Glycerol

"Glycerine" and "Glycerin" redirect here. For the Bush song, see Glycerine (song).
Glycerol
Names
IUPAC name
propane-1,2,3-triol
Other names
glycerin
glycerine
propanetriol
1,2,3-trihydroxypropane
Identifiers
ATC code A06AG04
A06AX01, QA16QA03
56-81-5 Yes
ChEBI CHEBI:17522 Yes
ChEMBL ChEMBL692 Yes
ChemSpider 733 Yes
DrugBank DB04077 Yes
Jmol-3D images Image
KEGG D00028 Yes
PubChem 753
UNII PDC6A3C0OX Yes
Properties
Molecular formula
 C3H8O3
Molar mass 92.09 g·mol−1
Appearance colorless liquid
hygroscopic
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)[1]
miscible[2]
Vapor pressure 0.003 mmHg (50°C)[2]
1.4746
Viscosity 1.412 Pa·s[3]
Hazards
MSDS External MSDS
NFPA 704
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g., canola oil Health code 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g., sodium chloride Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
1
0
0
Flash point 160 °C (320 °F; 433 K) (closed cup)
176 °C (349 °F; 449 K) (open cup)
US health exposure limits (NIOSH):
TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)[2]
None establisher[2]
N.D.[2]
Supplementary data page
Refractive index (n),
Dielectric constant (εr), etc.
Thermodynamic
data
 Phase behaviour
solid–liquid–gas
UV, IR, NMR, MS
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 Yes verify (what is: Yes/?)
Infobox references

Glycerol /ˈɡlɪsərɒl/[4] (also called glycerine or glycerin; see spelling differences) is a simple polyol (sugar alcohol) compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids known as triglycerides. Glycerol is sweet-tasting and generally considered non-toxic.

Production

Glycerol can be either synthetic, or derived from plants (usually soybeans) or animals (usually tallow). It can also be a blend of both animal and vegetable oils. Approximately 950,000 tons per year are produced in the United States and Europe; 350,000 tons of glycerol were produced per year in the United States alone from 2000–2004.[5] Production will increase as the EU directive 2003/30/EC is implemented, which requires the replacement of 5.75% of petroleum fuels with biofuel across all Member States by 2010, as glycerol is a byproduct in the production of biodiesel. It is projected that by the year 2020, production will be six times more than demand.[6]

From fats and oils

Triglycerides found in fats and oils are by definition esters of glycerol with long-chain carboxylic acids; the hydrolysis (saponification) or transesterification of these triglycerides produces stoichiometric quantities of glycerol. In this scheme, glycerol is produced as a co-product in the production of long-chain carboxylate salts used as soaps (see soap-making):

It is also a byproduct of the production of biodiesel via transesterification. This form of crude glycerol is often dark in appearance with a thick, syrup-like consistency. Triglycerides (1) are treated with an alcohol such as ethanol (2) with catalytic base to give ethyl esters of fatty acids (3) and glycerol (4):

Glycerol from triglycerides is produced on a large scale, but the crude product is of variable quality, with a low selling price of as low as 1–8 U.S. cents per pound in 2011. It can be purified, but the process is expensive. As a result, a good fraction of crude glycerol is disposed of as waste. Some glycerol is burned for energy, but the heat value is low.[7]

Crude glycerol from the hydrolysis of triglycerides can be purified by treatment with activated carbon to remove organic impurities, alkali to remove unreacted glycerol esters, and ion exchange to remove salts. High purity glycerol (> 99.5%) is obtained by multi-step distillation; vacuum is helpful due to the high boiling point of glycerol (290 °C).[6]

Synthetic glycerol

Synthetic glycerol refers to material obtained from non-triglyceride sources. Glycerol may also be produced by various routes from propylene. The epichlorohydrin process is the most important; it involves the chlorination of propylene to give allyl chloride, which is oxidized with hypochlorite to dichlorohydrins, which reacts with a strong base to give epichlorohydrin. Epichlorohydrin is then hydrolyzed to give glycerol. Chlorine-free processes from propylene include the synthesis of glycerol from acrolein and propylene oxide.[6]

Because of the emphasis on biodiesel, where glycerol is a waste product, the market for glycerol is depressed, and these old processes are no longer economical on a large scale. Due to the glycerol glut, efforts are being made to convert glycerol to its precursors, such as acrolein and epichlorohydrin.[8] (See the Chemical intermediate section of this article.)

Applications

Food industry

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.[9] As a sugar substitute, it has approximately 27 kilocalories per teaspoon (sugar has 20) and is 60% as sweet as sucrose. It does not feed the bacteria that form plaques and cause dental cavities. As a food additive, glycerol is labeled as E number E422. It is added to icing (frosting) to prevent it from setting too hard.

As used in foods, glycerol is categorized by the American Dietetic Association as a carbohydrate. The U.S. Food and Drug Administration (FDA) carbohydrate designation includes all caloric macronutrients excluding protein and fat. Glycerol has a caloric density similar to table sugar, but a lower glycemic index and different metabolic pathway within the body, so some dietary advocates accept glycerol as a sweetener compatible with low carbohydrate diets.

It is also recommended as an additive when using polyol sweeteners such as erythritol and xylitol which have a cooling effect, due to its heating effect in the mouth, if the cooling effect is not wanted.[10]

Pharmaceutical and personal care applications

Glycerol suppositories used as laxatives

Glycerol is used in medical and pharmaceutical and personal care preparations, mainly as a means of improving smoothness, providing lubrication and as a humectant. 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 U.S. FDA among the sugar alcohols as a caloric macronutrient.

Glycerol is a component of glycerin soap. Essential oils are added for fragrance. This kind of soap is used by people with sensitive, easily irritated skin because it prevents skin dryness with its moisturizing properties. It draws moisture up through skin layers and slows or prevents excessive drying and evaporation.

Glycerol can be used as a laxative when introduced into the rectum in suppository or small-volume (2–10 ml) (enema) form; it irritates the anal mucosa and induces a hyperosmotic effect.[11]

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.[12]

Botanical extracts

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 either a scientific (they have three hydroxyl groups), or consumable sense, but should in all instances more accurately be referred to as "Alcohol-Removed" products. Fluid extract manufacturers often extract herbs in hot water before adding glycerol to make glycerites.[13][14]

When used as a primary 'true' alcohol-free (e.g. no ethanol ever being used) botanical extraction solvent in innovative non-tincture based 'dynamic' 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. That glycerol possesses such high extractive power assumes it is utilized with dynamic methodologies as opposed to standard passive 'tincturing' methodologies that are better suited to alcohol. Glycerol possesses the intrinsic property of not denaturing or rendering a botanical's constituents inert (as alcohols – i.e. ethyl (grain) alcohol, methyl (wood) alcohol, etc., 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. Both glycerol and ethanol are viable preserving agents. Glycerol is bacteriostatic in its action, and ethanol is bactericidal in its action.[15][16][17]

Electronic cigarette liquid

Vegetable glycerin is a common component of e-liquid, a solution used with electronic cigarettes that is heated with an atomizer to produce aerosol vapor in order to deliver flavors and optionally nicotine.

Antifreeze

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 disrupts the crystal lattice formation of ice unless the temperature is significantly lowered. The minimum freezing point temperature is at about −36 °F / −37.8 °C 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.[18][19]

In the laboratory, glycerol is a common component of solvents for enzymatic reagents stored at temperatures below 0 °C 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 bacteria, nematodes, and mammalian embryos.

Internal Combustion Fuel

Used to power diesel generators supplying electricity for the FIA Formula E series of electric race cars.[20]

Chemical intermediate

Glycerol is used to produce nitroglycerin, which is an essential ingredient of various explosives such as dynamite, gelignite, and propellants like cordite. Reliance on soap-making to supply co-product glycerol made it difficult to increase production to meet wartime demand. Hence, synthetic glycerol processes were national defense priorities in the days leading up to World War II. Nitroglycerin, also known as glyceryl trinitrate (GTN) is commonly used to relieve angina pectoris, taken in the form of sub-lingual tablets, or as an aerosol spray.

Allyl iodide, a chemical building block for polymers, preservatives, organometallic catalysts, and pharmaceuticals, can be synthesized by using elemental phosphorus and iodine on glycerol.[21]

A great deal of research is being conducted to try to make value-added products from crude glycerol (typically containing 20% water and residual esterification catalyst) obtained from biodiesel production.[22] The use of crude glycerol as an additive to biomass for a renewable energy source when burned or gasified is also being explored.

Metabolism

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. 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 (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:

Glycerol Glycerol kinase Glycerol-3-phosphate Glycerol-3-phosphate dehydrogenase Dihydroxyacetone phosphate Triosephosphate isomerase Glyceraldehyde 3-phosphate
ATP ADP NAD+ NADH
NAD+ NADH

The enzyme glycerol kinase is present mainly in the liver and kidneys, but also in other body tissues, including muscle and brain.[30][31][32] In adipose tissue, glycerol 3-phosphate is obtained from dihydroxyacetone phosphate (DHAP) with the enzyme glycerol-3-phosphate dehydrogenase.

Glycerol has very low toxicity when ingested; its LD50 oral dose for rats is 12600 mg/kg and 8700 mg/kg for mice.

Historical cases of contamination with diethylene

On 4 May 2007, the US Food and Drug Administration advised all US makers of medicines to test all batches of glycerol for the toxic diethylene glycol.[33] This followed an occurrence of hundreds of fatal poisonings in Panama resulting from a falsified import customs declaration by Panamanian ex/im firm Aduanas Javier de Gracia Express, S. A.. The cheaper diethylene glycol was relabeled as the more expensive glycerol.[34][35]

See also

References

  1. ↑ Lide, D. R., ed. (1994). CRC Handbook of Data on Organic Compounds (3rd ed.). Boca Raton, FL: CRC Press. p. 4386.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 "NIOSH Pocket Guide to Chemical Hazards #0302". National Institute for Occupational Safety and Health (NIOSH).
  3. ↑ Segur, J. B.; Oberstar, H. E. (1951). "Viscosity of Glycerol and Its Aqueous Solutions". Industrial & Engineering Chemistry 43 (9): 2117. doi:10.1021/ie50501a040.
  4. ↑ Oxford dictionary: definition of glycerol (British & World English)
  5. ↑ Nilles, Dave (2005). "A Glycerin Factor". Biodiesel Magazine.
  6. ↑ 6.0 6.1 6.2 Christoph, Ralf; Schmidt, Bernd; Steinberner, Udo; Dilla, Wolfgang; Karinen, Reetta (2006). "Glycerol". Ullmann's Encyclopedia of Industrial Chemistry. Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a12_477.pub2. ISBN 3527306730.
  7. ↑ Sims, Bryan (25 October 2011). "Clearing the Way for Byproduct Quality: Why quality for glycerin is just as important for biodiesel". Biodiesel Magazine.
  8. ↑ Yu, Bin (2014). "Glycerol". Synlett 25 (4): 601. doi:10.1055/s-0033-1340636.
  9. ↑ Stevens, Alan. "Preserving flowers and decorative foliages with glycerin and dye" (PDF).
  10. ↑ Nikolov, Ivan. "Functional Food Design Rules".
  11. ↑ "Glycerin Enema". Drugs.com. Retrieved 17 November 2012.
  12. ↑ "Glycerin (Oral Route)". Mayo Foundation for Medical Education and Research. Retrieved 17 November 2012.
  13. ↑ Long, Walter S. (14 January 1916 – 13 January 1917). "The Composition of Commercial Fruit Extracts". Transactions of the Kansas Academy of Science 28: 157–161. doi:10.2307/3624347. JSTOR 3624347.
  14. ↑ Does Alcohol Belong In Herbal Tinctures? newhope.com
  15. ↑ Lawrie, James W. (1928) GLYCEROL AND THE GLYCOLS – Production, Properties and Analysis. The Chemical Catalog Company, Inc., New York, NY.
  16. ↑ Leffingwell, Georgia and Lesser, Miton (1945) GLYCERIN – Its Industrial and Commercial Applications. Chemical Publishing Co., Inc., Brooklyn, NY.
  17. ↑ The Manufacture of GLYCEROL – Vol. III (1956). The Technical Press, LTD., London, UK.
  18. ↑ Hudgens, R. Douglas; Hercamp, Richard D.; Francis, Jaime; Nyman, Dan A.; Bartoli, Yolanda (2007). "An Evaluation of Glycerin (Glycerol) as a Heavy Duty Engine Antifreeze/Coolant Base". doi:10.4271/2007-01-4000.
  19. ↑ Proposed ASTM Engine Coolant Standards Focus on Glycerin. Astmnewsroom.org. Retrieved on 15 August 2012.
  20. ↑ Formula E uses pollution-free glycerine to charge cars. fiaformulae.com. 13 September 2014
  21. ↑ Datta, Rasek Lal (1914). "The Preparation of Allyl Iodide". Journal of the American Chemical Society 36 (5): 1005–1007. doi:10.1021/ja02182a023.
  22. ↑ Johnson, Duane T.; Taconi, Katherine A. (2007). "The glycerin glut: Options for the value-added conversion of crude glycerol resulting from biodiesel production". Environmental Progress 26 (4): 338. doi:10.1002/ep.10225.
  23. ↑ Marshall, A. T. and Haverkamp, R. G. (2008). "Production of hydrogen by the electrochemical reforming of glycerol-water solutions in a PEM electrolysis cell". International Journal of Hydrogen Energy 33 (17): 4649–4654. doi:10.1016/j.ijhydene.2008.05.029.
  24. ↑ Melero, Juan A.; Van Grieken, Rafael; Morales, Gabriel; Paniagua, Marta (2007). "Acidic mesoporous silica for the acetylation of glycerol: Synthesis of bioadditives to petrol fuel". Energy Fuels 21 (3): 1782–1791. doi:10.1021/ef060647q.
  25. ↑ "Dow achieves another major milestone in its quest for sustainable chemistries" (Press release). Dow Chemical Company. 15 March 2007.
  26. ↑ Ott, L.; Bicker, M.; Vogel, H. (2006). "The catalytic dehydration of glycerol in sub- and supercritical water: a new chemical process for acrolein production". Green Chemistry 8 (2): 214–220. doi:10.1039/b506285c.
  27. ↑ Watanabe, Masaru; Iida, Toru; Aizawa, Yuichi; Aida, Taku M.; Inomata, Hiroshi (2007). "Acrolein synthesis from glycerol in hot-compressed water". Bioresource Technology 98 (6): 1285–1290. doi:10.1016/j.biortech.2006.05.007. PMID 16797980.
  28. ↑ Yazdani, S. S. and Gonzalez, R. (2007). "Anaerobic fermentation of glycerol: a path to economic viability for the biofuels industry". Current Opinion in Biotechnology 18 (3): 213–219. doi:10.1016/j.copbio.2007.05.002. PMID 17532205. Lay summary – ScienceDaily (27 June 2007).
  29. ↑ "Dow Epoxy advances glycerine-to-epichlorohydrin and liquid epoxy resins projects by choosing Shanghai site" (Press release). Dow Chemical Company. 26 March 2007.
  30. ↑ Tildon, J. T.; Stevenson Jr, J. H.; Ozand, P. T. (1976). "Mitochondrial glycerol kinase activity in rat brain". The Biochemical journal 157 (2): 513–6. PMC 1163884. PMID 183753.
  31. ↑ Newsholme, E. A.; Taylor, K (May 1969). "Glycerol kinase activities in muscles from vertebrates and invertebrates". Biochem. J. 112 (4): 465–74. PMC 1187734. PMID 5801671.
  32. ↑ Jenkins, B. T.; Hajra, A. K. (1976). "Glycerol Kinase and Dihydroxyacetone Kinase in Rat Brain". Journal of Neurochemistry 26 (2): 377–385. doi:10.1111/j.1471-4159.1976.tb04491.x. PMID 3631.
  33. ↑ "FDA Advises Manufacturers to Test Glycerin for Possible Contamination". U.S. Food and Drug Administration. 4 May 2007. Retrieved 8 May 2007.
  34. ↑ Walt Bogdanich (6 May 2007). "From China to Panama, a Trail of Poisoned Medicine". New York Times. Retrieved 8 May 2007.
  35. ↑ "10 Biggest Medical Scandals in History". 20 February 2013.

External links