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Alcohols have the -OH functional group, and ethanol is written CH₃CH₂OH or C₂H₅OH. Free GCSE AQA Chemistry notes with key terms and 10 questions answered.
Introduction to Alcohols and Their Structure is part of Reactions of alkenes and alcohols (chemistry only) in GCSE AQA Higher Chemistry.
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Alcohols have the -OH functional group, and ethanol is written CH₃CH₂OH or C₂H₅OH. Free GCSE AQA Chemistry notes with key terms and 10 questions answered.
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Use this lesson to stay inside Reactions of alkenes and alcohols (chemistry only) while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Introduction to Alcohols and Their Structure before testing yourself from memory.
You've probably heard of ethanol - it's in alcoholic drinks and hand sanitiser. But what makes ethanol an 'alcohol' from a chemistry perspective? The answer lies in a specific group of atoms that gives all alcohols their characteristic properties.
The -OH group is called a functional group because it's the part that makes alcohols 'function' in chemical reactions. Think of it like a chemical signature - whenever you see -OH attached to a carbon chain, you know you're dealing with an alcohol.
**Trap**: Many students confuse the -OH in alcohols with the OH⁻ ions in bases. They're completely different - the -OH in alcohols is covalently bonded to carbon, not floating around as an ion.
Alcohols form a homologous series - a family of compounds with the same functional group but different chain lengths. You need to know the first four members by name and structure.
Here are the four alcohols you must memorise:
Methanol: CH₃OH (1 carbon)Ethanol: CH₃CH₂OH or C₂H₅OH (2 carbons)Propanol: CH₃CH₂CH₂OH or C₃H₇OH (3 carbons)Butanol: CH₃CH₂CH₂CH₂OH or C₄H₉OH (4 carbons)
Notice the pattern: each alcohol has one more CH₂ group than the previous one. The -OH group is always at the end of the chain in these simple alcohols.
Alcohols can be represented in several ways. Examiners expect you to recognise all these formats and convert between them.
Method 1: Full structural formulaShows every atom and every bond. For ethanol:
H-C-C-O-H with all H atoms shown| |H H
Method 2: Condensed structural formulaGroups atoms together: CH₃CH₂OH
Method 3: Molecular formulaJust shows the total number of each atom: C₂H₆O
The key skill is spotting the -OH group in any representation. In exam questions, they might give you CH₃CH₂CH₂OH or C₃H₇OH - both represent propanol.
Worked example: Drawing the Structural Formula for Butanol
Draw the full structural formula for butanol (C₄H₉OH)
Answer: CH₃CH₂CH₂CH₂OH or H₃C-CH₂-CH₂-CH₂OH
Exam questions test your ability to spot alcohols from their names or formulae. Here's your strategy for guaranteed marks.
From names: Any compound ending in '-ol' is an alcohol. Methanol, ethanol, propanol, butanol - the '-ol' suffix is the giveaway.
From formulae: Look for the -OH group. It might be written as CH₃OH, C₂H₅OH, or even just shown in a structural diagram.
**Common exam trap**: Questions might show you compounds like CH₃COOH (propanoic acid) or CH₃CH₂Cl (chloroethane) alongside alcohols. The key is spotting that -OH group.
Alcohol questions appear regularly in GCSE papers. Here's how to maximise your marks and avoid the traps that catch most students.
When naming alcohols: Always use the exact names - methanol, ethanol, propanol, butanol. Don't write 'methyl alcohol' or 'ethyl alcohol' - you won't get the mark.
When drawing structures: Make sure the -OH group is clearly shown. Writing C₂H₅OH is fine, but if you draw the full structure, every bond must be visible.
When identifying functional groups: The question might ask 'What functional group is present in ethanol?' The answer is '-OH' or 'hydroxyl group', not just 'alcohol'.
Exam precision note: In Introduction to Alcohols and Their Structure - Alcohols - Reactions of alkenes and alcohols (chemistry only), always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
Ethanol can be written as CH3CH2OH. This formula shows clearly that the hydroxyl group is attached to the carbon chain, which is the key structural feature that identifies an alcohol.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Ethanol can be written as CH₃CH₂OH or C₂H₅OH, and its molecular formula is C₂H₆O. It has 2 carbon atoms and the -OH functional group, and it is found in alcoholic drinks and hand sanitiser.
A homologous series is a family of organic compounds with the same functional group and similar chemical properties, where each member differs from the next by a CH₂ unit. Methanol, ethanol, propanol and butanol form a homologous series.
The formula of methanol is CH₃OH. It is the first alcohol in the homologous series, with 1 carbon atom and the -OH functional group, and its name ends in '-ol' like all alcohols.
The functional group of alcohols is -OH, the hydroxyl group. It is covalently bonded to a carbon atom and determines the chemical behaviour and properties of alcohols.
The formula of propanol is CH₃CH₂CH₂OH, also written C₃H₇OH. It is the third alcohol in the homologous series, with 3 carbons and one more CH₂ group than ethanol.
An alcohol has a carbon chain with an -OH (hydroxyl) functional group covalently bonded to a carbon atom. In simple alcohols like ethanol, CH₃CH₂OH, the -OH group is at the end of the chain, and it determines how alcohols behave in reactions.
To draw butanol, draw a chain of 4 carbon atoms, put the -OH group on the end carbon, then add hydrogen atoms so each carbon has 4 bonds. This gives CH₃CH₂CH₂CH₂OH, with 4 carbons, 10 hydrogens and 1 oxygen.
The first four alcohols are methanol (CH₃OH), ethanol (C₂H₅OH), propanol (C₃H₇OH) and butanol (C₄H₉OH). They form a homologous series, with each member having one more CH₂ group than the one before.
An alcohol can be recognised by a name ending in -ol, such as ethanol, or by an -OH group in its formula, such as CH₃CH₂OH or C₂H₅OH. The -OH in alcohols is covalently bonded to carbon and is not an OH⁻ ion.
The -OH in an alcohol is not the same as the OH⁻ ion in bases. In alcohols the -OH group is covalently bonded to carbon, not floating around as an ion, and confusing the two is a common mistake.