Ethylene glycol
Ethylene glycol (monoethylene glycol (MEG), 1,2-ethanediol, IUPAC name: ethane-1,2-diol) is an alcohol with two -OH groups (a diol), a chemical compound widely used as an automotive antifreeze. In its pure form, it is an odorless, colorless, syrupy, sweet tasting, toxic liquid.
Production
Ethylene glycol is produced from ethylene, via the intermediate ethylene oxide. Ethylene oxide reacts with water to produce ethylene glycol according to the chemical equation
This reaction can be catalyzed by either acids or bases, or can occur at neutral pH under elevated temperatures. The highest yields of ethylene glycol occur at acidic or neutral pH with a large excess of water. Under these conditions, ethylene glycol yields of 90% can be achieved. The major byproducts are the ethylene glycol oligomers diethylene glycol, triethylene glycol, and tetraethylene glycol.
This molecule has been observed in space.[1]
Uses
Coolant
The major use of ethylene glycol is as an antifreeze in, for example, automobiles and personal computers. Due to its low freezing point, it is also used as a deicing fluid for windshields and aircraft. Ethylene glycol is also commonly used in chilled water air conditioning systems that place either the chiller or air handlers outside, or systems that must cool below the freezing temperature of water.
Hydrate inhibition
Ethylene glycol is being widely used to inhibit the formation of natural gas clathrates in long multiphase pipelines that convey natural gas from remote gas fields back to an onshore processing facility. Ethylene glycol can be recovered from the natural gas and reused as an inhibitor after a purification treatment that removes water and inorganic salts.
Manufacturing
Ethylene glycol has become increasingly important in the plastics industry for the manufacture of polyester fibers and resins, including polyethylene terephthalate, which is used to make plastic bottles for soft drinks. The antifreeze capabilities of ethylene glycol have made it an important component of vitrification mixtures for low-temperature preservation of biological tissues and organs.
Minor uses of ethylene glycol include the manufacture of capacitors, as a chemical intermediate in the manufacture of 1,4-dioxane and as an additive to prevent corrosion in liquid cooling systems for personal computers.
Chemistry
Ethylene glycol may also be used as a protecting group for carbonyl groups in organic synthesis. Reacting a ketone or aldehyde with ethylene glycol will, with acid catalyst (e.g. p-toluenesulfonic acid; BF3·Et2O), give a cyclic acetal — a 1,3-dioxolane, which is resistant to bases and other nucleophiles. The 1,3-dioxolane protecting group can thereafter be removed, e.g. by further acid hydrolysis.[2] In this example, isophorone was protected using ethylene glycol with p-toluenesulfonic acid in moderate yield. Water was removed by azeotropic distillation to shift the equilibrium to the right.[3]
Geothermal Systems
Ethylene glycol is commonly used in geothermal heating/cooling systems. The ethylene glycol is the fluid that is passed through the loops of hoses in geothermal systems to transport heat through the use of a geothermal heat pump. The ethylene glycol either gains energy from the source (lake, ocean, water well) or dissipates heat to the source depending if the system is being used for heating or cooling.
Laboratory use
Ethylene glycol is commonly used in laboratories to precipitate out proteins in solution. This is often an intermediary step in fractionation, purification and/or crystallization. It can be used to protect functional groups from reacting during organic synthesis. To get the functional group back to its original composition, simply add water and acid.
Ethylene glycol is commonly used as a preservative for specimens in schools, frequently during dissection. It is said to be safer than formaldehyde, but the safety is questionable.
Other applications
Ethylene glycol's high boiling point and affinity for water makes it an ideal desiccant for natural gas production. In the field, excess water vapor is usually removed by glycol dehydration. Ethylene glycol flows down from the top of a tower and meets a rising mixture of water vapor and hydrocarbon gases from the bottom. The glycol chemically removes the water vapor, allowing dry gas to exit from the top of the tower. The glycol and water are separated, and the glycol cycles back through the tower.
Instead of removing water ethylene glycol can also be used to depress the temperature at which hydrates are formed. The purity of glycol used for hydrate suppression (mono-ethylene glycol) is typically around 80%, whereas the purity of glycol used for dehydration (tri-ethylene glycol) is typically 95-99+%. Moreover, the injection rate for hydrate suppression is much lower than the circulation rate in a glycol dehydration tower.
Ethylene glycol is also used in the manufacture of some vaccines, but it is not itself present in these injections. It is used as a minor (1–2%) ingredient in shoe polish and also in some inks and dyes. Ethylene glycol has seen some use as a rot and fungal treatment for wood, both as a preventative and a treatment after the fact. It has been used in a few cases to treat partially rotted wooden objects to be displayed in museums. It is one of only a few treatments that are successful in dealing with rot in wooden boats, and is relatively cheap. Ethylene glycol may also be one of the minor ingredients in screen cleaning solutions, along with the main ingredient isopropyl alcohol.
Toxicity
The major danger from ethylene glycol is ingestion. Due to its sweet taste, children and animals will sometimes consume large quantities of it if given access to antifreeze.
Industrial hazards
Ethylene glycol can begin to breakdown at 230° – 250°F (110° – 121°C). Note that breakdown can occur when the system bulk (average) temperature is below these limits because surface temperatures in heat exchangers and boilers can be locally well above these temperatures.
The electrolysis of ethylene glycol solutions with a silver anode results in an exothermic reaction. In the Apollo 1 fire catastrophe a coolant consisting of ethylene glycol and water was implicated as a possible cause via this reaction. An ethylene glycol–water mixture can be ignited and burns in an atmosphere of pure low pressure oxygen.[citation needed]
History
Ethylene glycol was first prepared in 1859 by the French chemist Charles-Adolphe Wurtz. It was produced on a small scale during World War I as a coolant and as an ingredient in explosives. Widespread industrial production began in 1937 when ethylene oxide, a component in its synthesis, became cheaply available.
When first introduced it created a minor revolution in aircraft design because when used in place of water as an engine coolant, its higher boiling point allowed for smaller radiators operating at higher temperatures. Prior to the widespread availability of ethylene glycol, many aircraft manufacturers tried to use evaporative cooling systems which used water at high pressure. Invariably, these proved to be rather unreliable and were easily damaged in combat because they took up large amounts of room on the plane, where they were easily hit by gunfire.
References
- ^ J. M. Hollis, F. J. Lovas, P. R. Jewell, L. H. Coudert (2002-05-20). "Interstellar Antifreeze: Ethylene Glycol". The AstroPhysical Journal. 571: L59–L62. doi:10.1086/341148.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Theodora W. Greene, Peter G. M. Wuts. Protective Groups in Organic Synthesis (Third Edition ed.). John Wiley & Sons. pp. 312–322. ISBN 0-471-16019-9.
{{cite book}}:|edition=has extra text (help) - ^ J. H. Babler, N. C. Malek and M. J. Coghlan (1978). "Selective
hydrolysis of α,β- and β,γ-unsaturated ketals: method for deconjugation of β,β-disubstituted α,β-unsaturated ketones". J. Org. Chem. 43 (9): 1821–1823. doi:10.1021/jo00403a047.
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