Glycerol

Glycerol
IUPAC name propan-1,2,3-triol
Other names glycerin glycerine propane-1,2,3-triol 1,2,3-propanetriol 1,2,3-trihydroxypropane glyceritol glycyl alcohol
Identifiers
CAS number	56-81-5 Y
PubChem	753
ChemSpider	733
ATC code	A06AG04,A06AX01, QA16QA03
SMILES OCC(O)CO
Properties
Molecular formula	C3H5(OH)3
Molar mass	92.09382 g/mol
Appearance	clear, colorless liquid hygroscopic
Odor	odorless
Density	1.261 g/cm³
Melting point	17.8 °C (64.2°F)
Boiling point	290 °C (554°F)[1]
Refractive index (nD)	1.4746
Viscosity	1.2 Pa·s
Hazards
MSDS	JT Baker
NFPA 704	1 1 0
Flash point	160 °C (closed cup) 176 °C (open cup)
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Glycerol is an organic compound, also called glycerin or glycerine. It is a colourless, odourless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydrophilic hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol substructure is a central component of many lipids. Glycerol is sweet-tasting and of low toxicity.

Synthesis and production

Glycerol (3D model), showing the atoms and the lone electron pairs (in pink) associated with the oxygen atoms (in red)

Glycerol forms the backbone of triglycerides, and can be produced by saponification of animal fats, e.g. a byproduct of soap-making. It also is a byproduct of the production of biodiesel via transesterification. Because of the emphasis on biodiesel, the market for glycerol is depressed, and the old epichlorohydrin process for glycerol synthesis is no longer economical. Approximately 950,000 tons per annum are produced in the USA and Europe. 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^[2].

Glycerol is also produced as a byproduct of refining of cooking and salad oils, and various brands (e.g, NOW) are sold to the retail market as "Pure Vegetable Source" glycerin, 100% pure, which is safe for ingestion.

Applications

Foods 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. As a sugar substitute, it has approximately 27 calories per teaspoon and is 60 percent as sweet as sucrose. Although it has about the same food energy as table sugar, it does not raise blood sugar levels, nor does it feed the bacteria that form plaques and cause dental cavities. As a food additive, glycerol is labeled as E number E422.

Glycerol is also used to manufacture mono- and di-glycerides for use as emulsifiers, as well as polyglycerol esters going into shortenings and margarine.

It is also used as a humectant (along with propylene glycol labelled as E1520 and/or E422) in the production of snus, a Swedish style smokeless tobacco product that the Swedish government subjects to the same regulations as "food" because it is used orally.

Categorization

Glycerol is currently categorized by the American Dietetic Association as a carbohydrate. The U.S. FDA carbohydrate designation includes all caloric macronutrients excluding protein and fat. Glycerin 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 glycerin as a sweetener compatible with low carbohydrate diets.

Organic chemistry

In organic synthesis, glycerol is used as a readily available prochiral building block. Even if glycerol with no substitutions is symmetrical, and carbon atoms 1 and 3 are exchangeable, once one of them forms an ester or ether bond, the two are no longer exchangeable. Further bond formation and lysis may lead to products substituted solely at the third carbon; due to such circumstances, to maintain both full description and conformance to the chemistry naming rules (which require carbon counting to minimize ordinal numbers of substituents), the carbons are named sn-1, sn-2, and sn-3, with "sn" standing for "sterospecifical numbering".^[3]

Physical properties

Like ethylene glycol and propylene glycol, glycerol dissolved in water disrupts the hydrogen bonding between water molecules such that the mixture cannot form a stable crystal structure unless the temperature is significantly lowered. The minimum freezing point temperature is at about -36 °F / -37.8 °C corresponding to 60-70 % glycerol in water, as shown in the table below. Thus, glycerol has anti-freeze properties.

Foams and polymers

Glycerol is one of the major raw materials for the manufacture of polyol-based flexible foams, and to a lesser extent rigid polyurethane foams.

Nitroglycerine

Glycerol is used to produce nitroglycerin, or glycerol-trinitrate (GTN), which is an essential ingredient of smokeless gunpowder and 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 defence priorities in the days leading up to World War II. GTN is commonly used to relieve angina pectoris, taken in the form of sub-lingual tablets, or as an aerosol spray.

Research laboratory usage

Glycerol is a common component of solvents for enzymatic reagents stored at temperatures below zero degrees Celsius due to the depression of the freezing temperature of solutions with high concentrations of glycerol. It is also dissolved in water to reduce damage by ice crystals to laboratory organisms that are stored in frozen solutions, such as bacteria, nematodes, and fruit flies. Samples are loaded into agarose gel electrophoresis mixed in loading buffers that mainly consist of glycerol; when the sample is injected into wells, the glycerol causes the solution to sink through the running buffer to the bottom of the well.

Glycerol is also used by manufacturers to condition dialysis membranes and must be completely washed out prior to use. This is particularly important when the dissolved organic carbon (DOC) has to be measured in solution afterward. This is the case for the laboratory studies on the complexation of radionuclides by humic substances.

Pharmaceutical, botanical manufacture 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 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. It is also an ingredient in cigarettes that is used as a humectant. For human consumption, glycerol is classified by the U.S. FDA among the sugar alcohols as a caloric macronutrient.

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 a substitute for ethanol as a solvent in preparing herbal extractions. It is less extractive when utilized in tincture methodology and is approximately 30% more slowly absorbed by the body resulting in a much lower glycemic load. Fluid extract manufacturers often extract herbs in hot water before adding glycerin to make glycerites.^[4][5][6]

When used as a primary true alcohol-free botanical extraction solvent in innovative non-tincture based methodologies, Glycerol, as a glycerin and water base extractive solvent mix, has been shown, both in literature and through innovative extraction applications, to possess a high degree of extractive versatility for botanicals including removal of numerous constituents and complex compounds, often equal to or greater than that for ethanol. Glycerol is a stable preserving agent for botanical extracts that, when utilized in proper concentrations in an extraction solvent base, not only preserves safety and purity but does not allow inverting or REDOX of a finished extract's constituents over many years. Both Glycerol and ethanol are viable preserving agents. Glycerol is bacteriostatic in its action, and ethanol is bactericidal in its action. However, Glycerol possesses no secondary denaturing or inert rendering effects on a botanical extract's constituents, hence, Glycerol's preferred use in making many botanical extracts and use in pharmaceuticals where this quality is required.^[7][8][9]

Used as a laxative when introduced into the rectum in suppository or small-volume (2to10ml)(enema) form; irritates the anal mucosa and induces a hyperosmotic effect.

Glycerol is a component of glycerol soap, which is made from denatured alcohol, glycerol, sodium castorate (from castor), sodium cocoate, sodium tallowate, sucrose, and water. Sometimes one adds sodium laureth sulfate, or essential oils 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. It is possible to make glycerol soap at home.

Topical pure or nearly pure glycerol is an effective treatment for psoriasis, burns, bites, cuts, rashes, bedsores, and calluses.^{[Needs footnote.]} It can be used orally to eliminate halitosis, as it is a contact bacterial desiccant.^{[Needs footnote.]} The same property makes it very helpful with periodontal disease; it penetrates biofilm quickly and eliminates bacterial colonies.^{[Needs footnote.]}

Alternative chemical and fuel feedstock

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, as an alternative to disposal by incineration.

• Hydrogen gas production unit^[10]
• Glycerine acetate (as a potential fuel additive)^[11]
• Conversion to propylene glycol^[12]
• Conversion to acrolein^[13][14]
• Conversion to ethanol^[15]
• Conversion to epichlorhydrin,^[16] a raw material for epoxy resins

Specialized and potential uses

• Glycerol is used for an evaporative fogging agent as an alternative to Propylene Glycol in some solutions for "electronic cigarette" use, mostly for those with a physiological sensitivity or other health concerns.
• It is also used in de-icing/anti-icing fluids, as in vitrification of blood cells for storage in liquid nitrogen.
• In motion-picture production, glycerol is used as a non-evaporating substitute for perspiration or tears on actors. It is also used in some types of stage blood.
• Compost additive
• For tincture extraction and preservation of essential oils and chemicals from herbs
• Citric acid production
• Cosmetic bonding agent for makeup, including: eye shadow, lipstick, lipgloss, lotions and eyedrops
• Glycerol may be used as antifreeze for plants, if mixed with water in a 10 percent solution. It is believed to be effective at temperatures near -18 °C
• Glycerol can also be used as a bodybuilding supplement to increase nitric oxide (NO) uptake in the muscles and act in a similar way to the amino acid arginine to create swelling in the working muscle for the individual
• Glycerin, when poured on potassium permanganate, will combust making it a useful firelighting tool.
• Glycerine is used as an anti-drying agent in watercolor paints.
• Glycerine is used as a preservative to keep tobacco mixture for use in hookahs moist.

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 . In animals, wherein glycerol is derived from glucose (e.g., humans and other mammals), glycerol is sometimes not considered a true gluconeogenic substrate, as it cannot be used to generate new glucose.

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.

Historical cases of contamination with diethylene glycol

On May 4, 2007, the US Food and Drug Administration advised all US makers of medicines to test all batches of glycerine for the toxic diethylene glycol.^[17] This follows an occurrence of 100 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.^[18] Glycerine 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.

Additional physical properties

Its surface tension is 64.00 mN/m at 20 °C , and it has a temperature coefficient of -0.0598 mN/(m K).^[19]

See also

• Biodiesel by-product
• Epichlorohydrin
• Nitroglycerin
• Oleochemicals
• Saponification
• Transesterification

References

• U.S. Health.gov dietary guidelines

External links

• Glycerol on BioChemInfo.org
• Absolute alcohol using glycerol