Alkenes react with concentrated sulfuric acid in the cold through an addition reaction, leading to the formation of alkyl hydrogensulphates.
This reaction is a characteristic way alkenes, with their carbon-carbon double bonds, undergo addition reactions when exposed to strong acids like sulfuric acid under specific conditions. It's a fundamental process in organic chemistry, demonstrating the electrophilic nature of alkenes.
Reaction Mechanism and Product Formation
When an alkene comes into contact with concentrated sulfuric acid at low temperatures, the acid acts as an electrophile, initiating the reaction:
- Protonation: The double bond of the alkene (a region of high electron density) attacks a proton (H⁺) from the sulfuric acid. This leads to the formation of a carbocation intermediate.
- Nucleophilic Attack: The remaining hydrogensulphate ion (HSO₄⁻) then acts as a nucleophile, attacking the positively charged carbon atom of the carbocation.
The resulting product is an alkyl hydrogensulphate. The structure of this product is crucial for understanding the reaction. While it might sometimes be represented generally as R-HSO₄ (e.g., CH₃CH₂HSO₄ for ethene), the precise connectivity shows how all atoms are linked up. This means the alkyl group (R) is bonded to an oxygen atom, which is then bonded to the sulfur atom of the sulfate group (R-O-SO₃H). This specific linkage is key to understanding the molecule's chemical properties and subsequent reactions.
Key Conditions
The conditions under which this reaction occurs are specific and important for the formation of alkyl hydrogensulphates:
- Concentrated Sulfuric Acid: Using concentrated sulfuric acid is vital. Diluted sulfuric acid would lead to a different reaction, typically hydration, producing alcohols instead of alkyl hydrogensulphates.
- Cold Temperature: The reaction typically proceeds at low temperatures (e.g., 0°C or room temperature). This helps to favor the desired addition product, control the reaction rate, and prevent undesirable side reactions or decomposition of the product.
Example: Ethene and Sulfuric Acid
A prime example of this reaction is the interaction between ethene (C₂H₄) and concentrated sulfuric acid, which produces ethyl hydrogensulphate:
| Reactant (Alkene) | Reagent | Conditions | Product (Alkyl Hydrogensulphate) |
|---|---|---|---|
| Ethene (CH₂=CH₂) | Concentrated H₂SO₄ | Cold | Ethyl hydrogensulphate |
The chemical equation illustrating this specific reaction is:
CH₂=CH₂ + H₂SO₄ (conc.) → CH₃CH₂OSO₃H
This product, ethyl hydrogensulphate, is an important intermediate in industrial processes. For instance, it can be subsequently hydrolyzed (reacted with water and heat) to produce ethanol and regenerate sulfuric acid, making this a useful pathway for alcohol production.
Summary of Reaction Characteristics
- Type of Reaction: Electrophilic Addition
- Reactants: Alkene and Concentrated Sulfuric Acid
- Conditions: Cold Temperature
- Product: Alkyl hydrogensulphate (specifically R-O-SO₃H, showing the oxygen linkage)
This reaction highlights the versatility of alkenes in organic synthesis and their reactivity towards strong acids under controlled conditions, serving as a basis for various industrial applications.
