1. Industrial Production of Ethanol
Alcohols contain the hydroxyl functional group (\(-\text{OH}\)). Primary (\(1^\circ\)), secondary (\(2^\circ\)), and tertiary (\(3^\circ\)) alcohols are classified by the number of carbon atoms attached to the carbon bearing the \(-\text{OH}\) group.
| Comparison Factor | Fermentation of Glucose | Direct Hydration of Ethene |
|---|---|---|
| Feedstock | Carbohydrates (sugar cane, starch) - Renewable | Crude oil fraction cracking - Non-renewable |
| Reaction Equation | \(\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2\) | \(\text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \rightleftharpoons \text{C}_2\text{H}_5\text{OH}\) |
| Conditions | Yeast enzymes, anaerobic, \(35^\circ\text{C}\), atmospheric pressure | Conc \(\text{H}_3\text{PO}_4\) catalyst, \(300^\circ\text{C}\), \(60\text{ atm}\) steam |
| Process Type | Slow batch process | Fast continuous process |
| Product Purity | Impure aqueous solution (max ~15%), requires fractional distillation | Pure ethanol (>95% purity) |
2. Oxidation Pathways of Alcohols
The standard laboratory oxidising agent is acidified potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4\)). When oxidation occurs, orange dichromate ions (\(\text{Cr}_2\text{O}_7^{2-}\)) are reduced to green chromium(III) ions (\(\text{Cr}^{3+}\)):
\[ \text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6e^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O} \quad (\text{Orange} \rightarrow \text{Green}) \]
Primary Alcohols (\(1^\circ\))
- Partial Oxidation to Aldehyde: Distillation apparatus with gentle heating. The aldehyde has a lower boiling point than the alcohol (no hydrogen bonding between aldehyde molecules) and distils off immediately before further oxidation: \[ \text{CH}_3\text{CH}_2\text{OH} + [\text{O}] \xrightarrow{\text{distil}} \text{CH}_3\text{CHO} + \text{H}_2\text{O} \]
- Full Oxidation to Carboxylic Acid: Heating under reflux with excess acidified dichromate. Vapours condense and drip back into the flask to ensure complete oxidation: \[ \text{CH}_3\text{CH}_2\text{OH} + 2[\text{O}] \xrightarrow{\text{reflux}} \text{CH}_3\text{COOH} + \text{H}_2\text{O} \]
Secondary & Tertiary Alcohols
- Secondary Alcohols (\(2^\circ\)): Oxidised to ketones under reflux: \[ \text{CH}_3\text{CH(OH)CH}_3 + [\text{O}] \rightarrow \text{CH}_3\text{COCH}_3 + \text{H}_2\text{O} \] Ketones resist further oxidation.
- Tertiary Alcohols (\(3^\circ\)): Resist oxidation completely. There is no hydrogen atom attached to the carbon atom bearing the \(-\text{OH}\) group. The solution remains orange.
3. Elimination (Dehydration) to Form Alkenes
Heating an alcohol with an acid catalyst (concentrated sulfuric acid or concentrated phosphoric acid) at \(170^\circ\text{C}\) eliminates water to produce an alkene:
\[ \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{conc } \text{H}_2\text{SO}_4, 170^\circ\text{C}} \text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \]
This reaction enables the production of addition polymers from renewable biological ethanol feedstocks without relying on petroleum cracking.
4. Distinguishing Aldehydes from Ketones
| Test Reagent | Active Chemical Species | Observation with Aldehydes | Observation with Ketones |
|---|---|---|---|
| Tollens' Reagent | Ammoniacal silver nitrate \([\text{Ag(NH}_3)_2]^+\) | Silver mirror forms on flask walls (\(\text{Ag}^+ + e^- \rightarrow \text{Ag}(s)\)) | No reaction (solution remains colourless) |
| Fehling's Solution | Aqueous copper(II) tartrate complex | Blue solution forms brick-red precipitate of \(\text{Cu}_2\text{O}(s)\) | No reaction (solution remains deep blue) |
5. Practice Questions
Reagent: Potassium dichromate(VI) acidified with dilute sulfuric acid. (1 mark)
Apparatus: Distillation apparatus (heating with a Liebig condenser set downwards for distillation). (1 mark)
Reason: Ethanal has a lower boiling point (\(21^\circ\text{C}\)) than ethanol because ethanal molecules cannot form hydrogen bonds. Ethanal distils off immediately as it forms, preventing it from being further oxidised to ethanoic acid. (1 mark)