Question

Describe the industrial manufacture of ethanol. Give also the chemical equation of reactions. What is formed after dehydrogenation of ethanol? Write the mechanism of acidic dehydration of ethanol to get ethene.

Hint:

Fermentation is biological; industrially, ethene hydration is faster. Cu gives dehydrogenation, H\(_2\)SO\(_4\) gives dehydration.

Answer 1

Step 1: Industrial manufacture of ethanol.
(i) Fermentation: Glucose is converted into ethanol using enzymes. \[ C_6H_{12}O_6 \xrightarrow{\text{zymase}} 2C_2H_5OH + 2CO_2 \] (ii) From ethene: Ethene reacts with steam at 300°C in presence of phosphoric acid catalyst. \[ C_2H_4 + H_2O \xrightarrow{H_3PO_4, 300^\circ C} C_2H_5OH \] Step 2: Dehydrogenation of ethanol.
On heating with Cu at 573 K: \[ C_2H_5OH \xrightarrow{Cu, 573K} CH_3CHO + H_2 \] Thus, ethanol gives acetaldehyde.
Step 3: Dehydration mechanism (acidic).
\[ C_2H_5OH \xrightarrow{H_2SO_4, 443K} C_2H_4 + H_2O \] \underline{Mechanism:} 1. Protonation of OH group → formation of oxonium ion.
2. Loss of water → carbocation formation.
3. Rearrangement → elimination of H\(^+\) to give ethene.
Step 4: Conclusion.
- Ethanol industrially: fermentation/hydration of ethene.
- Dehydrogenation product = acetaldehyde.
- Dehydration (acidic) = ethene.