Alcohols

Summary: Alcohols contain the -OH functional group (CnH2n+1OH). Ethanol (C2H5OH) is made by fermentation (glucose → ethanol + CO2, yeast, anaerobic, ~35 degrees C) or hydration of ethene (C2H4 + H2O, 300 degrees C, 60 atm, H3PO4 catalyst). Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16

What are Alcohols?

Alcohols are a homologous series with the general formula:

CnH2n+1OH (or R-OH, where R = alkyl group)

The functional group is the hydroxyl group (-OH). This is a covalent bond between oxygen and hydrogen (O-H), attached to a carbon atom.

Key structural point: The -OH group is different from the hydroxide ion (OH-). The hydroxyl group in alcohols is covalently bonded to the carbon chain — alcohols are not ionic and are not alkaline. They do not dissociate in water to produce OH- ions.

The First Three Alcohols

Drawing displayed formulas:

• Methanol (CH3OH): One carbon atom bonded to three H atoms and one -OH group: H3C-OH
• Ethanol (C2H5OH): Two carbon chain with -OH attached to one end: CH3-CH2-OH
• Propanol (C3H7OH): Three carbon chain with -OH attached to one end: CH3-CH2-CH2-OH

When drawing displayed formulas of alcohols, the O-H bond must be shown clearly — draw the full bond: C-O-H, not just text next to a carbon.

Production of Ethanol

Ethanol (C2H5OH) can be produced by two methods.

Method 1: Fermentation (Biological Method)

Equation:

C6H12O6 (aq) --(yeast, 35 °C, anaerobic)--> 2C2H5OH (aq) + 2CO2 (g)
Glucose                                   Ethanol

Conditions and key facts:

• Yeast provides enzymes (zymase) that catalyse the reaction
• Temperature: 35 °C (warm but not hot — too hot kills the yeast enzymes; too cold and the reaction is too slow)
• Anaerobic (no oxygen) — if oxygen is present, yeast respires aerobically making CO2 and H2O instead
• Batch process — fermentation stops when ethanol concentration reaches about 15% (ethanol kills the yeast at higher concentrations)
• The ethanol is then purified by fractional distillation to obtain pure ethanol

Raw material: Glucose from plants (sugar cane, sugar beet) or from starch (corn, potatoes) after hydrolysis.

Method 2: Hydration of Ethene (Industrial Method)

Equation:

C2H4 (g) + H2O (g) --(H3PO4, 300 °C, 60 atm)--> C2H5OH (g)
Ethene         Steam                               Ethanol

Conditions and key facts:

• Catalyst: Phosphoric acid (H3PO4)
• Temperature: 300 °C
• Pressure: 60 atmospheres
• Continuous process — ethene and steam are continuously fed in, ethanol is continuously removed
• Product is pure — no fractional distillation needed

Raw material: Ethene from cracking of crude oil fractions (non-renewable).

Comparison Table: Fermentation vs Hydration

Fermentation uses a renewable resource (plants) but produces impure ethanol requiring further processing. Hydration of ethene uses a non-renewable resource but produces pure ethanol in a continuous, efficient process.

Reactions of Alcohols

Combustion

Alcohols burn in oxygen to produce carbon dioxide and water.

Ethanol:

C2H5OH (l) + 3O2 (g) --> 2CO2 (g) + 3H2O (l)

General pattern for balancing alcohol combustion:

1. Balance carbon atoms: number of CO2 = number of C in alcohol
2. Balance hydrogen atoms: number of H2O = half the number of H in alcohol
3. Balance oxygen atoms: O2 needed = (2 x CO2 + H2O - 1)/2 (the -1 accounts for the one O already in the -OH group)

Ethanol as a biofuel: Ethanol produced by fermentation is mixed with petrol (“gasohol”) and used as a fuel. It is considered carbon-neutral because the CO2 released during combustion was recently absorbed by the plants during photosynthesis.

Oxidation of Ethanol

Ethanol can be oxidised to ethanoic acid:

C2H5OH + 2[O] --> CH3COOH + H2O
Ethanol             Ethanoic acid

Oxidising agents (either can be used):

• Acidified potassium manganate(VII) (KMnO4/H2SO4): colour change from purple to colourless
• Acidified potassium dichromate(VI) (K2Cr2O7/H2SO4): colour change from orange to green

Conditions:

• Warm gently
• The oxidising agent must be acidified (with dilute sulfuric acid, H2SO4)

Observations during oxidation:

• With KMnO4: purple solution turns colourless
• With K2Cr2O7: orange solution turns green

This oxidation reaction provides a route from ethanol (in alcoholic drinks) to ethanoic acid (vinegar) — this is why wine left open to air turns sour (ethanol is oxidised by oxygen and bacteria to ethanoic acid).

Uses of Ethanol

Key Points

• General formula: CnH2n+1OH
• Functional group: hydroxyl (-OH) — covalently bonded, NOT the hydroxide ion
• First three: methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH)
• Fermentation: C6H12O6 ⇒ 2C2H5OH + 2CO2 (yeast, 35 degrees C, anaerobic, batch, ~15% max)
• Hydration: C2H4 + H2O ⇒ C2H5OH (H3PO4, 300 degrees C, 60 atm, continuous, pure)
• Combustion: C2H5OH + 3O2 ⇒ 2CO2 + 3H2O
• Oxidation: ethanol + [O] ⇒ ethanoic acid (acidified KMnO4: purple ⇒ colourless; acidified K2Cr2O7: orange ⇒ green)
• Ethanol uses: solvent, biofuel, alcoholic drinks, antiseptic

Key Concepts from Past Papers

• Fractional distillation depends on boiling point
• Petroleum is heated and vaporised; vapour enters at bottom of fractionating column
• Ethanol can be produced by fermentation (renewable) or hydration of ethene (non-renewable)

Keywords from Past Papers

methane, ethene, point, bonds, carbon, fractions, energy, hydrocarbons, boiling, idea, structure, water, alcohol, addition, poly

Related Notes

• Organic Chemistry Fundamentals
• Alkanes
• Alkenes
• Carboxylic Acids

Sources

• OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry (Alcohols and Ethers)], Rice University (free, CC BY 4.0)
• BBC Bitesize GCSE Chemistry — [Alcohols], BBC (free educational resource)
• Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry (Alcohols), Cambridge Assessment International Education
• CK-12 Chemistry for High School — [Chapter 25: Alcohols], CK-12 Foundation (free, CC BY-NC 3.0)

Past Paper Sources

• 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
• 0620/32 Feb/March 2018: Q66(a)(i) (1m)
• 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
• 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
• 0620/32 Feb/March 2021: Q66(c)(i) (1m)
• 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
• 0620/32 Feb/March 2023: Q66(a)(iv) (2m)
• 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m)
• 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
• 0620/33 May/June 2017: Q44(a)(ii) (1m)
• 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+3 more)
• 0620/33 May/June 2021: Q44(a)(iii) (1m), Q44(d)(i) (1m), Q44(d)(ii) (1m) (+1 more)

Common Misconceptions