Making Alcohols (Hydration of Ethene - Higher Tier)

Hydration of Ethene

Ethanol can be produced industrially by the hydration of ethene, which is an addition reaction where ethene reacts with steam (water vapour).

The reaction is:

$\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH}$

Here, ethene (an alkene) reacts with steam to form ethanol (an alcohol).

This reaction requires specific conditions:

• Catalyst: Phosphoric acid ($\text{H}_3\text{PO}_4$) supported on silica.
• Temperature: Around 300°C.
• Pressure: Approximately 60–70 atmospheres.

These conditions speed up the reaction and increase the yield of ethanol.

This method produces ethanol continuously and is widely used in industry because it gives a high yield and purity of ethanol.

Reaction Mechanism

The hydration of ethene proceeds via an electrophilic addition mechanism, typical of alkenes reacting with polar molecules.

Key steps in the mechanism:

1. Electrophile formation: The double bond in ethene is rich in electrons and attracts an electrophile. In this case, the electrophile is a proton ($\text{H}^+$) from the acid catalyst.
2. Carbocation intermediate: The proton adds to one of the carbon atoms in the double bond, breaking the double bond and forming a positively charged carbocation intermediate on the other carbon.
3. Nucleophilic attack: A water molecule ($\text{H}_2\text{O}$) then attacks the carbocation, bonding to the positively charged carbon.
4. Proton transfer: The attached water molecule loses a proton ($\text{H}^+$) to give the neutral ethanol molecule.

This mechanism explains why the reaction requires an acid catalyst and why the double bond is reactive.

For example, the proton adds to one carbon of the double bond, creating a carbocation on the other carbon, which is then attacked by water to form ethanol.

Industrial Production of Ethanol by Hydration

The hydration of ethene is carried out industrially as a continuous process, meaning reactants are continuously fed in and products continuously removed.

Key points about the industrial process:

• Ethene source: Ethene is obtained from cracking crude oil fractions, making it widely available.
• Conditions: High temperature (~300°C) and high pressure (~60–70 atm) with phosphoric acid catalyst on silica.
• Advantages: Produces ethanol with high yield and high purity, suitable for industrial uses such as solvents, fuels, and chemical feedstocks.
• Economic importance: This method is faster and more efficient than fermentation, allowing large-scale ethanol production.

The continuous process contrasts with batch processes like fermentation, which are slower and produce lower purity ethanol.