Glycerin

Glycerin

"Glycerine" and "Glycerin" redirect here. For the Bush song, see Glycerine (song).
Template:Chembox ChEMBL
Glycerol
Identifiers
CAS number 56-81-5 YesY
PubChem 753
ChemSpider 733 YesY
UNII PDC6A3C0OX YesY
DrugBank DB04077
KEGG D00028 YesY
ChEBI CHEBI:17522 YesY
ATC code QA03
Jmol-3D images Image 1
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, 291 K, 64 °F

Boiling point

290 °C, 563 K, 554 °F ([2])

Refractive index (nD) 1.4746
Viscosity 1.412 Pa·s[1]
Hazards
MSDS External MSDS
NFPA 704
1
1
0
Flash point 160 °C (320 °F) (closed cup)
176 °C (349 °F) (open cup)
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Glycerol (or glycerine, glycerin) 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 of low toxicity.

Production

Approximately 950,000 tons per annum are produced in the USA and Europe; 350,000 tons of glycerol were produced per year in the United States alone from 2000–2004.[3] 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.[4]

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 glycerin 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.[5]

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).[4]

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.[4]

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 (see #Chemical intermediate).

Applications

Food industry

In foods 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 leaves.[6] 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 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.

Pharmaceutical and personal care applications

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. With similar benefits, glycerin is a common ingredient in many bath salts recipes. However, some assert that due to glycerin's moisture absorbing properties, it can be more of a hindrance than a benefit.[7]

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.[8]

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 a useful initial emergency treatment of severely elevated eye pressure.[9]

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. Glycerol is approximately 30% more slowly absorbed by the body resulting in a much lower glycemic load. 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 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 glycerin to make glycerites.[10][11]

When used as a primary 'true' alcohol-free (e.g. no alcohol (i.e. 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 possess such high extractive power assumes that glycerol, with its tri-atomic structure, is utilized with dynamic methodologies as opposed to standard passive 'tincturing' methodologies that are better suited to alcohol's di-atomic structure. Glycerol possesses the intrinsic property of not denaturing or rendering a botanical's constituents inert (as di-atomic alcohols – i.e. ethanolic (grain) alcohol, methanolic (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.[12][13][14]

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.[15][16]

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.

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 glycerine made it difficult to increase production to meet wartime demand. Hence, synthetic glycerin 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.

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.[17] The use of crude glycerin 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. In some organisms, the glycerol component can be converted into glucose by the liver and, thus, provide energy for cellular metabolism.

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 only in the liver. 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.[25]

Historical cases of contamination with diethylene glycol

Glycerol and diethylene glycol are similar in appearance, smell, and taste. The US Federal Food, Drug, and Cosmetic Act was passed following the 1937 "Elixir sulfanilamide" incident of poisoning caused by diethylene glycol contamination of medicine.

On May 4, 2007, the US Food and Drug Administration advised all US makers of medicines to test all batches of glycerol for the toxic diethylene glycol.[26] This followed an occurrence of hundreds of fatal poisonings in Panama resulting from a Chinese factory deliberately falsifying records in order to export the cheaper diethylene glycol as the more expensive glycerol.[27]

See also

References

Template:Sugar alcohols

External links

  • CDC - NIOSH Pocket Guide to Chemical Hazards - Glycerin (mist)
  • Process flow diagram of hydrolysis process to produce glycerol from fats/oils