Ethene (ethylene) and Ethyne (acetylene) are fundamental hydrocarbons with significant industrial and laboratory applications. Their preparation methods vary depending on the scale and desired purity, often involving specific chemical reactions tailored to their unique structures.
Preparing Ethene (Ethylene)
Ethene (C₂H₄), the simplest alkene, is characterized by a carbon-carbon double bond. It is primarily prepared through the dehydration of ethanol or by the thermal cracking of larger hydrocarbons.
Dehydration of Ethanol
This is a common laboratory method for producing ethene. Ethanol undergoes an elimination reaction in the presence of a strong dehydrating agent at elevated temperatures.
- Reactants: Ethanol (CH₃CH₂OH)
- Reagents: Concentrated sulfuric acid (H₂SO₄) or aluminum oxide (Al₂O₃)
- Conditions: Heating to approximately 170°C for sulfuric acid, or 350-400°C for aluminum oxide.
The reaction for dehydration using sulfuric acid is:
CH₃CH₂OH → CH₂=CH₂ + H₂O
Sulfuric acid acts as a catalyst and a dehydrating agent, removing water from the ethanol molecule. For industrial production, aluminum oxide or phosphoric acid on alumina are often preferred catalysts due to easier separation and less corrosive byproducts.
Cracking of Hydrocarbons
Industrially, ethene is predominantly produced through the steam cracking of petroleum fractions (like naphtha or gas oil) or natural gas liquids (like ethane and propane). This process involves breaking down larger alkane molecules into smaller, unsaturated hydrocarbons, including ethene.
- Process: Alkanes are heated to very high temperatures (750-950°C) in the presence of steam, which prevents coke formation.
- Example: Ethane (C₂H₆) cracking yields ethene and hydrogen:
C₂H₆ → CH₂=CH₂ + H₂
This method accounts for the vast majority of global ethene production, supplying the chemical industry with a key building block for plastics and other organic compounds.
Preparing Ethyne (Acetylene)
Ethyne (C₂H₂), also known as acetylene, is the simplest alkyne, featuring a carbon-carbon triple bond. It is notably reactive and used in various applications, from welding to organic synthesis.
From Calcium Carbide
The most common industrial and laboratory method for producing ethyne involves the reaction of calcium carbide with water. This reaction is straightforward and generates ethyne gas directly.
- Reactants: Calcium carbide (CaC₂) and water (H₂O)
- Conditions: Ambient temperature.
The reaction is vigorous and exothermic:
CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂
Calcium carbide is typically produced by heating lime (calcium oxide) and coke (carbon) in an electric arc furnace. This method is cost-effective and provides a reliable source of ethyne.
Via 1,2-Dibromoethane (from Ethene)
Ethyne can also be synthesized through a two-step process originating from ethene. This method involves the successive dehydrohalogenation of a dihaloalkane.
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Bromination of Ethene: Initially, ethene is treated with bromine, undergoing an addition reaction to form 1,2-Dibromoethane (also known as ethylene dibromide). This reaction occurs readily at room temperature.
CH₂=CH₂ + Br₂ → BrCH₂-CH₂Br -
Dehydrohalogenation: The intermediate 1,2-Dibromoethane is then subjected to heating with an excess amount of alcoholic potassium hydroxide (KOH). This strong base facilitates a successive dehydrohalogenation reaction, where two molecules of hydrogen bromide (HBr) are removed from the 1,2-Dibromoethane molecule, yielding ethyne.
BrCH₂-CH₂Br + 2KOH (alc.) → C₂H₂ + 2KBr + 2H₂O
This method provides a laboratory pathway to synthesize ethyne starting from ethene, showcasing the interconversion possibilities within hydrocarbon chemistry.
| Method | Starting Material | Key Reagent/Condition | Product | Primary Use |
|---|---|---|---|---|
| Dehydration of Ethanol | Ethanol | Conc. H₂SO₄, 170°C | Ethene | Lab Synthesis |
| Cracking of Hydrocarbons | Alkanes (e.g., Ethane) | High Temperature (Steam) | Ethene | Industrial |
| From Calcium Carbide | Calcium Carbide | Water | Ethyne | Industrial/Lab |
| Via 1,2-Dibromoethane | Ethene (via Bromine) | Alcoholic KOH, Heat | Ethyne | Lab Synthesis |
These methods highlight the versatility in organic synthesis, allowing for the preparation of these crucial unsaturated hydrocarbons through various chemical transformations.
