Glycerol

Glycerol is a type of organic polyol compound. It's a colourless, odourless, viscous liquid that is sweet-tasting and non-toxic, but it is not recommended for deliberate ingestion unless used as a suppository. The glycerol backbone is present in glycerides, which are lipids with a glycerol backbone. It's commonly used in FDA-approved wound and burn procedures because of its antimicrobial and antiviral properties. It may also be used as a reliable indicator of liver disease. It is most commonly used in the food industry as a sweetener and as a humectant in pharmaceutical formulations. Glycerol is hygroscopic and miscible with water due to the presence of three hydroxyl groups.

Structure of Glycerol

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Chemical structure

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Ball and stick structure

Glycerol is a triol with a propane structure that is replaced by hydroxy (OH) groups at positions 1, 2, and 3.  Despite being achiral, glycerol is prochiral in reactions involving one of the two main alcohols. Thus, in substituted derivatives, the stereospecific numbering prefixes the stem name of the molecule with an "sn-" prefix. It works as an osmolyte, a solvent, a detergent, a human metabolite, an algal metabolite, a Saccharomyces cerevisiae metabolite, an Escherichia coli metabolite, and a mouse metabolite, among other things. It's a combination of an alditol and a triol.

Glycerol Production

Glycerol is found in triglycerides, which are glycerol esters of long-chain carboxylic acids, and are derived from plant and animal sources. Soybeans and palm trees are popular plant sources. Glycerol from triglycerides is processed on a large scale. Purification is possible, but the procedure is costly. Glycerol is burned for energy, but it has a low heat value. These triglycerides are hydrolyzed, saponified, or transesterified to produce glycerol and fatty acid derivatives. Glycerol and fatty sodium salt or soap can be made by saponifying triglycerides with sodium hydroxide.

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Treatment with activated carbon to extract organic impurities, alkali to remove unreacted glycerol esters, and ion exchange to remove salts may all be used to purify crude glycerol obtained through the hydrolysis of triglycerides. Multi-step distillation produces high purity glycerol, which necessitates the use of a vacuum chamber due to its high boiling point (around 290 °C).

Glycerol Synthesis

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Glycerol can be made from propene in a variety of ways, but it is typically not cost-effective. The most significant is the “epichlorohydrin process”.

• This involves chlorinating propylene to produce allyl chloride. 

• The above-produced allyl chloride is then oxidised with hypochlorite to produce dichlorohydrins.

• This then reacts with a strong base to produce epichlorohydrin. 

• Glycerol is generated after the epichlorohydrin is hydrolyzed. 

The synthesis of glycerol from acrolein and propylene oxide are examples of chlorine-free propylene processes. The demand for glycerol is depressed as a result of large-scale production of biodiesel from fats, where glycerol is a waste product. As a result, synthetic methods are not cost-effective. Due to an abundance of glycerol, attempts are being made to transform it into synthetic precursors including acrolein and epichlorohydrin.

Types of Glycerine

Natural Glycerine- Natural glycerine, also called glycerol or glycerine, is a transparent liquid derived from soybean, coconut, or palm oils. Examples- vegetable glycerine.

Synthetic Glycerine- The substance made from petrochemical raw materials such as propylene, allyl chloride, epichlorohydrin, glycerol chlorohydrins, acrolein, or allyl alcohol is known as synthetic glycerol. Examples- Borax glycerin, syrup glycerol or glycerine (syp glycerol).

Use of Glycerol

• Glycerin is an FDA-approved wound medication that is mildly antimicrobial and antiviral. Glycerin in an 85 per cent solution has bactericidal and antiviral properties, according to the Red Cross. The glycerin used in wound treatment shows decreased inflammation after about 2 hours. As a result, it's commonly used in wound care materials, such as glycerin-based hydrogel sheets for burns and other wounds. It is used to package donor skin used in skin grafts and is certified for all forms of wound treatment except third-degree burns. Since no topical treatment for third-degree burns has been accepted, this restriction is not exclusive to glycerin.

• Glycerol is often used in medicinal, pharmaceutical, and personal care products to improve smoothness, lubrication, and as a humectant.

• The use of glycerine as a topical treatment for ichthyosis and xerosis has proven to be effective. Allergic immunotherapies, cough syrups, elixirs, and expectorants, toothpaste, mouthwashes, skincare products, shaving cream, hair care products, soaps, and water-based personal lubricants all contain it. Glycerine is used as a tablet keeping agent in stable dosage forms such as capsules. The United States Food and Drug Administration classifies glycerol as a caloric macronutrient among the sugar alcohols for human consumption. Glycerol is also used in blood banks to keep red blood cells alive until they are frozen.

• Glycerol is one of the ingredients in glycerin soap. For fragrance, essential oils are used. Because of its moisturising properties, this type of soap is preferred by people who have sensitive, easily irritated skin. It pulls moisture up through the layers of the skin, preventing or slowing excessive drying and evaporation.

• Glycerol acts as a laxative by irritating the anal mucosa and causing a hyperosmotic reaction, which causes the colon to expand by drawing water into it, resulting in evacuation. It can be used as a suppository or a small-volume (2–10 ml) enema without being diluted. Alternatively, it can be given as a high volume enema in a dilute solution, such as 5%.

• Glycerol can cause a rapid, temporary decrease in the internal pressure of the eye when taken orally, generally combined with fruit juice to mask the sweet taste. This can help treat highly elevated eye pressure in the short term.

• In the field of bioprinting, glycerol has also been used as a part of bio-ink formulations. The presence of glycerol in the bio-ink adds viscosity without incorporating large protein, starch, or glycoprotein molecules.

• Glycerol prevents tannins from precipitating in ethanol extracts of plants when used in "tincture" process extractions, specifically as a 10% solution. It's also used in herbal extractions as an alcohol-free alternative to ethanol as a solvent. When used in a traditional tincture process, it is less extractive. The alcohol in alcohol-based tinctures may be extracted and replaced with glycerol for preservation purposes. Since glycerol comprises three hydroxyl groups, such products are not "alcohol-free" in the science or FDA regulatory context. Manufacturers of fluid extracts often extract herbs in hot water before adding glycerol to create glycerites.

• Glycerol has been shown to have a high degree of extractive flexibility for botanicals when used as a primary "real" alcohol-free botanical extraction solvent in non-tincture dependent methodologies, including the removal of various constituents and complex compounds, with an extractive power that can rival that of alcohol and water-alcohol solutions. The fact that glycerol has such high extractive strength implies that it is used with dynamic methodologies rather than the more traditional passive "tincturing" methods that are best suited to alcohol. Glycerol, unlike alcohols, has the property of not denaturing or making the constituents of a botanical inert. Glycerol is a stable preserving agent for botanical extracts that, when used at the right concentrations in an extraction solvent base, prevents inverting or mitigates reduction-oxidation (redox nature) of a finished extract's constituents over time. Both glycerol and ethanol can be used as preservatives. Glycerol has a bacteriostatic effect, whereas ethanol has a bactericidal effect.

• In electronic cigarettes, glycerine is commonly used to produce vapour. Glycerine is a common component of e-liquid, a solvent used with electronic vaporizers, along with propylene glycerol (electronic cigarettes). The glycerol is heated in an atomizer (a heating coiled material made of Kanthal wire) to create the aerosol that delivers nicotine to the consumer.

• Glycerol, like ethylene glycerol and propylene glycerol, is a non-ionic kosmotrope that competes with water-water hydrogen bonds by forming strong hydrogen bonds with water molecules. The formation of ice is disrupted by this interaction. The minimum freezing point temperature is about 36 degrees Fahrenheit (38 degrees Celsius), which corresponds to 70 per cent glycerol in water.

• Before being substituted by ethyl glycerol, which has a lower freezing point, glycerol was used as an anti-freeze in automotive applications. Although a glycerol-water mixture has a higher minimum freezing point than an ethylene glycol-water mixture, glycerol is not poisonous and is being reconsidered for use in automotive applications.

• Due to the depression of the freezing point, glycerol is a common component of solvents for enzymatic reagents stored at temperatures below 0 °C in the laboratory. It's also used as a cryoprotectant, in which glycerol is dissolved in water to protect laboratory organisms stored in frozen solutions, such as fungi, bacteria, nematodes, and mammalian embryos, from ice crystal damage.

• Glycerol is used to make nitroglycerin, which is a key component of explosives like dynamite and gelignite, as well as propellants like cordite. Because of the reliance on soap-making to supply co-product glycerol, increasing production to meet wartime demand was difficult. As a result, in the years leading up to World War II, synthetic glycerol processes were high on the national defence priority list. 

• Glyceryl trinitrate (GTN), also known as nitroglycerin, is a drug that is widely used to treat angina pectoris. It comes in the form of sublingual tablets or an aerosol spray.

• Glycerol is used to dampen vibration in pressure gauges. External vibrations, such as those from compressors, motors, and pumps, cause harmonic vibrations in Bourdon gauges, causing the needle to travel excessively and giving inaccurate readings. Excessive needle swinging can damage internal gears and other components, resulting in premature wear. As glycerol is poured into a gauge to replace the air room, the harmonic vibrations transmitted to the needle are reduced, extending the gauge's lifespan and reliability.

Did You Know That?

• Diabetes causes high blood glycerol levels, which are thought to be the cause of decreased fertility in patients with diabetes and metabolic syndrome. Diabetic patients have three times the amount of glycerol in their blood than healthy people. The treatment of testes with direct glycerol has been shown to result in a substantial long-term reduction in sperm count.

• Glycerol is also broken down in the liver. Since the rate of absorption by the liver is considered a reliable indicator of liver health, glycerol injections may be used as a simple test for liver harm. Cirrhosis and fatty liver disease also cause a decrease in glycerol metabolism.