What is Glycol Definition Types Structure Reactions and Uses
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.
About Glycol
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
Industrial Route
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.
Biological Routes
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.
Historical Routes
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.
Chemical Reactions
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.
Antifreeze
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.
FAQs on Glycol in Chemistry Structure Properties and Applications
1. What is glycol in chemistry?
A glycol is a type of diol—an organic compound that contains two hydroxyl (–OH) groups attached to different carbon atoms. Glycols are derived from alkanes by replacing two hydrogen atoms with –OH groups. For example, ethylene glycol (C2H6O2) has the structure HO–CH2–CH2–OH. In organic chemistry, glycols are important alcohols used in antifreeze, polymers, and solvents.
2. What is the formula of ethylene glycol?
The chemical formula of ethylene glycol is C2H6O2. Its structural formula is HO–CH2–CH2–OH, showing two hydroxyl (–OH) groups attached to adjacent carbon atoms. It is also called ethane-1,2-diol according to IUPAC nomenclature. Ethylene glycol is the simplest glycol and is widely used as antifreeze.
3. What is the difference between glycol and alcohol?
The main difference is that a glycol contains two hydroxyl (–OH) groups, while a typical alcohol contains only one –OH group.
- Alcohol: One –OH group (e.g., ethanol, C2H5OH).
- Glycol (diol): Two –OH groups on different carbon atoms (e.g., ethylene glycol, C2H6O2).
4. How is ethylene glycol prepared from ethene?
Ethylene glycol is prepared from ethene (C2H4) by oxidation to ethylene oxide followed by hydrolysis.
- Step 1 (Oxidation): 2C2H4(g) + O2(g) → 2C2H4O(g) (ethylene oxide)
- Step 2 (Hydrolysis): C2H4O + H2O(l) → C2H6O2(l)
5. What are the common types of glycols?
The most common types of glycols (diols) are ethylene glycol and propylene glycol.
- Ethylene glycol (C2H6O2): Used in antifreeze and polyester production.
- Propylene glycol (C3H8O2): Used in food, cosmetics, and pharmaceuticals.
6. Why does ethylene glycol have a high boiling point?
Ethylene glycol has a high boiling point because it forms strong intermolecular hydrogen bonds due to its two –OH groups. Each molecule can form multiple hydrogen bonds, increasing intermolecular attraction. As a result, its boiling point (~197 °C) is much higher than ethanol (78 °C), even though their molar masses are comparable.
7. What are the uses of glycol in chemistry and industry?
Glycols are widely used as antifreeze, solvents, and polymer precursors in chemistry and industry.
- Antifreeze: Ethylene glycol lowers the freezing point of water in car radiators.
- Polyester production: Reacts with terephthalic acid to form PET.
- Solvent: Used in inks, resins, and paints.
- Humectant: Propylene glycol retains moisture in cosmetics and food.
8. Is ethylene glycol toxic?
Yes, ethylene glycol is toxic if ingested because it is metabolized into harmful acids in the body. In the liver, it is oxidized to glycolic acid and oxalic acid, which can cause kidney damage and metabolic acidosis. In contrast, propylene glycol is much less toxic and is used in food and pharmaceuticals.
9. How does glycol react with carboxylic acids?
Glycols react with carboxylic acids to form esters through esterification reactions. For example, ethylene glycol reacts with acetic acid as follows:
HO–CH2–CH2–OH + 2CH3COOH → CH3COO–CH2–CH2–OOCCH3 + 2H2O.
Because glycols have two –OH groups, they can form diesters and are important in polymer formation such as polyesters.
10. What is the IUPAC name of glycol?
The IUPAC name of the simplest glycol is ethane-1,2-diol. In systematic nomenclature, glycols are named as alkanediols, indicating two –OH groups and their positions. For example:
- Ethylene glycol → ethane-1,2-diol
- Propylene glycol → propane-1,2-diol
