Ethanol by Hydration of Ethene

Industrially, ethanol is made on a large scale by the hydration of ethene with steam:CH₂=CH₂(g) + H₂O(g) ⇌ CH₃CH₂OH(g)Conditions: a concentrated phosphoric acid catalyst (H₃PO₄), a temperature of about 300 °C and a pressure of about 60 to 70 atm. Only about 5% of the ethene reacts on each pass, so unreacted ethene is recycled, giving an overall conversion of about 95%.Mechanism: acid-catalysed electrophilic addition
1. The electrons in the C=C double bond attack an H⁺ ion from the acid. The H bonds to one carbon, forming a carbocation.
2. A lone pair on the oxygen of a water molecule attacks the positive carbon.
3. The protonated alcohol loses an H⁺ ion to form ethanol. The H⁺ is regenerated, so the acid is a catalyst.Ethene is obtained by cracking fractions from crude oil, so this route uses a finite resource.Advantages: a fast, continuous process that gives almost pure ethanol with a high atom economy.
Disadvantages: uses non-renewable crude oil; the high temperature and pressure need a lot of energy and expensive equipment.Worked example: calculate the atom economy of making ethanol from ethene and steam.
Atom economy = (M_r of desired product ÷ total M_r of reactants) × 100
= 46 ÷ (28 + 18) × 100 = 100%, because ethanol is the only product.