3.2Unit 1 · Physical & Organic Chemistry
Introducing organic chemistry — Edexcel IGCSE Chemistry Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 7 parts Lesson 2 of 5 1 quiz
Lesson summary
Explain the terms hydrocarbon, homologous series, functional group and isomerism.Carbon forms four covalent bonds, allowing chains, branches and rings.Stems1 C meth-, 2 C eth-, 3 C prop-, 4 C but-, 5 C pent-.Members of a homologous series differ by CH2 and show trends in physical…
- What is organic chemistry?
- Organic compounds
- Classifying organic reactions
- Empirical formula of organic molecules
- Isomers in different families
- More naming rules
Section 1 of 7Introducing organic chemistry
Essential knowledge
What is organic chemistry?
It is the study of carbon compounds. Carbon has four outer electrons and forms four covalent bonds, so it can bond to itself to make chains, branches and rings, giving millions of compounds.
Organic compounds
Organic compounds are grouped into homologous series. Members share a general formula and functional group, differ by CH2, have similar chemical reactions and show a gradual change in physical properties such as boiling point.
Formulae
Formulae the molecular formula gives atom numbers (C2H6O); the structural formula shows groupings (CH3CH2OH); the displayed formula shows every bond; the general formula represents the whole series.
Naming organic compounds
Naming organic compounds the stem tells the number of carbon atoms in the longest chain; the ending tells the functional group. Position numbers locate a double bond or –OH group, e.g. propan-1-ol, but-2-ene. Branches are named as methyl, ethyl, etc.
Structural isomers have the same molecular formula but different structures, e.g. butane and 2-methylpropane (C4H10), or but-1-ene and but-2-ene (C4H8).
Classifying organic reactions
Classifying organic reactions in substitution one atom is replaced by another (alkanes + halogens in UV). In addition two molecules become one (alkenes + bromine, hydrogen or steam). Combustion is reaction with oxygen. Addition polymerisation joins many alkene monomers.
Saturated compounds contain only single C–C bonds; unsaturated compounds contain at least one C=C double bond. Only unsaturated compounds undergo addition reactions.
Empirical formula of organic molecules
The empirical formula is the simplest whole-number ratio of atoms. Many organic molecules have a different empirical and molecular formula: butane C4H10 has empirical formula C2H5, and ethene C2H4 has CH2.
Isomers in different families
Isomers can belong to different homologous series. C3H6 can be propene (an alkene with C=C) or cyclopropane (a ring with only single bonds). Different functional groups give different chemical properties, e.g. only propene decolourises bromine water.
More naming rules
The stem hex- means six carbons. Numbers show where a group is attached, and di- shows two identical groups, e.g. 1,2-dibromoethane has a bromine atom on each of the two carbons.
Incomplete combustion equations
With limited oxygen, a hydrocarbon forms carbon monoxide or carbon (soot) and water, e.g. 2CH4 + 3O2 → 2CO + 4H2O, or CH4 + O2 → C + 2H2O.
Section 2 of 7Process & sequence
1
Find the longest continuous carbon chain and choose the stem (meth-, eth-, prop-, but-, pent-).
2
Identify the functional group and select the ending (-ane, -ene, -ol, -oic acid).
3
Number the chain from the end that gives the functional group the lowest number.
4
Name any branches (methyl, ethyl) and give their position numbers.
5
Write numbers and words separated by hyphens, e.g. but-2-ene or 2-methylpropane.
Naming an organic compoundwrite these steps in this order; each step is a marking point.
1
Find longest carbon chain
2
Choose stem from carbon count
3
Identify functional group ending
4
Number from nearest group end
5
Add branch names and positions
Section 3 of 7Compare
| Series | Functional group | General formula | Suffix / example |
|---|---|---|---|
| Alkanes | C–C single bonds only | CnH2n+2 | -ane, ethane |
| Alkenes | C=C | CnH2n | -ene, ethene |
| Alcohols | –OH | CnH2n+1OH | -ol, ethanol |
| Carboxylic acids | –COOH | CnH2n+1COOH | -oic acid, ethanoic acid |
| Esters | –COO– | CnH2nO2 | -oate, ethyl ethanoate |
| Type | What happens | Example |
|---|---|---|
| Substitution | One atom/group replaced by another | CH4 + Br2 → CH3Br + HBr |
| Addition | Two molecules join; C=C becomes C–C | C2H4 + Br2 → C2H4Br2 |
| Combustion | Reacts with oxygen | CH4 + 2O2 → CO2 + 2H2O |
| Polymerisation | Many monomers join | n ethene → poly(ethene) |
Section 4 of 7Worked examples
1Draw and name the two structural isomers of C4H10.
- Straight chainCH3CH2CH2CH3, butane.
- BranchedCH3CH(CH3)CH3, a three-carbon chain with a methyl on carbon 2, 2-methylpropane.
- Both have 4 C and 10 H but different structures.
2Addition
Classify the reaction C2H4 + H2O → C2H5OH.
- Two molecules combine to form one product and the C=C double bond becomes a single bond, so it is an addition reaction (hydration of ethene).
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Organic chemistry · Hydrocarbon · Functional group · Homologous series · Molecular formula · Displayed formula · Structural formula · General formula · Isomers · Addition reaction
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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