Ethylene Glycol: Synthesis, Properties & Reaction Chart
Ethylene Glycol ($\text{HO-CH}_2\text{-CH}_2\text{-OH}$), also known as ethane-1,2-diol, is the most important dihydric alcohol. It is a vital industrial chemical used as an antifreeze and a precursor in the production of polyesters (Terylene).
1. Preparation of Ethylene Glycol
Ethylene glycol is primarily prepared from ethylene via hydroxylation or through the hydration of ethylene oxide.
Key Synthesis Routes
- Baeyer’s Reagent: Cold, dilute alkaline $\text{KMnO}_4$ solution reacts with ethylene to give glycol (Syn-addition).
- Industrial Method: Catalytic oxidation of ethylene to Ethylene Oxide, followed by acid-catalyzed hydration.
- From 1,2-Dibromoethane: Hydrolysis using aqueous $\text{Na}_2\text{CO}_3$ or $\text{NaOH}$.
2. Physical Properties & Applications
Due to the presence of two hydroxyl groups, ethylene glycol exhibits very strong intermolecular hydrogen bonding, leading to unique physical characteristics.
High-Yield Trends
- Viscosity: Highly viscous liquid (syrupy) due to extensive H-bonding.
- Boiling Point: High boiling point ($197^\circ\text{C}$), much higher than ethanol ($78^\circ\text{C}$).
- Antifreeze: Added to car radiators because it lowers the freezing point of water and raises its boiling point.
- Sweet Taste: It is sweet but highly poisonous (toxic) if ingested.
3. Chemical Reactions
Ethylene glycol can react with one or both $-\text{OH}$ groups. Its oxidation behavior is particularly important for JEE and NEET, as it produces a variety of intermediates.
Critical Oxidation Pathway
Oxidation with Conc. $\text{HNO}_3$ or alkaline $\text{KMnO}_4$ proceeds as follows:
- Periodic Acid ($\text{HIO}_4$): Cleaves the $\text{C-C}$ bond to give two molecules of Formaldehyde ($\text{HCHO}$).
- Dehydration: Heating with $\text{ZnCl}_2$ yields Acetaldehyde; heating alone gives Diethylene glycol.
- Dioxane Formation: Dehydration with conc. $\text{H}_2\text{SO}_4$ yields 1,4-Dioxane (a cyclic ether).
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