Alcohols — AQA GCSE Chemistry
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Alcohols explained
Alcohols covers 5 specification points, including Alcohols contain the functional group –OH.
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and Methanol, ethanol, propanol and butanol are…. Open a point below for its explanation and revision guidance.
Alcohols contain the functional group –OH.
The –OH group is the functional group of alcohols. It consists of an oxygen atom covalently bonded to a hydrogen atom, and that oxygen is also bonded to a carbon atom of the hydrocarbon chain. The –OH group gives alcohols their characteristic properties, including solubility in water and reactions such as combustion and oxidation. In ethanol, CH₃CH₂OH, the –OH is attached to a carbon that already has two hydrogen atoms and one carbon neighbour, so the oxygen forms two single covalent bonds and retains two lone pairs. Recognising the –OH group allows you to identify an alcohol from a displayed formula, to name it using the -ol suffix, and to predict that it will react with sodium and be oxidised by acidified potassium dichromate(VI). The position of the –OH on the chain matters: propan-1-ol and propan-2-ol are structural isomers with different properties.
Methanol, ethanol, propanol and butanol are the first four members of a homologous series of alcohols.
A homologous series is a family of compounds with the same functional group and a general formula, in which each member differs from the next by a CH₂ unit and shows a gradual change in physical properties. For alcohols the general formula is CₙH₂ₙ₊₁OH. The first four members are methanol, CH₃OH; ethanol, CH₃CH₂OH; propanol, CH₃CH₂CH₂OH; and butanol, CH₃CH₂CH₂CH₂OH. Each successive member has one more carbon and two more hydrogen atoms than the previous one. As the chain length increases, boiling point rises and solubility in water decreases, while the –OH group continues to give the same characteristic reactions. Propanol and butanol each have structural isomers in which the –OH is on a different carbon, so the position of the functional group must be specified in the name.
Aqueous solutions of ethanol are produced when sugar solutions are fermented using yeast.
Fermentation is the breakdown of sugar by yeast to produce ethanol and carbon dioxide. Yeast provides enzymes that catalyse the reaction. A sugar solution, such as glucose solution, is warmed to about 25–35 °C and kept under anaerobic conditions; the yeast enzymes convert glucose into ethanol and carbon dioxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. The ethanol formed is in aqueous solution because it is produced in water, and it can be separated by fractional distillation. Temperatures that are too high denature the enzymes and stop fermentation, while oxygen encourages aerobic respiration instead of fermentation. The reaction is exothermic and produces bubbles of carbon dioxide, which can be tested with limewater.
Students should know the conditions used for fermentation of sugar using yeast.
Fermentation converts sugars such as glucose into ethanol and carbon dioxide using yeast. The yeast provides enzymes that catalyse the reaction. The conditions are: a warm temperature, typically around 25–35 °C, but not so hot that enzymes denature; absence of oxygen, because oxygen would allow aerobic respiration instead; and a source of sugar in solution. A typical method mixes glucose solution with yeast in a flask, fits a bung and delivery tube to keep air out and collect gas, and keeps the mixture warm. The gas produced turns limewater milky, confirming carbon dioxide, while the liquid contains ethanol, which can be separated by fractional distillation.
Students should be able to recognise alcohols from their names or from given formulae.
Alcohols are a homologous series of organic compounds containing the –OH functional group. Their names end in -ol, so methanol, ethanol, propanol and butanol are alcohols. Their formulae show the –OH group attached to a carbon chain; for example, ethanol is C₂H₅OH and propanol is C₃H₇OH. Recognising an alcohol means identifying the –OH group in a displayed, structural or molecular formula, or using the -ol ending in a name. The general formula for alcohols with one –OH group is CₙH₂ₙ₊₁OH. Students should distinguish alcohols from alkanes, which have no –OH group, and from carboxylic acids, which contain –COOH.
Your focus
- State that alcohols contain the –OH functional group.
- Recognise the –OH group in displayed and structural formulae of alcohols.
- Explain how the –OH group determines the characteristic reactions of alcohols.
Show all 15 objectives
- Name and write the formulae of the first four alcohols in the homologous series.
- Describe how successive members of the alcohol series differ by a CH₂ unit.
- Explain trends in boiling point and solubility as the alcohol chain length increases.
- Describe how yeast ferments sugar solution to produce aqueous ethanol and carbon dioxide.
- Write the word and balanced symbol equations for the fermentation of glucose.
- Explain why warm, anaerobic conditions are needed for fermentation.
- Describe the conditions needed for fermentation of sugar using yeast.
- Explain why each condition is necessary for fermentation.
- Identify the products of fermentation and describe a test for carbon dioxide.
- Recognise the –OH functional group in names and formulae.
- Classify a compound as an alcohol from its name or formula.
- Distinguish alcohols from alkanes and carboxylic acids.
Alcohols exam tips
Marking Points
- Identify the –OH group as the functional group present in all alcohols.
- Describe the –OH group as an oxygen atom bonded to a hydrogen atom and to a carbon atom of the molecule.
- Recognise the –OH group in a displayed formula and use it to classify the compound as an alcohol.
- Name alcohols using the -ol suffix and, where needed, a number to show the position of the –OH group.
- Link the –OH group to characteristic alcohol reactions such as reaction with sodium and oxidation by acidified potassium dichromate(VI).
- Define a homologous series as a family of compounds with the same functional group and a general formula, differing by CH₂ units.
- Name the first four alcohols in order as methanol, ethanol, propanol and butanol.
- Give the molecular formulae CH₄O, C₂H₆O, C₃H₈O and C₄H₁₀O, or the general formula CₙH₂ₙ₊₁OH.
- Explain that each successive member has one more carbon atom and two more hydrogen atoms than the previous member.
- Describe the trend in physical properties, such as increasing boiling point and decreasing solubility in water, as chain length increases.
- Recognise that propanol and butanol have structural isomers depending on the position of the –OH group.
- State that fermentation uses yeast to convert sugar into ethanol and carbon dioxide.
- Give the word equation glucose → ethanol + carbon dioxide, or the balanced symbol equation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
- Describe the conditions: a sugar solution, yeast, a warm temperature of about 25–35 °C and the absence of oxygen.
- Explain that enzymes in yeast catalyse the fermentation reaction.
- State that the ethanol produced is in aqueous solution and can be concentrated by fractional distillation.
- Describe the test for the carbon dioxide produced, such as bubbling it through limewater to form a white precipitate.
- States that yeast is added to a sugar solution, for example glucose solution.
- States that the mixture must be kept warm, typically around 25–35 °C, so enzymes work at a suitable rate.
- States that oxygen must be excluded, often by using an airlock or bung, so anaerobic respiration occurs.
- States that the products are ethanol and carbon dioxide.
- Explains that yeast enzymes catalyse the conversion of sugar to ethanol and carbon dioxide.
- Describes a test for carbon dioxide, such as bubbling the gas through limewater and observing a milky precipitate.
- Identifies the –OH functional group in a displayed, structural or molecular formula.
- Uses the -ol ending in a name as evidence that the compound is an alcohol.
- Recognises ethanol as C₂H₅OH and propanol as C₃H₇OH.
- Distinguishes alcohols from alkanes, which contain only C–C and C–H bonds.
- Distinguishes alcohols from carboxylic acids, which contain the –COOH group.
- Applies the general formula CₙH₂ₙ₊₁OH to simple alcohols with one –OH group.
Examiner Tips
- 💡When identifying an alcohol, locate the oxygen and check that it is bonded to both a hydrogen atom and a carbon atom.
- 💡In displayed formulae, draw the O–H bond explicitly so the functional group is unmistakable.
- 💡Use the -ol ending in names and add a number only when the –OH could be in more than one position.
- 💡Learn the first four names and formulae as a sequence so you can move up or down the series quickly.
- 💡When asked for a trend, quote a specific property and state the direction of change with increasing chain length.
- 💡Check the general formula by counting atoms in a known member such as ethanol before applying it to another alcohol.
- 💡Learn the balanced equation for fermentation and check that carbon, hydrogen and oxygen atoms balance on both sides.
- 💡When describing conditions, mention yeast, a sugar solution, a warm temperature and the absence of oxygen.
- 💡Link the carbon dioxide produced to a limewater test to show how the gas can be identified.
- 💡Link each condition to its purpose: warmth for enzyme activity, no oxygen for anaerobic conditions, sugar as the reactant.
- 💡When describing a practical, name the sugar solution and the yeast, and state how oxygen is kept out.
- 💡If asked to identify a gas, describe the limewater test and the result rather than naming the gas alone.
- 💡Circle or underline the –OH group in a displayed formula before deciding whether the compound is an alcohol.
- 💡Check the name ending: -ol indicates an alcohol, while -ane indicates an alkane and -oic acid indicates a carboxylic acid.
- 💡When writing formulae, keep the –OH group intact rather than separating the oxygen and hydrogen.
Common Mistakes
- Confusing the –OH group in alcohols with the hydroxide ion, OH⁻; correct this by stating that alcohols contain a covalently bonded –OH group, not a hydroxide ion.
- Treating –OH as a separate ion attached to the chain; correct this by drawing the oxygen bonded to both the carbon chain and the hydrogen atom.
- Assuming every molecule containing oxygen is an alcohol; correct this by checking that the oxygen is bonded to hydrogen and to a carbon atom.
- Ignoring the position of the –OH group when naming; correct this by numbering the carbon chain so the –OH has the lowest possible number.
- Writing the general formula as CₙH₂ₙOH; correct this by writing CₙH₂ₙ₊₁OH for alcohols.
- Stating that all members have identical physical properties; correct this by describing the gradual trend in boiling point and solubility.
- Naming the third member as propyl alcohol only; correct this by using the systematic name propanol, adding a number if the –OH position varies.
- Assuming each member differs by one carbon atom only; correct this by stating that each member differs by a CH₂ unit.
- Stating that fermentation produces only ethanol; correct this by including carbon dioxide as the other product.
- Using very high temperatures to speed up fermentation; correct this by keeping the mixture warm but not hot, because high temperatures denature the yeast enzymes.
- Writing the product as ethanoic acid; correct this by writing ethanol, because fermentation of sugar produces ethanol, not ethanoic acid.
- Claiming that oxygen is needed; correct this by stating that anaerobic conditions favour fermentation.
- Saying the mixture should be boiled or heated strongly; correction: keep it warm, around 25–35 °C, because higher temperatures denature the yeast enzymes.
- Claiming oxygen is needed; correction: oxygen must be excluded so that anaerobic fermentation occurs rather than aerobic respiration.
- Stating that fermentation produces only ethanol; correction: carbon dioxide is also produced and can be tested with limewater.
- Confusing the –OH group in an alcohol with the hydroxide ion, OH⁻; correction: the alcohol functional group is a covalent –OH group, not a separate hydroxide ion.
- Treating any compound containing oxygen as an alcohol; correction: look specifically for the –OH group attached to carbon, and note that carboxylic acids contain –COOH.
- Mixing up the formulae of methanol and ethanol; correction: methanol is CH₃OH and ethanol is C₂H₅OH.