1.8.2Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Functional Groups and Homologous Series — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 28 of 36 Unit quiz
Section 1 of 5Key Definitions
Organising organic compounds into families makes it possible to predict their behaviour.
Functional group
- An atom or group of atoms in a molecule that is responsible for its characteristic chemical reactions.
Homologous series
- A family of compounds with the same functional group and general formula, differing by one CH₂ unit from the next member.
Chemical properties
- Members of a homologous series react in similar ways because they share the same functional group.
Physical properties
- Boiling points and other physical properties change gradually as the carbon chain gets longer.
Hydrocarbon
- A compound containing only hydrogen and carbon atoms.
Section 2 of 5Common Functional Groups
Common Functional Groups
Alkane
- Single C–C bonds only, general formula CₙH₂ₙ₊₂, and suffix -ane.
Alkene
- A carbon-carbon double bond C=C, general formula CₙH₂ₙ, and suffix -ene.
Alcohol
- A hydroxyl group –OH on a carbon chain, general formula CₙH₂ₙ₊₁OH, and suffix -ol.
Halogenoalkane
- A halogen atom replacing hydrogen, general formula CₙH₂ₙ₊₁X, with prefix chloro-, bromo- or iodo-.
Aldehyde and ketone
- Both contain a C=O group; an aldehyde has it at the end of the chain (–CHO) and a ketone within the chain.
Carboxylic acid
- The –COOH group, with suffix -oic acid.
| Homologous series | Functional group | Example |
|---|---|---|
| Alkane | C–C | Ethane, C₂H₆ |
| Alkene | C=C | Ethene, C₂H₄ |
| Alcohol | –OH | Ethanol, C₂H₅OH |
| Halogenoalkane | –Br | Bromoethane, C₂H₅Br |
Section 3 of 5Types of Formula
1
The empirical formula gives the simplest whole number ratio of atoms, so ethane C₂H₆ has the empirical formula CH₃.
2
The molecular formula gives the actual number of each atom, such as C₄H₁₀ for butane.
3
The structural formula shows the arrangement of groups without every bond, for example CH₃CH₂CH₂CH₃.
4
The displayed formula shows every atom and every bond in the molecule.
5
The skeletal formula shows only the carbon backbone as lines, with functional groups labelled and hydrogens on carbon not shown.
Section 4 of 5General Formulae in Use
General Formulae in Use
Finding formula
- For an alkane with 6 carbons, 2n + 2 gives 14 hydrogens, so the formula is C₆H₁₄.
Checking unsaturation
- Having fewer hydrogens than 2n + 2 shows the molecule contains a double bond or ring.
Saturated
- A saturated compound has only single bonds between carbon atoms.
Aliphatic
- Compounds with carbon in chains or non-aromatic rings are classed as aliphatic, whereas benzene-based compounds are aromatic.
Section 5 of 5Exam Checklist & Tips
1.8.2 – Key Points for the Exam
- Functional group The group of atoms that gives an organic molecule its characteristic reactions.
- Homologous series A family of organic compounds with the same functional group and general formula.
- General formula An algebraic formula that applies to every member of a homologous series.
- Saturated Containing only single carbon-carbon bonds.
- Skeletal formula A simplified representation showing only the carbon skeleton and functional groups.
Common Mistakes
- Saying the members of a homologous series have identical physical properties.
- Writing C₂H₅OH as the formula of an alkane.
- Forgetting that a ketone has C=O inside the chain and an aldehyde has it at the end.
- Including hydrogens attached to carbon in a skeletal formula.
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