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Alcohols and phenols: Class 12 Chemistry
Classification and naming of alcohols, phenols and ethers, preparation, hydrogen bonding and physical properties, reactions of alcohols and phenols, acidity, tests to distinguish them, and uses of methanol and ethanol.
Key points
Alcohols have the general formula R-OH, where the OH group is bonded to a saturated sp³ carbon. In phenols the OH group is bonded directly to a benzene ring (C₆H₅OH). Ethers have the formula R-O-R'. Alcohols are primary (RCH₂OH), secondary (R₂CHOH) or tertiary (R₃COH), depending on the carbon that holds the OH group. Alcohols with two or three OH groups are called diols and triols, such as ethane-1,2-diol and propane-1,2,3-triol (glycerol).
IUPAC names replace the final e of the alkane by ol and number the OH group: CH₃CH₂OH is ethanol, CH₃CH(OH)CH₃ is propan-2-ol. Ethers are named as alkoxyalkanes, for example CH₃OCH₂CH₃ is methoxyethane. Alcohols and ethers with the same formula are functional group isomers, such as ethanol and methoxymethane (C₂H₆O).
Preparation of alcohols:
- Hydration of alkenes with steam and a phosphoric acid catalyst: C₂H₄ + H₂O → C₂H₅OH.
- Hydrolysis of alkyl halides with aqueous KOH.
- Reduction of aldehydes and ketones to primary and secondary alcohols (with LiAlH₄ or NaBH₄).
- Reaction of Grignard reagents with aldehydes and ketones, followed by water.
- Fermentation of sugars by yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (zymase enzyme, about 30 to 35 °C).
Alcohols have much higher boiling points than alkanes or ethers of similar mass because their molecules form hydrogen bonds with each other. Ethanol boils at 78 °C, while methoxymethane boils at about −24 °C. Lower alcohols dissolve in water in all proportions because they form hydrogen bonds with water. Solubility falls as the alkyl chain gets longer. Phenol is a colourless crystalline solid and is only slightly soluble in water.
Reactions of alcohols involving the O-H bond:
- With sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. Hydrogen gas is released.
- With carboxylic acids (esterification) in the presence of conc. H₂SO₄: R-COOH + R'OH ⇌ R-COOR' + H₂O.
- Alcohols are very weak acids, much weaker than water. They do not react with NaOH.
Reactions of alcohols involving the C-O bond and oxidation:
- With HX or PCl₅ or SOCl₂ the OH group is replaced by a halogen. Tertiary alcohols react fastest with HCl and ZnCl₂ (Lucas test: tertiary at once, secondary in about five minutes, primary does not react in the cold).
- Dehydration with conc. H₂SO₄ at about 170 °C gives an alkene. At about 140 °C with excess alcohol it gives an ether, 2C₂H₅OH → C₂H₅OC₂H₅ + H₂O.
- Oxidation: a primary alcohol gives an aldehyde and then a carboxylic acid, a secondary alcohol gives a ketone, and a tertiary alcohol resists oxidation. Acidified K₂Cr₂O₇ turns from orange to green.
Phenol is more acidic than alcohols. The phenoxide ion is stabilised because its negative charge spreads into the benzene ring by resonance. Phenol reacts with NaOH to give sodium phenoxide, but it is a weaker acid than carboxylic acids and carbonic acid, so it does not react with sodium hydrogencarbonate. Alcohols give neither reaction.
The OH group activates the benzene ring toward electrophilic substitution at the ortho and para positions. Phenol reacts with bromine water at once to give a white precipitate of 2,4,6-tribromophenol. Nitration with dilute HNO₃ gives 2-nitrophenol and 4-nitrophenol, and with conc. HNO₃ gives 2,4,6-trinitrophenol (picric acid). Phenol also gives a violet colour with neutral iron(III) chloride.
Ethers are fairly unreactive. They are good solvents and diethyl ether was used as an anaesthetic. The Williamson synthesis prepares them from sodium alkoxide and an alkyl halide: C₂H₅ONa + C₂H₅Br → C₂H₅OC₂H₅ + NaBr.
Uses: methanol as a solvent, fuel additive and for making methanal. It is poisonous and can cause blindness. Ethanol is used in drinks (in countries where permitted), as a solvent, in medicines, as fuel and as a disinfectant. Denatured spirit is ethanol made undrinkable by additives. Phenol is used to make plastics (Bakelite), dyes, drugs and disinfectants, and glycerol is used in cosmetics, medicines and explosives.
Study tip: Prepare a table of alcohol and phenol reactions with the reagent in one column and the observation in the next. Questions on comparison of acidity and on tests to distinguish compounds are very common, and they are won by quoting the observation (colour, precipitate or gas) and not only the name of the reagent.
Definitions and flashcards
Say the meaning first, then open the card to check.
Alcohol
An organic compound with an OH group on a saturated carbon atom.
Phenol
A compound with an OH group attached directly to a benzene ring.
Hydrogen bonding
Attraction between an H atom bonded to O (or N, F) and a lone pair on another such atom.
Esterification
Reaction of an alcohol with a carboxylic acid to give an ester and water.
Dehydration
Removal of water from a compound, as in alcohol to alkene.
Fermentation
Breakdown of sugars by enzymes of yeast to give ethanol and carbon dioxide.
Denatured spirit
Ethanol made unfit for drinking by adding poisonous substances.
Lucas reagent
Concentrated HCl with anhydrous ZnCl₂, used to distinguish alcohol classes.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why does ethanol have a higher boiling point than methoxymethane? (2 marks)
Both have the formula C₂H₆O. Ethanol molecules form hydrogen bonds with one another through the OH group, which need extra energy to break. Methoxymethane has no O-H bond and has only weak dipole forces.
Why is phenol more acidic than ethanol? (2 marks)
When phenol loses H⁺ it forms the phenoxide ion, whose negative charge is spread over the ring by resonance, so the ion is stable. The ethoxide ion has the charge on oxygen alone, and the ethyl group pushes electrons toward it, making it less stable.
How would you distinguish phenol from ethanol by two tests? (2 marks)
Add neutral FeCl₃ solution: phenol gives a violet colour and ethanol gives none. Add bromine water: phenol gives a white precipitate of 2,4,6-tribromophenol while ethanol does not.
Write the products when propan-1-ol and propan-2-ol are oxidised with acidified potassium dichromate. (3 marks)
Propan-1-ol is a primary alcohol and is oxidised first to propanal and then to propanoic acid. Propan-2-ol is a secondary alcohol and is oxidised to propanone (a ketone), which resists further oxidation. In both cases the orange dichromate turns green.
Explain how dehydration of ethanol gives different products with conc. H₂SO₄. (3 marks)
At about 170 °C with excess acid, one molecule of water is removed from each ethanol molecule and ethene forms: C₂H₅OH → C₂H₄ + H₂O. At about 140 °C with excess ethanol, two ethanol molecules lose one water molecule between them and diethyl ether forms: 2C₂H₅OH → C₂H₅OC₂H₅ + H₂O.
Describe the industrial preparation of ethanol by fermentation and by hydration of ethene. (5 marks)
Fermentation: sugars from molasses are diluted, yeast is added and the mixture is kept at about 30 to 35 °C. Enzymes of yeast (zymase) convert glucose into ethanol and CO₂: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The liquid is then distilled and fractionated to about 95% ethanol. Hydration: ethene and steam are passed over phosphoric acid supported on silica at high temperature and pressure: C₂H₄ + H₂O → C₂H₅OH. This gives purer ethanol and is faster, but needs petroleum as raw material.
Numericals with worked steps
Calculate the mass of ethanol obtained by fermentation of 90 g of glucose, assuming complete conversion. (C = 12, H = 1, O = 16)
- C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
- Molar mass of glucose = 72 + 12 + 96 = 180 g/mol.
- Moles of glucose = 90 / 180 = 0.5 mol.
- Moles of ethanol = 2 × 0.5 = 1 mol.
- Molar mass of ethanol = 46 g/mol, so mass = 46 g.
Answer: 46 g of ethanol
Find the mass of CO₂ and the volume of O₂ at STP involved when 9.2 g of ethanol burns completely. (1 mol gas = 22.4 dm³)
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
- Moles of ethanol = 9.2 / 46 = 0.2 mol.
- Moles of CO₂ = 2 × 0.2 = 0.4 mol; mass = 0.4 × 44 = 17.6 g.
- Moles of O₂ = 3 × 0.2 = 0.6 mol; volume = 0.6 × 22.4 = 13.44 dm³.
Answer: 17.6 g of CO₂ and 13.44 dm³ of O₂
Calculate the percentage of oxygen by mass in methanol, CH₃OH. (C = 12, H = 1, O = 16)
- Molar mass = 12 + 4 + 16 = 32 g/mol.
- Mass of oxygen = 16 g.
- Percentage = 16 / 32 × 100 = 50%.
Answer: 50% oxygen
4.7 g of phenol reacts with excess bromine water to give 2,4,6-tribromophenol. Find the mass of the product. (C = 12, H = 1, O = 16, Br = 80)
- C₆H₅OH + 3Br₂ → C₆H₂Br₃OH + 3HBr.
- Molar mass of phenol = 72 + 6 + 16 = 94 g/mol; moles = 4.7 / 94 = 0.05 mol.
- Ratio 1 : 1, so moles of product = 0.05 mol.
- Molar mass of C₆H₂Br₃OH = 72 + 2 + 240 + 17 = 331 g/mol.
- Mass = 0.05 × 331 = 16.55 g.
Answer: 16.55 g of 2,4,6-tribromophenol
Common mistakes
- Saying phenol is an alcohol. The OH group on a benzene ring changes the properties completely.
- Believing that phenol reacts with NaHCO₃ and gives CO₂. It is too weak an acid.
- Writing that tertiary alcohols oxidise easily. They resist mild oxidation.
- Mixing the dehydration conditions: 170 °C gives ethene and 140 °C gives ether.
- Forgetting that hydrogen bonding is the reason for high boiling point and water solubility.