You might associate the word 'alcohol' entirely with drinks, but in chemistry, it describes an entire family of versatile compounds used in everything from hand sanitisers to race car fuels. Alcohols form a homologous series, meaning they are a family of organic compounds that follow the same general formula: .
Each successive member of this series differs from the previous one by a single unit. All alcohols share the same functional group, which is an atom or group of atoms that dictates how the molecule reacts chemically. For alcohols, this is the hydroxyl group (). Note that this is a covalently bonded group and does not behave like the hydroxide ion () found in strong alkalis.
To correctly identify and draw alcohols in an exam, you must understand three ways of representing them:
Examiners expect you to recall the specific formulae and draw the displayed structures for the first four members. For propanol and butanol, you only need to know the straight-chain versions (where the group is on the end carbon).
1. Methanol (1 Carbon)
2. Ethanol (2 Carbons)
3. Propanol (3 Carbons)
4. Butanol (4 Carbons)
Draw the displayed formula for butanol, ensuring you meet all Edexcel mark scheme requirements.
Step 1: Draw the carbon backbone. Butanol has 4 carbon atoms. Draw them in a straight chain connected by single covalent bonds: .
Step 2: Attach the functional group. Add the oxygen atom to the end carbon. Crucial Edexcel Rule: You must draw the single bond between the oxygen and the hydrogen ().
Step 3: Fill remaining bonds with hydrogen. Every carbon atom must have exactly four bonds. Add hydrogen atoms above, below, and to the sides of the carbon chain until all slots are filled. (See structure for Butanol above).
Students frequently lose marks in displayed formulae by writing -OH at the end of the chain. You MUST explicitly draw the bond between the oxygen and hydrogen (-O-H) to get the mark.
Always write the molecular formula of an alcohol with the OH clearly separated at the end (e.g., write C2H5OH instead of C2H6O) to prove you know it contains a hydroxyl group.
Do not waste time learning complex branched isomers for this topic; for propanol and butanol, Edexcel only requires you to know the straight-chain versions (propan-1-ol and butan-1-ol).
Homologous series
A family of organic compounds with the same general formula, similar chemical properties, and a clear trend in physical properties.
Functional group
A specific atom or group of atoms within a molecule that determines its characteristic chemical reactions.
Hydroxyl group
The -OH functional group that is characteristic of all alcohol molecules.
Molecular formula
A chemical formula showing the actual number and type of each atom present in a molecule.
Structural formula
A written representation that shows the arrangement of atoms in a molecule group by group, without drawing out the individual bonds.
Displayed formula
A graphical representation of a molecule showing all the atoms and every single covalent bond between them as lines.
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Homologous series
A family of organic compounds with the same general formula, similar chemical properties, and a clear trend in physical properties.
Functional group
A specific atom or group of atoms within a molecule that determines its characteristic chemical reactions.
Hydroxyl group
The -OH functional group that is characteristic of all alcohol molecules.
Molecular formula
A chemical formula showing the actual number and type of each atom present in a molecule.
Structural formula
A written representation that shows the arrangement of atoms in a molecule group by group, without drawing out the individual bonds.
Displayed formula
A graphical representation of a molecule showing all the atoms and every single covalent bond between them as lines.