What is Glycol?
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
1. What is a glycol and how is it structurally different from a simple alcohol?
A glycol is an organic compound belonging to the alcohol family that is characterized by the presence of two hydroxyl (-OH) groups attached to different carbon atoms. This makes it a type of diol. In contrast, a simple alcohol, like ethanol (CH₃CH₂OH), has only one hydroxyl group. The presence of two -OH groups gives glycols a higher boiling point and greater water solubility compared to alcohols with similar molecular weight.
2. What is the chemical structure and IUPAC name for the most common glycol?
The most common and simplest glycol is ethylene glycol. Its IUPAC name is ethane-1,2-diol. The chemical formula is C₂H₆O₂, and its structural formula is HO-CH₂-CH₂-OH, which clearly shows the two hydroxyl groups attached to adjacent carbon atoms.
3. What are the primary industrial applications of ethylene glycol?
Ethylene glycol has two main industrial uses. The most significant applications are:
- Antifreeze Formulations: It is a key component in automotive antifreeze and coolants because it lowers the freezing point and raises the boiling point of water.
- Polyester Production: It serves as a monomer, along with terephthalic acid, in the manufacturing of polyethylene terephthalate (PET), a plastic widely used for bottles and fibres.
4. How does ethylene glycol work as an antifreeze agent in vehicle cooling systems?
Ethylene glycol functions as an antifreeze by disrupting the hydrogen bonding between water molecules. This disruption is a colligative property known as freezing point depression. When mixed with water, the glycol molecules interfere with the formation of the crystal lattice structure of ice, thereby lowering the temperature at which the solution freezes. A mixture of 60% ethylene glycol and 40% water can lower the freezing point to approximately -45°C.
5. Why is propylene glycol used in foods and cosmetics while ethylene glycol is highly toxic?
The difference in safety comes down to how they are metabolized in the body. Ethylene glycol is oxidized to form toxic compounds, primarily glycolic acid and then oxalic acid, which can cause severe metabolic acidosis and kidney failure. In contrast, propylene glycol (propane-1,2-diol) is metabolized into lactic acid, a naturally occurring substance in the body that is easily processed and eliminated. This non-toxic pathway makes propylene glycol safe for use as a solvent and preservative in foods, pharmaceuticals, and cosmetics.
6. How is ethylene glycol commercially prepared from ethene as per the NCERT syllabus?
The commercial production of ethylene glycol is a two-step process starting from ethene (C₂H₄).
1. First, ethene undergoes catalytic oxidation to form an intermediate called ethylene oxide (an epoxide).
2. Then, this ethylene oxide is hydrolyzed by reacting it with water, which opens the epoxide ring to form ethylene glycol (ethane-1,2-diol). The reaction is: C₂H₄O (ethylene oxide) + H₂O → HO-CH₂-CH₂-OH (ethylene glycol).
7. In organic synthesis, how can ethylene glycol be used as a protecting group for aldehydes and ketones?
Ethylene glycol is used to protect carbonyl groups (C=O) in aldehydes and ketones from reacting with nucleophiles or bases during a multi-step synthesis. When the carbonyl compound is treated with ethylene glycol in the presence of an acid catalyst, it forms a stable cyclic structure called a 1,3-dioxolane or a cyclic acetal. This protecting group is resistant to basic and nucleophilic conditions. After the desired reactions on other parts of the molecule are complete, the protecting group can be easily removed by acid hydrolysis, regenerating the original carbonyl group.
8. What physical property of glycols makes them useful as both solvents and humectants?
The key physical property is their ability to form strong hydrogen bonds, thanks to their two hydroxyl (-OH) groups. This allows them to:
- Act as effective solvents by dissolving a wide range of substances, including those that are not easily soluble in water or simple alcohols.
- Function as humectants (moisture-retaining agents) by attracting and holding water molecules from the air, which is a valuable property in products like cosmetics and foods.
9. Are 'glycol' and 'glycerol' the same thing? Explain the key structural difference.
No, glycol and glycerol are not the same, although both are types of polyhydroxy alcohols. The key difference is the number of hydroxyl (-OH) groups. A glycol (specifically ethylene glycol) is a diol, meaning it has two hydroxyl groups (ethane-1,2-diol). Glycerol, on the other hand, is a triol, meaning it has three hydroxyl groups (propane-1,2,3-triol). This structural difference also leads to different physical properties, such as viscosity, with glycerol being significantly more viscous than ethylene glycol.
