laboratory, glycerol is a common component of solvents for enzymatic reagents stored at temperatures below 0 °C (32 °F) due to the depression of the freezing temperature. It is also used as a cryoprotectant where the glycerol is dissolved in water to reduce damage by ice crystals to laboratory organisms that are stored in frozen solutions, such as fungi, bacteria, nematodes, and mammalian embryos. Some organisms like the moor frog produce glycerol to survive freezing temperatures during hibernation.[38]

Chemical intermediate

[edit]

Glycerol is used to produce a variety of useful derivatives.

Nitration gives nitroglycerin, an essential ingredient of various explosives such as dynamite, gelignite, and propellants like cordite. Nitroglycerin under the name glyceryl trinitrate (GTN) is commonly used to relieve angina pectoris, taken in the form of sub-lingual tablets, patches, or as an aerosol spray.

Trifunctional polyether polyols are produced from glycerol and propylene oxide.

Oxidation of glycerol affords mesoxalic acid.[39] Dehydrating glycerol affords hydroxyacetone.

Chlorination of glycerol gives the 1-chloropropane-2,3-diol:

HOCH(CH2OH)2 + HCl → HOCH(CH2Cl)(CH2OH) + H2O

The same compound can be produced by hydrolysis of epichlorohydrin.[40]

Epoxidation by reaction with epichlorohydrin and a Lewis acid yields Glycerol triglycidyl ether.[41][42]

Vibration damping

[edit]

Glycerol is used as fill for pressure gauges to damp vibration. External vibrations, from compressors, engines, pumps, etc., produce harmonic vibrations within Bourdon gauges that can cause the needle to move excessively, giving inaccurate readings. The excessive swinging of the needle can also damage internal gears or other components, causing premature wear. Glycerol, when poured into a gauge to replace the air space, reduces the harmonic vibrations that are transmitted to the needle, increasing the lifetime and reliability of the gauge.[43]

Niche uses

[edit]

Entertainment industry

[edit]

Glycerol is used by set decorators when filming scenes involving water to prevent an area meant to look wet from drying out too quickly.[44]

Glycerine is also used in the generation of theatrical smoke and fog as a component of the fluid used in fog machines as a replacement for glycol, which has been shown to be an irritant if exposure is prolonged.

Ultrasonic couplant

[edit]

Glycerol can be sometimes used as replacement for water in ultrasonic testing, as it has favourably higher acoustic impedance (2.42 MRayl versus 1.483 MRayl for water) while being relatively safe, non-toxic, non-corrosive and relatively low cost.[45]

Internal combustion fuel

[edit]

Glycerol is also used to power diesel generators supplying electricity for the FIA Formula E series of electric race cars.[46]

Research on additional uses

[edit]

Research continues into potential value-added products of glycerol obtained from biodiesel production.[47] Examples (aside from combustion of waste glycerol):

Metabolism

[edit]

Glycerol is a precursor for synthesis of triacylglycerols and of phospholipids in the liver and adipose tissue. When the body uses stored fat as a source of energy, glycerol and fatty acids are released into the bloodstream.

Glycerol is mainly metabolized in the liver. Glycerol injections can be used as a simple test for liver damage, as its rate of absorption by the liver is considered an accurate measure of liver health. Glycerol metabolism is reduced in both cirrhosis and fatty liver disease.[59][60]

Blood glycerol levels are highly elevated during diabetes, and is believed to be the cause of reduced fertility in patients who suffer from diabetes and metabolic syndrome. Blood glycerol levels in diabetic patients average three times higher than healthy controls. Direct glycerol treatment of testes has been found to cause significant long-term reduction in sperm count. Further testing on this subject was abandoned due to the unexpected results, as this was not the goal of the experiment.[61]

Circulating glycerol does not glycate proteins as do glucose or fructose, and does not lead to the formation of advanced glycation endproducts (AGEs). In some organisms, the glycerol component can enter the glycolysis pathway directly and, thus, provide energy for cellular metabolism (or, potentially, be converted to glucose through gluconeogenesis).

Before glycerol can enter the pathway of glycolysis or gluconeogenesis (depending on physiological conditions), it must be converted to their intermediate glyceraldehyde 3-phosphate in the following steps:

ATP
ADP
Rightward reaction arrow with minor substrate(s) from top left and minor product(s) to top right

The enzyme glycerol kinase is present mainly in the liver and kidneys, but also in other body tissues, including muscle and brain.[62][63][64] In adipose tissue, glycerol 3-phosphate is obtained from dihydroxyacetone phosphate with the enzyme glycerol-3-phosphate dehydrogenase.

Toxicity and safety

[edit]

Glycerol has very low toxicity when ingested; its LD50 oral dose for rats is 12600 mg/kg and 8700 mg/kg for mice. It does not appear to cause toxicity when inhaled, although changes in cell maturity occurred in small sections of lung in animals under the highest dose measured. A sub-chronic 90-day nose-only inhalation study in Sprague Dawley rats exposed to 0.03, 0.16 and 0.66 mg of glycerin per liter of air for 6-hour continuous sessions revealed no treatment-related toxicity other than minimal metaplasia of the epithelium lining at the base of the epiglottis in rats exposed to 0.66 mg/L glycerin.[65][66]

Glycerol intoxication

[edit]

Excessive consumption by children can lead to glycerol intoxication.[67] Symptoms of intoxication include hypoglycemia, nausea and a loss of consciousness. While intoxication as a result of excessive glycerol consumption is rare and its symptoms generally mild, occasional reports of hospitalization have occurred.[68] In the United Kingdom in August 2023, manufacturers of syrup used in slush ice drinks were advised to reduce the amount of glycerol in their formulations by the Food Standards Agency to reduce the risk of intoxication.[69] A 2025 study reported that between 2018 and 2024, at least 21 children aged 2–7 in the UK and Ireland received emergency treatment for symptoms of glycerol intoxication following the consumption of slush ice drinks.[70][71]

Food Standards Scotland advises that slush ice drinks containing glycerol should not be given to children under the age of 4, owing to the risk of intoxication. It also recommends that businesses do not use free refill offers for the drinks in venues where children under the age of 10 are likely to consume them, and that products should be appropriately labelled to inform consumers of the presence of glycerol.[72]

Historical cases of contamination with diethylene glycol

[edit]

On 4 May 2007, the FDA advised all U.S. makers of medicines to test all batches of glycerol for diethylene glycol contamination.[73] This followed an occurrence of hundreds of fatal poisonings in Panama resulting from a falsified import customs declaration by Panamanian import/export firm Aduanas Javier de Gracia Express, S. A. The cheaper diethylene glycol was relabeled as the more expensive glycerol.[74][75] Between 1990 and 1998, incidents of DEG poisoning reportedly occurred in Argentina, Bangladesh, India, and Nigeria, and resulted in hundreds of deaths. In 1937, more than one hundred people died in the United States after ingesting DEG-contaminated elixir sulfanilamide, a drug used to treat infections.[76]

Etymology

[edit]

The origin of the gly- and glu- prefixes for glycols and sugars is from Ancient Greek γλυκύς glukus which means sweet.[77] Name glycérine was coined ca. 1811 by Michel Eugène Chevreul to denote what was previously called "sweet principle of fat" by its discoverer Carl Wilhelm Scheele. It was borrowed into English ca. 1838 and in the 20th c. displaced by 1872 term glycerol featuring an alcohols' suffix -ol.

Properties

[edit]

Table of thermal and physical properties of saturated liquid glycerin:[78][79]

Temperature (°C) Density (kg/m3) Specific heat (kJ/kg·K) Kinematic viscosity (m2/s) Conductivity (W/m·K) Thermal diffusivity (m2/s) Prandtl number Bulk modulus (K−1)
0 1276.03 2.261 8.31×10−3 0.282 9.83×10−8 84700 4.7×10−4
10 1270.11 2.319 3.00×10−3 0.284 9.65×10−8 31000 4.7×10−4
20 1264.02 2.386 1.18×10−3 0.286 9.47×10−8 12500 4.8×10−4
30 1258.09 2.445 5.00×10−4 0.286 9.29×10−8 5380 4.8×10−4
40 1252.01 2.512 2.20×10−4 0.286 9.14×10−8 2450 4.9×10−4
50 1244.96 2.583 1.50×10−4 0.287 8.93×10−8 1630 5.0×10−4

See also

[edit]

References

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

Glycerol

View on Grokipedia
from Grokipedia
Glycerol, also known as glycerin or , is a trihydric alcohol with the molecular formula C₃H₈O₃ consisting of a chain bearing three hydroxyl groups. Glycerol is the scientific or IUPAC name for the pure compound propane-1,2,3-triol (C₃H₈O₃); glycerin is the common commercial name, typically referring to a version with at least 95% purity. In everyday use, especially in cosmetics, food, and pharmaceuticals, the terms are often used synonymously. It appears as a clear, colorless, odorless, and viscous liquid that is hygroscopic and exhibits a sweet taste approximately 0.6 times as intense as . First isolated in 1779 by Swedish chemist from the of fats, glycerol is now predominantly obtained as a coproduct of manufacturing via of triglycerides in oils or animal fats, with synthetic production from also feasible. Its physical properties include a of 1.261 g/cm³ at 20°C, a melting point of 18°C, and a of 290°C (with decomposition). Glycerol's versatility stems from its non-toxicity, water solubility, and ability to form bonds, enabling applications as a and in foods, , and pharmaceuticals; as an ; and as a precursor in synthesizing compounds like for explosives. Over 1500 uses have been documented, underscoring its industrial significance despite occasional historical contamination risks in impure forms.

Structure and Properties

Molecular Structure

Glycerol possesses the molecular formula C₃H₈O₃ and the systematic IUPAC name propan-1,2,3-triol (EC number 200-289-5). Its structure consists of a linear three-carbon propane backbone with hydroxyl (-OH) groups attached to each carbon atom at positions 1, 2, and 3, represented as HOCH₂CH(OH)CH₂OH. This trihydric alcohol configuration features primary hydroxyl groups on the terminal carbons and a secondary hydroxyl group on the central carbon, enabling extensive hydrogen bonding. The molecule exhibits a plane of bisecting the central C-H and C-OH bonds, rendering it achiral despite the potential for in substituted derivatives. In its most stable conformation, the carbon chain adopts a gauche arrangement to minimize steric repulsion between the hydroxyl groups, as determined by quantum chemical calculations and spectroscopic . The molecular weight is 92.09 g/mol, with bond lengths typical of aliphatic alcohols: C-O approximately 1.43 and O-H around 0.96 .

Physical Properties

Glycerol is a clear, colorless, odorless, syrupy at , which solidifies upon cooling but often supercools to remain liquid below its . It is hygroscopic, readily absorbing atmospheric moisture. Key physical properties are summarized below:
  • Molecular formula: C₃H₈O₃
  • Molar mass: 92.09 g/mol
  • Density: 1.261 g/cm³ at 20 °C
  • : 18 °C
  • Boiling point: 290 °C (decomposes)
  • Vapor pressure: Low, contributing to its stability
  • Dynamic viscosity: 1.5 Pa·s at 20 °C
  • Refractive index: 1.475 at 20 °C
  • Chromatic dispersion: Glycerol exhibits normal dispersion (refractive index decreases with increasing wavelength), which can be modeled using Cauchy's empirical equation n(λ) = A + B/λ² + C/λ⁴ (where λ is the wavelength in micrometers), with fitted coefficients A, B, and C depending on temperature and the wavelength range; detailed fits are available in studies of hygroscopic liquids including glycerol.
  • Flash point: 177 °C (open cup)
  • Autoignition temperature: 393 °C
  • Solubility: Miscible with and ; soluble in acetone and dioxane; sparingly soluble in hydrocarbons
Glycerol exhibits high thermal stability up to its and low volatility, with a dielectric constant of approximately 42.5 at 25 °C.

Chemical Properties

Glycerol, a 1,2,3-propanetriol, exhibits chemical reactivity primarily through its three hydroxyl groups—two primary at C1 and C3, and one secondary at C2—enabling it to participate in typical reactions such as esterification, etherification, oxidation, , and . The primary hydroxyls display greater reactivity toward electrophiles and oxidants compared to the secondary group, influencing product selectivity in multi-step transformations. In esterification, glycerol reacts with free s or derivatives under acidic or enzymatic to form monoglycerides, diglycerides, and triglycerides, with equilibrium favoring partial esters unless water is removed. This stepwise process, governed by , is central to the synthesis of emulsifiers and byproducts, where excess fatty acid drives toward triesters. Dehydration of glycerol, typically catalyzed by strong acids like or solid acids such as heteropolyacids, proceeds via elimination of two molecules to produce (propenal, CH₂=CHCHO), a key industrial intermediate, with side products including acetol and under non-optimized conditions. Oxidation reactions vary by reagent and conditions: mild agents like yield or from selective oxidation, while stronger oxidants such as produce and tartronic acid; exhaustive oxidation can cleave to , CO₂, and . Glycerol's reducing power leads to violent exothermic reactions with solid oxidants like or , potentially explosive due to rapid heat evolution from multiple reactive hydroxyl sites. Nitration occurs when glycerol is treated with a mixture of concentrated nitric and sulfuric acids at low temperatures, forming trinitroglycerin (glyceryl trinitrate), a sensitive high resulting from substitution of all three hydroxyls. Halogenation, such as with HCl or , substitutes hydroxyls to chlorohydrins like 1-chloropropane-2,3-diol, while heated mixtures with gas can detonate. Glycerol demonstrates general stability toward and bases but is incompatible with strong oxidizers (e.g., , perchlorates), acylating agents like , and alkali metals like , often yielding vigorous or explosive responses; above 290°C releases and other volatiles.

Production

Natural Sources

Glycerol occurs ubiquitously in biological systems as the three-carbon backbone of glycerolipids, particularly triglycerides (triacylglycerols), which form the primary storage in animals, , and microorganisms. These molecules consist of one glycerol unit esterified to three chains via synthesis, enabling efficient and formation. In animals, glycerol is derived from fats such as (bovine) and (porcine), while in plants it is obtained from triglyceride-rich oils extracted from seeds, fruits, or kernels, including (yielding approximately 10% glycerol upon ), , and . of these natural s—whether enzymatic or in extraction processes—liberates free glycerol, which constitutes about 10% by weight of the triglyceride structure. Glycerol also arises naturally through metabolic pathways in living organisms, including de-esterification (lipolysis) of lipids during fat catabolism and as a byproduct of phospholipid turnover in cell membranes. Certain microorganisms, such as yeasts during alcoholic fermentation, produce glycerol as a compatible solute for osmotic stress response or redox balancing, contributing trace amounts in fermented products like beer, wine, honey, and vinegar. In archaea and some bacteria, glycerol-based phospholipids form unique membrane structures adapted to extreme environments. These biological occurrences underscore glycerol's fundamental role in lipid biochemistry across domains of life.

Industrial Synthesis

Industrial synthesis of glycerol historically relied on petrochemical processes derived from , developed as alternatives to natural extraction from fats and oils during periods of supply constraints, such as . The first synthetic production occurred in in 1943, followed by the in 1948, marking a shift toward chlorine-based routes that peaked at 50-60% of global glycerol supply in the and before declining due to the rise of byproducts. These methods involve multi-step reactions converting to intermediates like or , followed by or reduction to yield glycerol with efficiencies around 90%. The predominant route, known as the allyl chloride or chlorohydrin process, begins with high-temperature chlorination of at approximately 500°C to produce : C₃H₆ + → C₃H₅Cl + HCl. then undergoes chlorohydrin formation with and , yielding glycerol dichlorohydrin, which is hydrolyzed using (6% solution at 96°C) or caustic soda to glycerol, achieving yields of about 90% based on . For 1 metric ton of 99% glycerol, this requires roughly 625 kg , 2000 kg , 450 kg NaOH, and 450 kg lime, generating significant HCl and NaCl byproducts. Alternative chlorine-free processes include the acrolein route, where is oxidized to (CH₂=CHCHO), which reacts with to form , subsequently hydrolyzed to glycerol with 80-90% yield and producing acetone as a (about 990 kg per metric glycerol). Materials include 925 kg , 230 kg oxygen, 1100 kg , and 485 kg H₂O₂ per metric . Another pathway utilizes , produced via epoxidation of (often with ), which isomerizes to (80-85% yield) and then proceeds to and glycerol. These routes offer environmental advantages by avoiding but require precise for oxidation steps. Despite their technical maturity, synthetic processes have diminished in commercial prominence since the early 2000s, as transesterification generates glycerol more economically at scale, though synthetic methods remain viable for high-purity demands or regions with limited infrastructure. Ongoing research explores bio-based feedstocks or integrated complexes to revive efficiency, but current industrial synthesis emphasizes legacy pathways for consistency in quality control.

Byproduct from Biodiesel and Purification Advances

The process in , involving the reaction of triglycerides from oils or animal fats with in the presence of a catalyst such as , yields methyl esters () as the primary product and crude glycerol as a , typically constituting about 10% by weight of the input feedstock. For every 100 pounds of generated, approximately 10 pounds of crude glycerol are produced, with variations depending on feedstock composition and process efficiency. This byproduct stream, generated at a ratio of roughly 0.35 kg per gallon of , contains 80-90% glycerol but is contaminated with residual , water, inorganic salts from the catalyst, soaps formed via , free s, and organic matter. The rapid expansion of , driven by policy mandates and biofuel incentives in the and during the early , resulted in a significant oversupply of crude glycerol. Global production of biodiesel-derived crude glycerol escalated from 167 kilotons in 2003 to 2,000 kilotons by 2012, outpacing demand for refined glycerol and causing market prices to plummet from highs above $0.50 per pound in the mid- to below $0.05 per pound by 2009. This surplus shifted glycerol from a primarily synthetic or animal-derived commodity to one dominated by origins, comprising over 70% of total supply by the , and prompted research into valorization to mitigate economic burdens on biodiesel producers. Purification of crude glycerol is essential to achieve technical-grade (80-99% purity) or pharmaceutical-grade (>99.5% purity) standards suitable for industrial or consumer applications, as impurities reduce its value and limit uses. Traditional methods include recovery via or , followed by neutralization or acidification to precipitate soaps, salting-out to separate phases, and for glycerol concentration. Advances since the have focused on cost-effective, scalable techniques to handle the glycerol glut, such as membrane-based using or polymeric ultrafiltration and nanofiltration to remove salts and organics without chemical additives, achieving 95% glycerol recovery with pore sizes tailored to 5-20 nm. Ion-exchange resins and adsorption have been optimized for targeted impurity removal, while integrated approaches combine physicochemical pre-treatments—like acidification and —with microbial or catalytic conversions to bypass full purification for high-value derivatives like . Recent laboratory-scale demonstrations, including low-energy solvent extraction and , have reported purification costs reduced by 20-30% compared to conventional , enhancing economic viability amid fluctuating outputs. These innovations prioritize energy efficiency and minimal waste, addressing the causal linkage between scale-up and glycerol's low initial purity, which historically rendered untreated streams suitable only for low-end disposal or .

Applications

Food and Beverage Uses

Glycerol serves as a versatile in the form of a , , , and , enabling moisture retention, texture enhancement, and inhibition of microbial growth or crystallization in various products. , it is classified as (GRAS) under 21 CFR 182.1320 when employed in accordance with good manufacturing practices. Within the , glycerol is authorized as E422 under Regulation (EC) No 1333/2008, with the European Authority's 2017 re-evaluation determining no safety concerns from its use and deeming a numerical unnecessary due to its metabolic handling akin to endogenous production. In confectionery and baked goods, glycerol prevents sugar crystallization in icings, candies, , and fondants, yielding smoother textures and prolonged freshness. As a , it maintains in products like cakes, muffins, cookies, and waffles, reducing by limiting moisture migration and evaporation, thereby extending without altering flavor profiles significantly. Typical incorporation levels range from 1-5% in formulations to optimize tenderness and plasticity. In beverages, glycerol enhances , , and perceived sweetness in items such as soft drinks, syrups, and liqueurs, functioning as a for flavors and a thickener to impart body. It is particularly employed in ice or frozen drinks at concentrations around 10-15% to lower the freezing point, preserving a semi-liquid state and averting full solidification during storage or serving. This application aligns with its solubility properties and low freezing point of -46.7°C, facilitating stable emulsions and reduced formation.

Pharmaceutical and Medical Applications

Glycerol serves as a versatile in pharmaceutical formulations due to its , , and viscosity-modifying properties. In topical preparations such as ointments and creams, it functions as an emollient to enhance hydration and , particularly in conditions involving xerosis. In oral solutions, syrups, and parenteral products, glycerol acts as a and to improve stability and . It is also employed as a in capsule shells and a carrier for active ingredients like antibiotics and antiseptics. As a laxative, glycerol is commonly administered rectally via suppositories to relieve by drawing into the intestines and stimulating bowel movement, typically acting within minutes. These suppositories, available over-the-counter, are shaped for easy insertion and provide rapid osmotic effects without systemic absorption in most cases. Oral glycerol may also be used for similar purposes, though rectal forms predominate for acute relief. In medical therapeutics, intravenous or oral glycerol functions as an osmotic dehydrating agent to reduce in , with effective doses ranging from 0.25 to 2.0 g/kg body weight. This application leverages glycerol's metabolizability, potentially offering advantages over non-metabolized agents like in certain contexts, though its use is more prevalent in Asian clinical practice. Orally, glycerol reduces in by hyperosmotic mechanisms, as documented in clinical guidelines. Additionally, its viscous nature contributes to the soothing effects in cough syrups for acute respiratory conditions. Glycerol's safety profile in these applications is generally favorable at therapeutic doses, but high intravenous administration can lead to or renal effects, necessitating monitoring. supports its efficacy in targeted uses, though randomized trials remain limited for some indications like reduction.

Industrial and Chemical Uses

Glycerol is a vital chemical intermediate in the production of , formed by the of glycerol with a mixture of concentrated nitric and sulfuric acids at controlled temperatures below 30°C to prevent . This reaction, yielding approximately 1.5 tons of per ton of glycerol, produces a highly liquid used in , propellants, and blasting applications since its synthesis in 1847 by . In the coatings industry, glycerol serves as the primary in alkyd resin synthesis via the monoglyceride process, where excess glycerol undergoes with unsaturated vegetable oils (typically 40-60% oil length) followed by polycondensation with , resulting in that provide gloss, adhesion, and drying properties in paints, varnishes, and enamels accounting for over 70% of architectural coatings. Glycerol functions as a non-toxic and , forming aqueous solutions that depress the freezing point (e.g., 50% glycerol lowers it to -23°C) and raise the while minimizing in systems like heavy-duty engines and industrial chillers, particularly in biobased formulations revived post-2000 due to biodiesel glycerol surplus. As a feedstock for , glycerol undergoes gas-phase hydrochlorination at 200-250°C with HCl to form dichlorohydrin intermediates, followed by base-catalyzed dehydrochlorination yielding at selectivities above 90%, which is then polymerized into resins for composites, adhesives, and electrical laminates in a process scaled commercially since 2007. Glycerol also acts as an industrial for dissolving resins, inks, and alkaloids, and as a in and regenerated films, exploiting its high (290°C) and with and alcohols to stabilize formulations in processing and chemical .

Niche and Emerging Applications

Glycerol serves as a in , facilitating reactions under unconventional conditions such as - and microwave-assisted processes, which enhance reaction rates while minimizing environmental impact compared to traditional volatile organic solvents. In , crude glycerol from is increasingly utilized as a carbon source for microbial to produce high-value compounds, including (biodegradable plastics) and rhamnolipids (biosurfactants), addressing surplus glycerol disposal while enabling sustainable chemical production. Emerging energy applications include of glycerol to generate and fuel gases, as demonstrated in a 2023 process where glycerol undergoes two-stage conversion—initial for followed by hydrodeoxygenation—yielding renewable fuels with potential to integrate into economies. Glycerol functions as a permeating cryoprotectant in low-temperature preservation of biological materials, reducing formation and osmotic stress in applications such as rooster cryopreservation (where it mitigates mechanical damage during freezing) and human storage (with 70% concentrations preserving viability effectively). In advanced drug delivery, glycerosomes—lipid vesicles incorporating glycerol as an edge activator and penetration enhancer—improve flux and stability over conventional liposomes, with formulations showing enhanced for hydrophobic drugs in topical applications. Additionally, polyglycerol-based are emerging as (PEG) alternatives in mRNA-loaded nanoparticles, reducing anti-PEG activation while maintaining delivery efficacy. In electronic cigarettes, vegetable glycerin—a purified form of glycerol—is a primary component of e-liquids, serving as a delivery vehicle for nicotine and flavorings. It is vaporized using controlled heating coils in vaping devices to produce inhalable aerosols.

Metabolism

Human and Animal Metabolism

Glycerol, released from triglycerides via in , circulates in the bloodstream and is primarily metabolized in the liver of mammals, where it undergoes by to glycerol-3-phosphate (Gro3P), followed by oxidation to (DHAP) for entry into or . In humans, this process contributes to glucose production during , with the liver accounting for the majority of glycerol uptake; studies using [U-13C]glycerol infusions in fasted subjects show hepatic glycerol utilization supports reesterification and , though only 2-3% of basal hepatic glucose output derives from glycerol at rest. Ethanol inhibits this hepatic metabolism by competing for NAD+ in the dehydrogenase step, reducing glycerol conversion to DHAP. In peripheral tissues like muscle, glycerol release correlates with nonesterified fatty acid (NEFA) mobilization, but direct metabolism is limited due to low glycerol kinase expression; instead, it serves as a for rates. Glycerol also participates in broader metabolic networks, such as serving as a precursor for de novo synthesis in the liver via pyruvate and TCA cycle intermediates, potentially aiding . Aquaglyceroporins (e.g., AQP7 in adipose, AQP9 in liver) facilitate glycerol transport, influencing and linking to conditions like and through altered flux. Animal metabolism mirrors human pathways in non-ruminant mammals, with PPARα regulating hepatic glycerol kinase and dehydrogenase expression to control gluconeogenesis; disruptions lead to elevated circulating glycerol and lipid accumulation. In dogs, exercise boosts glycerol-derived gluconeogenesis fourfold in diabetic models versus ninefold in normals, highlighting insulin's regulatory role. Ruminants differ due to rumen microbial fermentation, converting supplemental glycerol partly to volatile fatty acids like propionate (gluconeogenic precursor) and butyrate, enhancing energy availability during early lactation or ketosis. In dairy cattle, oral glycerol supplementation (up to 10% diet) improves feed efficiency, alleviates ketosis by mimicking glucose metabolism, and yields apparent metabolizable energy of 3.14-3.58 kcal/g without toxicity at moderate doses. Carnivorous fish like rainbow trout exhibit conserved hepatic genes for glycerol metabolism at the lipid-glucose interface, though utilization efficiency varies by species. Overall, glycerol's catabolic flexibility supports adaptive energy provision across mammals, with liver dominance and pathway intersections underscoring its role beyond mere lipid breakdown.

Microbial and Biotechnological Utilization

Glycerol, particularly crude glycerol derived as a byproduct from biodiesel production, serves as an effective carbon and energy source for numerous microorganisms in biotechnological applications due to its highly reduced state, which enables higher yields of reduced products compared to glucose. Microbes such as bacteria from genera Clostridium, Klebsiella, and Escherichia utilize glycerol via oxidative or reductive pathways, often under anaerobic conditions, to produce value-added compounds including biofuels, organic acids, and biopolymers. This utilization addresses the surplus of low-value crude glycerol, with global biodiesel production generating approximately 4 kg of glycerol per 100 kg of biodiesel since the early 2000s expansion. In reductive fermentation, glycerol is converted to 1,3-propanediol (1,3-PDO), a precursor for polyesters and antifreeze, by natural producers like Clostridium butyricum and engineered strains of Escherichia coli, achieving titers up to 100 g/L under optimized conditions. For instance, Anaerobium acetethylicum ferments glycerol to ethanol and hydrogen with minimal byproducts, yielding 1.2 mol ethanol and 1.8 mol hydrogen per mol glycerol in batch cultures reported in 2017. Hydrogen production via dark fermentation with mixed cultures or Enterobacter species reaches rates of 200-500 mL H₂/g glycerol, enhanced by impurities like methanol in crude glycerol acting as co-substrates. Organic acids such as are produced aerobically or anaerobically; Actinobacillus succinogenes in microbial fuel cell-assisted systems converts glycerol to succinate at 45 g/L yield while generating , as demonstrated in a 2021 study integrating bioelectrochemical enhancement. (PHAs), biodegradable plastics, accumulate in bacteria like Pseudomonas and grown on glycerol, with accumulation up to 80% of cell dry weight under nutrient-limited conditions, leveraging glycerol's role in the bypass. Yeasts and actinomycetes further expand applications; osmotolerant yeasts like and Yarrowia lipolytica bioconvert glycerol to (up to 50% cellular content) and ethanol, while actinomycetes such as species yield secondary metabolites like antibiotics when glycerol outperforms glucose as a substrate, as shown in optimizations from 2015 onward. Challenges include inhibitor tolerance to crude glycerol impurities (e.g., salts, ), addressed through microbial strain engineering and fed-batch processes achieving 90% conversion efficiencies in industrial pilots. These processes promote a glycerol model, reducing reliance on for chemicals like 1,3-PDO, which saw commercial microbial production scale to thousands of tons annually by 2010 via and Genencor collaborations.

Safety and Toxicity

General Toxicology Profile

Glycerol demonstrates low across multiple routes of exposure. Oral LD50 values in rats range from 5.57 g/kg to 12.6 g/kg, while dermal LD50 in rabbits exceeds 10 g/kg, indicating minimal risk from incidental or skin contact at typical exposure levels. At lethal doses, primary effects include gastrointestinal irritation, hyperemia, and osmotic disturbances leading to or , but these require of volumes far exceeding normal dietary intake. LC50 in rats surpasses 570 mg/m³ over 1 hour, with mist forms posing low respiratory hazard under threshold limit values of 10 mg/m³. However, when vegetable glycerin, a form of glycerol used in vaping, is heated to high temperatures, it undergoes thermal degradation, producing harmful byproducts such as formaldehyde and acetaldehyde, which can cause lung irritation, inflammation, and oxidative stress upon inhalation. Chronic toxicity studies in rodents show no evidence of systemic adverse effects, carcinogenicity, or genotoxicity at dietary levels up to 20% of intake, though localized gastrointestinal irritation may occur at high concentrations due to its hyperosmotic properties. Glycerol is not classified as a carcinogen by regulatory bodies and lacks reproductive or developmental toxicity in available animal models. Human data align with low risk, with the lowest observed toxic oral dose reported at 1.428 g/kg in specific contexts, but population-wide safety is affirmed by its endogenous role in metabolism and absence of adverse outcomes in long-term food additive use. Vegetable-derived glycerin, commonly used in cosmetics and food products, acts as a humectant that draws moisture to the skin. It has an EWG score of 1–2, indicating low hazard, and is considered completely safe for topical and oral use in small amounts. Regulatory assessments confirm glycerol's safety profile, designating it as (GRAS) by the U.S. FDA for direct food use under 21 CFR 182.1320, with no specified by EFSA due to its negligible toxicity and metabolic integration. Mild or eye is possible with undiluted exposure, but it is non-sensitizing and requires no special handling beyond standard precautions. Overall, empirical data support glycerol's classification as a substance of low hazard, with risks confined to acute overload scenarios rather than routine exposure.

Glycerol Intoxication Cases

Glycerol intoxication, though rare given its general safety profile as a and pharmaceutical agent, has been documented primarily in case reports involving acute high-dose exposure. In young children, consumption of glycerol-containing slush ice drinks has led to a distinct clinical characterized by rapid onset of symptoms including reduced consciousness in 94% of cases, in 95%, metabolic ( in 94%, and pseudohypertriglyceridemia in 89%. A analysis of 21 pediatric cases in the and , spanning approximately 15 years up to 2025, identified these incidents as linked to sugar-free slushies where glycerol served as a and , with children aged 2–7 years presenting within 30–60 minutes of ingestion. All affected children required emergency hospitalization but recovered fully after supportive care, including glucose administration and monitoring, without long-term sequelae. In adults, intoxication cases are even less common and typically arise from medical or diagnostic misuse rather than dietary exposure. A notable instance involved a 72-year-old male who developed progressive neurological symptoms—such as , , and —approximately 4 hours after of 120 grams of glycerol for Menière's disease diagnostics, resulting in severe intoxication with elevated serum glycerol levels and requiring for clearance. This case highlighted glycerol's potential for osmotic effects leading to cerebral and imbalances at doses exceeding 1–2 grams per kilogram body weight, though the patient recovered after intensive treatment. Another reported adult case mimicked toxic alcohol poisoning due to exogenous glycerol ingestion, presenting with , osmolar gap, and anion gap , but resolved with supportive measures once identified. These incidents underscore dose-dependent risks, particularly in vulnerable populations: young children exhibit heightened sensitivity due to lower body mass and immature metabolic pathways, while adults tolerate higher amounts unless compounded by underlying conditions or rapid absorption. No fatalities from isolated glycerol intoxication have been reported in the reviewed medical literature, distinguishing it from more toxic polyols. Public health responses, including advisories from agencies like the UK Food Standards Agency, recommend limiting glycerol-containing slushies for children under 4 years and monitoring intake to prevent such events.

Historical Contamination Incidents

One of the primary risks associated with glycerol in pharmaceutical and food applications has been adulteration with (DEG), a cheaper industrial with similar physical properties but high , leading to acute upon ingestion. Such contaminations often occur when unscrupulous suppliers substitute or mix DEG into bulk glycerol shipments, exploiting lax testing in supply chains from regions with minimal regulatory oversight. These incidents have disproportionately affected developing countries, resulting in hundreds of fatalities, mostly among children consuming contaminated oral medications. In 1986, adulterated glycerol supplied to a hospital caused 14 deaths from renal failure among patients aged 10 to 76 who ingested it as a pharmaceutical ; the glycerol had been laced with a toxic substitute, highlighting early gaps in domestic verification in . A more extensive outbreak occurred in from November 1995 to June 1996, where DEG-contaminated glycerin, imported from via , was used to produce acetaminophen syrup, resulting in at least 85 pediatric deaths from acute renal failure; post-mortem analyses confirmed DEG levels up to 18% in the glycerin, far exceeding safe thresholds. The incident prompted international alerts on glycerol purity testing, revealing how intermediate brokers can obscure origins in global trade. The 2006 Panama incident involved over 100 deaths, primarily adults, from DEG-adulterated glycerin in cough syrups manufactured by local firms; the contaminated glycerin originated from Chinese suppliers mislabeling industrial-grade material as pharmaceutical-grade, with DEG concentrations reaching 99% in some batches, underscoring persistent vulnerabilities despite prior warnings. Similar patterns emerged in that year, with 18 fatalities from DEG in domestic medications, though not directly tied to glycerol. Subsequent events in the and , including contaminated syrups in (2008-2009, 57 child deaths) and (2022), reinforced the need for spectroscopic verification of glycerol, as DEG lacks distinguishing taste or appearance; these cases, often linked to substandard imports, have driven global pharmacopeial standards like USP <467> for residual solvents. Overall, these incidents demonstrate causal links between economic incentives for adulteration and regulatory enforcement failures, rather than inherent flaws in glycerol itself.

History and Etymology

Discovery and Early Development

Glycerol was first isolated in 1779 by Swedish chemist Carl Wilhelm Scheele during experiments involving the heating of olive oil with litharge (lead(II) oxide), which produced a colorless, sweet-tasting, viscous liquid as a byproduct of fat hydrolysis. Scheele noted its syrupy consistency, solubility in water and alcohol, and lack of volatility, distinguishing it from known substances, though he did not fully characterize its composition at the time. This accidental discovery arose from early investigations into saponification processes, where fats were decomposed into soaps and other components. Scheele detailed his findings in a 1783 scientific article, marking the initial recognition of glycerol as a distinct chemical entity derived from natural . Subsequent early development in the late 18th and early 19th centuries focused on refining isolation methods and elucidating its role in ; for instance, French Michel-Eugène Chevreul advanced understanding by systematically analyzing the products of fat in the 1810s, identifying glycerol as the non-fatty acid residue and coining the name "glycerin" from glykeros (sweet), based on its and properties. Chevreul's work, grounded in empirical decomposition of animal and vegetable fats, established glycerol's trihydroxy structure indirectly through repeated distillations and precipitations, laying foundational chemical insights without modern analytical tools. By the 1820s, glycerol's purity and yield improved via hydrolysis of fats, enabling small-scale production for initial applications like sweetening and preserving fruits, though commercial scalability remained limited until later industrial advances. These efforts reflected causal linkages between breakdown and formation, confirmed through reproducible heating and extraction protocols rather than speculative theories.

Industrial History and Naming

Glycerol, systematically named propane-1,2,3-triol, derives its common name from the French term glycérine, coined by chemist Michel-Eugène Chevreul in 1811 to reflect its sweet taste, rooted in the Greek glykys meaning "sweet." The suffix shift to "-ol" in "glycerol" distinguishes it from amines, as the "-ine" ending misleadingly implies nitrogen content, a convention formalized in later chemical nomenclature. Industrial production originated as a of fat for soap-making, with the process documented since ancient times but first scaled commercially in the early . In 1811, Chevreul outlined the initial industrial recovery of glycerol during treatment of fats to yield soaps and fatty acids. By 1860, dedicated plants emerged, processing triglycerides with steam or acids to separate glycerol, which constituted about 10% of output from animal and vegetable fats. Demand surged during for explosives, prompting the 1908 Neuberg process using () under anaerobic conditions with to yield up to 12% glycerol from sugars, marking the first microbial industrial bioproduction. Post-war, synthetic routes from via chlorination and gained traction in the 1940s, reducing reliance on natural fats until expansion in the 2000s revived glycerol as a low-cost by-product, though early industrial focus remained on and derivatives.

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