Glycol is any type of organic compound that belongs to the alcohol family, with two hydroxyl (-OH) groups attached to different carbon atoms in the molecule. Often, the word is applied to the simplest member of the class, which is ethylene glycol. Ethylene glycol, also known as 1,2-ethanediol and having the molecular formula HOCH2-CH2OH, is a colorless, oily liquid with a mild odour and sweet taste.
Glycol is commercially produced from ethylene oxide, which can be obtained from ethylene. Ethylene glycol is widely used as an antifreeze in the production of human-made fibres, low-freezing explosives, and brake fluid. Ethylene glycol, including some of its derivatives, is mildly toxic.
Propylene glycol, also known as 1,2-propanediol, has physical properties that are similar to ethylene glycol. However, unlike ethylene glycol, propylene glycol is not said to be toxic and is used extensively in cosmetics, foods, and oral hygiene products as a preservative, solvent, and also as a moisture-retaining agent. Propylene glycol is also manufactured in huge amounts from propylene oxide, which can be obtained from propylene.
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The other essential glycols are 1,3-butanediol, which can be used as a starting material for brake fluid manufacturing and of plasticizers for resins; 1,4-butanediol is used in polyester resins and in polyurethanes for plasticizers and coatings, and also for making butyrolactone, which is a valuable chemical and solvent intermediate; 2-methyl-2-propyl-1, 3-propanediol, made into meprobamate, which is used as a widely used tranquillizer; 2-ethyl-1, 3-hexanediol, which is an effective insect repellent.
Production of Glycol
Ethylene glycol is formed from ethylene (otherwise called ethene) via the intermediate ethylene oxide. The ethylene oxide reacts with water to form ethylene glycol as per the chemical equation given below.
C2H4O + H2O → HO−CH2CH2−OH
This chemical reaction is catalyzed either by acids or bases or can also take place at neutral pH under elevated temperatures. The ethylene glycol highest yield takes place at neutral or acidic pH with an excess of water. Under certain conditions, 90% yields of ethylene glycol can be achieved. The primary by-products are given as triethylene glycol, tetraethylene glycol, and oligomers diethylene glycol. The separation of these water and oligomers is energy-intensive. Annually, up to 6.7 million tonnes of ethylene glycol are produced.
The caterpillar of the Galleria mellonella, a Greater wax moth, has gut bacteria with the ability to degrade the polyethylene (PE) into the ethylene glycol.
As per most of the sources, in 1856, Charles-Adolphe Wurtz, a French chemist, first prepared ethylene glycol. He treated "ethylene iodide" (C2H4I2) first with the silver acetate and later hydrolyzed the resultant "ethylene diacetate" mixture with potassium hydroxide. And Wurtz named his new compound "glycol" due to its shared qualities with both glycerin (with three hydroxyl groups) and ethyl alcohol (with one hydroxyl group). In 1859, Wurtz also prepared the ethylene glycol through the hydration of ethylene oxide. There appears to have been no commercial application or manufacture of ethylene glycol before World War I, when it was synthesized in Germany from the ethylene dichloride and used as a substitute for glycerol in the industry of explosives.
Ethylene glycol can be used as a protecting group for carbonyl groups in organic synthesis. When an aldehyde or ketone is treated with ethylene glycol in the presence of an acid catalyst (for example, p-toluenesulfonic acid; BF3Et2O), a 1,3-dioxolane is formed that is resistant to bases and other nucleophiles. Thereafter, the 1, the 3-dioxolane protecting group is removed by the further process of acid hydrolysis. In this case, isophorone was protected in a moderate yield using ethylene glycol and p-toluenesulfonic acid. To shift the equilibrium to the right, water was removed by azeotropic distillation.
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Uses of Glycol
Primarily, ethylene glycol is used in antifreeze formulations (with a percentage of 50%) and as a raw material in polyester manufacturing like polyethylene terephthalate (PET) (with a percentage of 40%).
Coolant and Heat-Transfer Agent
The primary usage of ethylene glycol is the same as a medium for convective heat transfer—for example, liquid-cooled computers and automobiles. Ethylene glycol is a common coolant in chilled-water air-conditioning systems, where the air handlers or chillers are either outside or the water must be cooled to below freezing temperatures. Ethylene glycol is the fluid used in geothermal cooling and heating systems to transport heat using a geothermal heat pump. Depending on whether the system is being used for cooling or heating, the ethylene glycol either gains energy from the source (water well, lake, ocean) or dissipates the heat to the sink.
Pure ethylene glycol freezes at a temperature of −12 °C. But, when it is mixed with water, the mixture freezes at a very lower temperature. For example, a 60% ethylene glycol mixture and 40% of water freezes at a temperature of −45 °C. Diethylene glycol behaves in a similar manner. The freezing point depression of a few mixtures is explained as a colligative property of the solutions; however, concerning the highly concentrated mixtures, such as the deviations from ideal solution behaviour are expected because of the influence of the intermolecular forces.