C₂H₆O₂ — Ethylene Glycol

Ethylene glycol (C₂H₆O₂) is a simple organic compound belonging to the alcohol family. It is the simplest diol, meaning it contains two hydroxyl (–OH) groups attached to a two-carbon chain. Ethylene glycol is a colorless, odorless, viscous, and sweet-tasting liquid primarily used as an antifreeze in automotive engines and as a raw material in polyester manufacturing. Its chemical structure allows hydrogen bonding, which gives it unique physical properties such as a high boiling point and low freezing point.

Discovered in the 1850s, ethylene glycol became an industrially important compound during World War II due to its application in cooling aircraft engines. Today, it plays a crucial role in the production of polyethylene terephthalate (PET), used in plastic bottles and textile fibers, making it one of the most commercially important organic compounds globally.

Ethylene glycol consists of two carbon atoms, each bonded to hydroxyl groups, forming a dihydric alcohol. The molecular structure can be represented as:

Each carbon atom is sp³ hybridized, resulting in a tetrahedral geometry around both carbons. The molecule has strong hydrogen bonding capabilities due to the presence of two hydroxyl groups, leading to high viscosity and boiling point.

The two –OH groups also make the compound hygroscopic, meaning it readily absorbs water from the atmosphere. The polarity of ethylene glycol arises from the uneven distribution of electrons between the oxygen and hydrogen atoms in the hydroxyl groups, giving rise to dipole-dipole interactions. These properties make it a powerful coolant and solvent for polar and ionic substances.

Ethylene glycol is mainly produced from ethylene (C₂H₄), a fundamental petrochemical. The process involves two main steps:

• 1. Oxidation of Ethylene to Ethylene Oxide: Ethylene reacts with oxygen in the presence of a silver catalyst to form ethylene oxide.

• 2. Hydration of Ethylene Oxide: Ethylene oxide is then hydrolyzed with water to yield ethylene glycol.

This process provides high yields of ethylene glycol (90% or more). The resulting mixture also contains small amounts of diethylene glycol and triethylene glycol as by-products. These higher glycols have applications as solvents and plasticizers.

Alternative synthetic routes include hydrolysis of ethylene dichloride and catalytic hydrogenation of formaldehyde derivatives, though these are less common industrially due to cost inefficiency.

Physical Properties:

• Ethylene glycol is a viscous, clear liquid that mixes completely with water and alcohols.
• Has a low freezing point (−12.9°C) and high boiling point (197°C), making it suitable for thermal regulation applications.
• Exhibits strong hydrogen bonding due to hydroxyl groups, which enhances its solvation properties.
• Hygroscopic and absorbs moisture from the air.

Chemical Properties:

• 1. Reaction with Sodium: Reacts with sodium metal to produce hydrogen gas and sodium alkoxide.

\( 2Na + 2HOCH_2CH_2OH \rightarrow 2NaOCH_2CH_2OH + H_2 \uparrow \)

• 2. Oxidation: Ethylene glycol oxidizes to form glycolic acid, glyoxylic acid, and oxalic acid.

\( HOCH_2CH_2OH + O_2 \rightarrow HOCH_2COOH + H_2O \)

• 3. Dehydration: When heated with sulfuric acid, it dehydrates to form acetaldehyde.

\( HOCH_2CH_2OH \xrightarrow{H_2SO_4, heat} CH_3CHO + H_2O \)

• 4. Reaction with Carboxylic Acids: Forms esters, a key reaction in the production of polyester resins.

\( HOCH_2CH_2OH + 2RCOOH \rightarrow (RCOOCH_2CH_2OOC)R + 2H_2O \)

Ethylene glycol serves as a multipurpose industrial compound with wide applications:

• 1. Antifreeze and Coolants: The most significant use of ethylene glycol is in automotive antifreeze formulations. Its ability to depress the freezing point and raise the boiling point of water makes it ideal for maintaining engine temperatures in both hot and cold climates.
• 2. Polyester Manufacturing: Used as a precursor for the production of polyethylene terephthalate (PET), a polymer used in plastic bottles, textiles, and films.
• 3. Deicing Solutions: Applied to aircraft and runways to prevent ice formation.
• 4. Heat Transfer Medium: Used in heating, ventilation, and air conditioning (HVAC) systems for thermal energy transfer.
• 5. Solvent: Acts as a solvent in inks, dyes, and paints due to its high polarity and low volatility.
• 6. Pharmaceutical Applications: Used as a humectant and intermediate in certain drug formulations (though toxic if ingested directly).
• 7. Chemical Intermediate: Used to produce resins, plasticizers, and explosives.

Ethylene glycol is highly toxic if ingested, as it metabolizes in the body to form glycolic acid and oxalic acid, which can cause metabolic acidosis, kidney failure, and death. Despite its sweet taste, accidental poisoning is common, particularly in pets and wildlife.

Health Hazards:

• Inhalation of vapors can cause dizziness and respiratory irritation.
• Ingestion leads to nausea, vomiting, confusion, and renal failure.
• Prolonged skin contact may result in irritation and dermatitis.

Safety Precautions:

• Use protective gloves and goggles when handling.
• Store in clearly labeled, tightly closed containers away from children and animals.
• Ensure proper ventilation in working areas to prevent vapor buildup.
• Dispose of ethylene glycol-containing wastes following environmental regulations.

Due to its toxicity, ethylene glycol-based antifreezes are often replaced with propylene glycol-based alternatives in environmentally sensitive applications.