Ethanol — preparation

Ethanol, C₂H₅OH, is the oldest organic compound prepared in industry; the ancient Egyptians made it by fermenting sugars and starches thousands of years ago.

• 1.
• Alcoholic fermentation. About 20 % of the world's ethanol is made by fermenting starch and sugar materials (sugar cane, sugar beet, maize).
• In Egypt it is made from molasses (the sugar-rich residue of sugar refining, containing sucrose), for instance at the Egyptian Sugar and Distillation Company (Hawamdeya).
• Yeast is added:

• C₁₂H₂₂O₁₁ (sucrose) + H₂O —(invertase enzyme of yeast)→ C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
• C₆H₁₂O₆ —(zymase enzyme)→ 2C₂H₅OH + 2CO₂ The resulting dilute alcohol (about 10–15 %) is concentrated by fractional distillation (about 95–96 % ethanol, a constant-boiling mixture with water).

2. Catalytic hydration of ethene. The main industrial method in petroleum-producing countries: ethene obtained by cracking petroleum fractions is hydrated with steam in the presence of dilute sulphuric or phosphoric acid: CH₂=CH₂ + H₂O —(H₃PO₄ or dil. H₂SO₄, heat, pressure)→ CH₃CH₂OH Ethanol is therefore called a petrochemical (made from petroleum). Note: ethene is the only alkene that gives a primary alcohol by hydration; other alkenes follow Markovnikov's rule and give secondary or tertiary alcohols: CH₃CH=CH₂ + H₂O → CH₃CH(OH)CH₃ (2-propanol, secondary) (CH₃)₂C=CH–CH₃ + H₂O → (CH₃)₂C(OH)–CH₂CH₃ (2-methyl-2-butanol, tertiary)

3. General method: hydrolysis of alkyl halides. Heating an alkyl halide with aqueous alkali (KOH or NaOH) replaces the halogen with –OH: R–X + KOH(aq) → R–OH + KX

• CH₃CH₂Br + KOH → CH₃CH₂OH + KBr (primary alcohol)
• CH₃CHBrCH₃ + KOH → CH₃CH(OH)CH₃ + KBr (secondary)
• (CH₃)₃C–Br + KOH → (CH₃)₃C–OH + KBr (tertiary) Ease of hydrolysis: R–Cl < R–Br < R–I (the weaker the C–X bond, the easier). So alkyl iodides are hydrolysed most easily. Which halide for which alcohol? methanol ← CH₃X; 2-butanol ← 2-halobutane; 2-methyl-2-pentanol ← 2-halo-2-methylpentane.