What you'll learn
Alcohols, carbonyl compounds (aldehydes and ketones) and carboxylic acids are linked by a sequence of oxidation reactions, and together with their derivatives — esters, acyl chlorides and amides — they form much of organic synthesis. They also connect directly to Caribbean industries: the fermentation of sugar-cane molasses to ethanol for rum, the methanol produced from natural gas at Point Lisas, and the esters that give tropical fruits their flavours. This guide covers the classification and properties of alcohols, their reactions (combustion, oxidation, dehydration, esterification, reaction with sodium, substitution), fermentation, the structure and reactions of aldehydes and ketones, including nucleophilic addition and tests to distinguish them, and carboxylic acids and their derivatives.
CAPE questions frequently ask for reagents and conditions, observations in tests, reaction mechanisms (especially nucleophilic addition) and multi-step synthesis routes. Learning the connections between functional groups is the key.
Key terms and definitions
Alcohol — A compound containing the hydroxyl (–OH) group attached to a saturated carbon.
Primary, secondary, tertiary alcohol — The –OH carbon is attached to one, two or three other carbons.
Aldehyde — A carbonyl compound with the C=O group at the end of the chain (–CHO).
Ketone — A carbonyl compound with the C=O group within the chain.
Carboxylic acid — A compound containing the –COOH group.
Ester — A compound formed from a carboxylic acid and an alcohol, containing –COO–.
Oxidation (organic) — Gain of oxygen or loss of hydrogen.
Reflux — Continuous boiling and condensing of a reaction mixture so that volatile substances are not lost.
Distillation — Separating a volatile product by evaporating and condensing it as it forms.
Nucleophilic addition — A reaction in which a nucleophile attacks the δ+ carbon of a C=O group, followed by addition of H⁺.
Fermentation — The anaerobic conversion of sugars to ethanol and carbon dioxide by yeast enzymes.
Core concepts
Physical properties of alcohols
Alcohols have higher boiling points than alkanes of similar Mᵣ because their –OH groups form hydrogen bonds. Short-chain alcohols are soluble in water because they hydrogen-bond with water molecules; solubility decreases as the non-polar hydrocarbon chain gets longer.
Production of ethanol
Fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, using yeast at about 30–40 °C, in anaerobic conditions. In the Caribbean, molasses from sugar processing is fermented and then distilled to make rum. Fermentation uses a renewable resource but is a slow batch process and produces dilute ethanol that must be distilled.
Hydration of ethene: C₂H₄ + H₂O → C₂H₅OH, with a phosphoric acid catalyst at about 300 °C and 60–70 atm. It is fast, continuous and gives purer ethanol, but uses ethene from non-renewable crude oil or natural gas.
Reactions of alcohols
Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O — ethanol is used as a fuel and fuel additive.
With sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂ (bubbles of hydrogen; less vigorous than sodium with water).
Substitution to halogenoalkanes: with HBr (from NaBr and concentrated H₂SO₄), PCl₅ (steamy fumes of HCl — a test for –OH), or SOCl₂.
Dehydration to alkenes: heating with concentrated sulfuric or phosphoric acid, or passing vapour over hot aluminium oxide: C₂H₅OH → C₂H₄ + H₂O.
Esterification: an alcohol reacts with a carboxylic acid, warmed with a concentrated sulfuric acid catalyst: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Esters have sweet, fruity smells and are used as flavourings, perfumes and solvents.
Oxidation of alcohols
The oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄. Its colour changes from orange to green as Cr₂O₇²⁻ is reduced to Cr³⁺.
- Primary alcohols are oxidised to aldehydes (distil off the aldehyde as it forms), then to carboxylic acids (heat under reflux with excess oxidant). CH₃CH₂OH + [O] → CH₃CHO + H₂O; CH₃CHO + [O] → CH₃COOH
- Secondary alcohols are oxidised to ketones: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O.
- Tertiary alcohols are not oxidised (no hydrogen on the –OH carbon); the dichromate stays orange.
This difference is used to distinguish primary, secondary and tertiary alcohols.
The iodoform (tri-iodomethane) test
Compounds containing the CH₃CH(OH)– group (such as ethanol and propan-2-ol) or the CH₃CO– group (such as ethanal and methyl ketones) give a pale yellow precipitate of CHI₃ (with an antiseptic smell) when warmed with iodine in alkaline solution.
Aldehydes and ketones: the carbonyl group
The C=O bond is polar (Cᵟ⁺=Oᵟ⁻), so the carbon is attacked by nucleophiles.
Nucleophilic addition of HCN (from KCN and dilute acid):
- The cyanide ion (:CN⁻) attacks the δ+ carbon, and the π electrons of C=O move onto the oxygen, forming an intermediate anion.
- The negatively charged oxygen picks up H⁺, forming a hydroxynitrile (cyanohydrin).
CH₃CHO + HCN → CH₃CH(OH)CN (2-hydroxypropanenitrile). This extends the carbon chain by one. With aldehydes and unsymmetrical ketones, the product contains a chiral centre and is formed as a racemic mixture, because the planar carbonyl can be attacked from either side.
Reduction: NaBH₄ (sodium tetrahydridoborate) reduces aldehydes to primary alcohols and ketones to secondary alcohols.
Distinguishing aldehydes from ketones
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens' reagent ([Ag(NH₃)₂]⁺), warm | silver mirror | no change |
| Fehling's (or Benedict's) solution, warm | blue → brick-red precipitate (Cu₂O) | stays blue |
| Acidified K₂Cr₂O₇, warm | orange → green | stays orange |
Aldehydes are oxidised in these tests because they have a hydrogen on the carbonyl carbon; ketones are not easily oxidised.
2,4-Dinitrophenylhydrazine (Brady's reagent) gives an orange-yellow precipitate with both aldehydes and ketones — a test for the carbonyl group. The melting point of the purified precipitate can identify the specific compound.
Carboxylic acids
Carboxylic acids are weak acids: CH₃COOH ⇌ CH₃COO⁻ + H⁺. They react with:
- metals: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
- bases: CH₃COOH + NaOH → CH₃COONa + H₂O
- carbonates (effervescence of CO₂ — a test for –COOH): 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
- alcohols to form esters.
Reduction with LiAlH₄ in dry ether converts carboxylic acids to primary alcohols.
Acid derivatives
- Acyl chlorides (e.g. ethanoyl chloride, CH₃COCl) are made from acids using PCl₅ or SOCl₂. They are very reactive and react vigorously with water (forming the acid and HCl), alcohols (forming esters, faster and irreversibly), and ammonia or amines (forming amides).
- Esters are hydrolysed by heating with acid (reversible) or with alkali (saponification, irreversible, forming the carboxylate salt). Saponification of fats and oils with NaOH produces soap — a traditional process in Caribbean households using coconut oil and lye.
- Amides contain –CONH₂ and are found in proteins (peptide links).
Worked examples
Example 1: Distinguishing alcohols
Describe a test to distinguish propan-1-ol, propan-2-ol and 2-methylpropan-2-ol.
Warm each with acidified potassium dichromate(VI). Propan-1-ol and propan-2-ol turn it from orange to green; 2-methylpropan-2-ol does not (tertiary). Distil the products from the first two and test with Tollens' reagent: the product from propan-1-ol (propanal) gives a silver mirror; the product from propan-2-ol (propanone) does not.
Example 2: Oxidation conditions
How would you make ethanal rather than ethanoic acid from ethanol?
Use a limited amount of acidified potassium dichromate(VI) and distil the ethanal immediately as it forms, so it is not oxidised further.
Example 3: Nucleophilic addition
Name the product when propanone reacts with HCN and state the mechanism type.
2-hydroxy-2-methylpropanenitrile, formed by nucleophilic addition.
Example 4: Esterification
Name the ester formed from methanol and butanoic acid and write its structural formula.
Methyl butanoate, CH₃CH₂CH₂COOCH₃.
Example 5: Identifying the isomers of C₄H₁₀O
Four alcohols have the formula C₄H₁₀O. Describe tests to identify each.
| Alcohol | Class | Acidified K₂Cr₂O₇ | Product tested with Tollens' | Iodoform test |
|---|---|---|---|---|
| Butan-1-ol | primary | orange → green | aldehyde: silver mirror | negative |
| 2-Methylpropan-1-ol | primary | orange → green | aldehyde: silver mirror | negative |
| Butan-2-ol | secondary | orange → green | ketone: no mirror | positive (contains CH₃CH(OH)–) |
| 2-Methylpropan-2-ol | tertiary | stays orange | — | negative |
The two primary alcohols could then be distinguished by their boiling points, or by the melting points of derivatives.
Example 6: A two-step synthesis
Outline how ethanol can be converted into ethyl ethanoate using ethanol as the only organic starting material.
- Oxidise some of the ethanol to ethanoic acid by heating under reflux with excess acidified potassium dichromate(VI).
- Warm the ethanoic acid with more ethanol and a few drops of concentrated sulfuric acid to form ethyl ethanoate by esterification.
Example 7: Reactions of carboxylic acids — observations
Describe what is seen when a few drops of ethanoic acid are added to (a) sodium carbonate solution, (b) magnesium ribbon.
(a) Effervescence; the gas turns limewater milky (CO₂). (b) Effervescence and the magnesium slowly dissolves; the gas gives a squeaky pop with a lighted splint (H₂). Both reactions are slower than with hydrochloric acid of the same concentration, because ethanoic acid is weak and the concentration of H⁺ is lower.
Example 8: Making a chain longer
Explain why the reaction of ethanal with HCN is useful in organic synthesis.
The nucleophilic addition of HCN adds a carbon atom, converting ethanal (2 carbons) into 2-hydroxypropanenitrile (3 carbons). The nitrile group can then be hydrolysed to a carboxylic acid, giving 2-hydroxypropanoic acid (lactic acid), so the reaction is a route to longer-chain molecules.
Functional group tests at a glance
| Functional group | Test | Positive result |
|---|---|---|
| C=C | bromine water | orange-brown → colourless |
| –OH (alcohol) | PCl₅ | steamy fumes of HCl |
| Primary/secondary alcohol | acidified K₂Cr₂O₇, warm | orange → green |
| C=O (aldehyde or ketone) | 2,4-DNPH | orange-yellow precipitate |
| Aldehyde | Tollens' reagent, warm | silver mirror |
| Aldehyde | Fehling's solution, warm | blue → brick-red precipitate |
| CH₃CO– or CH₃CH(OH)– | iodine in NaOH, warm | pale yellow precipitate (CHI₃) |
| –COOH | Na₂CO₃(aq) | effervescence; CO₂ turns limewater milky |
Common mistakes and how to avoid them
- Saying tertiary alcohols are oxidised — they are not, under these conditions.
- Confusing reflux and distillation — reflux for full oxidation to the acid; distillation to collect the aldehyde.
- Getting test results wrong — Tollens' gives a silver mirror; Fehling's gives a brick-red precipitate; Brady's is positive for both aldehydes and ketones.
- Naming esters backwards — the alcohol part comes first.
- Drawing nucleophilic addition with the nucleophile attacking oxygen — it attacks the δ+ carbon.
- Forgetting the catalyst (concentrated H₂SO₄) in esterification.
Self-test questions
- Name the organic product when butan-2-ol is heated with acidified potassium dichromate(VI). Answer: Butanone.
- Which of these give a positive iodoform test: ethanol, methanol, propanone, propanal? Answer: Ethanol and propanone (both contain CH₃CH(OH)– or CH₃CO–).
- Write the equation for the alkaline hydrolysis of ethyl ethanoate with sodium hydroxide. Answer: CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH.
- Why does ethanol have a much higher boiling point than ethanal, although their Mᵣ values are similar? Answer: Ethanol molecules form hydrogen bonds through their O–H groups; ethanal has only permanent dipole–dipole and London forces, which are weaker.
How this links to your Internal Assessment
These functional groups offer rich SBA opportunities: preparing an ester and identifying it by smell, investigating the fermentation of different sugar sources (cane sugar, molasses, fruit juice) by measuring CO₂ production, carrying out functional group tests on unknown compounds, or determining the ethanoic acid content of vinegar by titration. Strong investigations control temperature and yeast quantity in fermentation, use appropriate safety measures for flammable organics, and explain observations using balanced equations.
Exam technique for Alcohols, carbonyls and carboxylic acids
- Learn reagent + conditions + product + observation for each reaction.
- Draw the nucleophilic addition mechanism with curly arrows from the lone pair on :CN⁻ and from the C=O π bond.
- In synthesis questions, map the route through functional groups and give conditions for each step.
- Name compounds carefully, including position numbers.
Quick revision summary
- Alcohols: hydrogen bonding raises boiling points; short chains are water-soluble.
- Ethanol from fermentation (yeast, 30–40 °C, anaerobic) or hydration of ethene (H₃PO₄, 300 °C, 60–70 atm).
- Oxidation with acidified K₂Cr₂O₇ (orange → green): primary → aldehyde (distil) → acid (reflux); secondary → ketone; tertiary no reaction.
- Iodoform test: yellow CHI₃ for CH₃CH(OH)– and CH₃CO– groups.
- Carbonyls: nucleophilic addition of HCN; reduction with NaBH₄.
- Aldehydes give positive Tollens' and Fehling's tests; Brady's reagent detects both aldehydes and ketones.
- Carboxylic acids are weak acids, react with carbonates (CO₂), and form esters, acyl chlorides and amides; alkaline hydrolysis of fats makes soap.