Which statement about a homologous series is correct?

Successive members differ by a –CH₂– unit

Its members have different functional groups

Its members react in unrelated ways

IGCSE Chemistry 0620 Organic Chemistry Notes

Overview

What you’ll learn

Draw displayed formulae, write general formulae of the four homologous series, and identify functional groups and isomers.
Describe how petroleum is separated into fractions and state the uses and trends of each fraction.
Describe the unreactivity of alkanes, the reactions of alkenes, cracking, and the aqueous bromine test.
Compare the two industrial routes to ethanol and describe the reactions of ethanoic acid and ester formation.
Describe addition and condensation polymerisation, the problems of plastic waste, and the structures of nylon, PET and proteins.

Syllabus checklist (0620)

Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.

Core — every candidate

11.1Draw and interpret displayed formulae, in which every atom and bond is shown.
11.1Write and interpret the general formulae of the homologous series: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH and carboxylic acids CₙH₂ₙ₊₁COOH.
11.1Identify a functional group as the atom or group of atoms that determines the chemical properties of a homologous series.
11.1State that a homologous series is a family of similar compounds with similar chemical properties, due to the same functional group.
11.1State that a saturated compound has molecules in which all carbon–carbon bonds are single bonds.
11.1State that an unsaturated compound has molecules containing one or more carbon–carbon bonds that are not single bonds.
11.2Name and draw the displayed formulae of methane, ethane, ethene, ethanol and ethanoic acid, and of the products of the reactions in topics 11.4–11.7.
11.2State the type of compound from an ending -ane, -ene, -ol or -oic acid, or from a molecular or displayed formula.
11.3Name the fossil fuels: coal, natural gas and petroleum.
11.3Name methane as the main constituent of natural gas.
11.3State that hydrocarbons are compounds that contain hydrogen and carbon only.
11.3State that petroleum is a mixture of hydrocarbons and describe its separation into useful fractions by fractional distillation.
11.3Describe the properties of fractions changing from the bottom to the top of the fractionating column: shorter chain length, higher volatility, lower boiling points and lower viscosity.
11.3Name the uses of the fractions: refinery gas for heating and cooking; gasoline fraction for cars; naphtha fraction as a chemical feedstock; kerosene fraction for jet fuel; diesel fraction for diesel engines; fuel oil for ships and home heating; lubricating oil for lubricants, waxes and polishes; bitumen for road surfaces.
11.4State that alkanes are saturated hydrocarbons in which all the bonds are single covalent bonds.
11.4Describe alkanes as generally unreactive, except in terms of combustion and substitution by chlorine.
11.5State that alkenes are unsaturated hydrocarbons containing a carbon–carbon double bond.
11.5Describe the manufacture of alkenes and of hydrogen by the cracking of larger alkanes, using high temperature and a catalyst.
11.5Describe the reasons for cracking, including the demand for shorter-chain alkenes and hydrogen.
11.5Describe the test to distinguish saturated from unsaturated hydrocarbons using aqueous bromine.
11.6Describe the manufacture of ethanol by fermentation of aqueous glucose at 25–35 °C using yeast and the absence of oxygen, and by the catalytic addition of steam to ethene at 300 °C and 6000 kPa (60 atmospheres) using an acid catalyst.
11.6Describe the combustion of ethanol.
11.6State the uses of ethanol as a solvent and as a fuel.
11.7Describe the reaction of ethanoic acid with metals, bases and carbonates, including the names and formulae of the salts produced.
11.8Define polymers as large molecules built up from many smaller molecules called monomers.
11.8Describe the formation of poly(ethene) as an example of addition polymerisation of ethene monomers.
11.8State that plastics are made from polymers.
11.8Describe how the properties of plastics influence the way they are used and disposed of.
11.8Describe the environmental challenges of plastics: disposal in landfill, accumulation in oceans, and the production of toxic gases when they are burned.

Supplement — Extended candidates only

11.1State that a structural formula unambiguously describes the arrangement of atoms, for example CH₂=CH₂, CH₃CH₂OH and CH₃COOCH₃.
11.1Define structural isomers as compounds with the same molecular formula but different structural formulae, including examples with the formula C₄H₁₀ and C₄H₈.
11.1Describe the general characteristics of a homologous series: the same functional group, the same general formula, successive members differing by a –CH₂– unit, a trend in physical properties, and similar chemical properties.
11.2Name and draw the structural and displayed formulae of unbranched alkanes, alkenes including but-1-ene and but-2-ene, alcohols including propan-1-ol, propan-2-ol, butan-1-ol and butan-2-ol, and carboxylic acids containing up to four carbon atoms.
11.2Name and draw the displayed formulae of unbranched esters made from unbranched alcohols and carboxylic acids, each containing up to four carbon atoms.
11.4State that in a substitution reaction one atom or group is replaced by another atom or group.
11.4Describe the photochemical reaction of alkanes with chlorine, in which ultraviolet light provides the activation energy, and draw the products limited to monosubstitution.
11.5State that in an addition reaction only one product is formed.
11.5Describe the addition reactions of alkenes with bromine and aqueous bromine, with hydrogen using a nickel catalyst, and with steam using an acid catalyst, and draw the products.
11.6Describe the advantages and disadvantages of manufacturing ethanol by fermentation and by the catalytic addition of steam to ethene.
11.7Describe the formation of ethanoic acid by the oxidation of ethanol with acidified aqueous potassium manganate(VII), and by bacterial oxidation during vinegar production.
11.7Describe the reaction of a carboxylic acid with an alcohol, using an acid catalyst, to form an ester.
11.8Identify the repeat units and/or linkages in addition and condensation polymers.
11.8Deduce the repeat unit of an addition polymer from a given alkene, and vice versa.
11.8Deduce the repeat unit of a condensation polymer from given monomers, and vice versa, limited to polyamides made from a dicarboxylic acid and a diamine, and polyesters made from a dicarboxylic acid and a diol.
11.8Describe the differences between addition and condensation polymerisation.
11.8Describe and draw the structures of nylon, a polyamide, and PET, a polyester.
11.8State that PET can be converted back into monomers and re-polymerised.
11.8Describe proteins as natural polyamides formed from amino acid monomers with the general structure given in the syllabus, and describe and draw the protein structure.

In 30 seconds

1A homologous series is a family of compounds with the same functional group and similar reactions; neighbours differ by a –CH₂– unit.
2General formulae: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH, carboxylic acids CₙH₂ₙ₊₁COOH.
3Petroleum is separated by fractional distillation; fractions at the top are shorter-chain, more volatile, lower-boiling and runnier.
4Alkanes are saturated and unreactive; alkenes have a C=C double bond and decolourise aqueous bromine.
5Ethanol is made by fermentation of glucose (yeast, 25–35 °C, no oxygen) or by hydrating ethene with steam (300 °C, 60 atm, acid catalyst).
6Addition polymerisation joins alkene monomers; condensation polymerisation (nylon, PET, proteins) releases a small molecule such as water.

Notes · 1 of 5

Organic families, formulae and functional groups

Carbon compounds come in families; the functional group calls the reactions.

A hydrocarbon contains hydrogen and carbon only. Hydrocarbons and their derivatives are grouped into homologous series — families whose members share the same functional group, the atom or grouping that decides the family’s reactions. Members share a general formula, next-door members differ by one –CH₂– unit, and physical properties change gradually across the family while reactions stay similar.

The four series you must know
Series	General formula	Functional group	Example
Alkanes	CₙH₂ₙ₊₂	None (all single bonds)	Methane CH₄
Alkenes	CₙH₂ₙ	C=C double bond	Ethene C₂H₄
Alcohols	CₙH₂ₙ₊₁OH	–OH	Ethanol C₂H₅OH
Carboxylic acids	CₙH₂ₙ₊₁COOH	–COOH	Ethanoic acid CH₃COOH

A saturated compound has only single carbon–carbon bonds; an unsaturated compound contains at least one double (or triple) bond. A displayed formula draws every atom and every bond, while a structural formula (Supplement) spells out the arrangement unambiguously in symbols, such as CH₃CH₂OH for ethanol or CH₂=CH₂ for ethene.

Supplement: structural isomers share one molecular formula but differ in how the atoms connect. C₄H₁₀ exists as straight-chain butane and branched 2-methylpropane; C₄H₈ exists as but-1-ene, but-2-ene and others. More branches usually mean a lower boiling point — a useful comparison between isomers.

Notes · 2 of 5

Naming organic compounds

The ending tells you the family; the start of the name counts the carbons.

Counting carbons and reading the ending
Prefix	Carbons	Ending	Family
meth-	1	-ane	alkane
eth-	2	-ene	alkene
prop-	3	-ol	alcohol
but-	4	-oic acid	carboxylic acid

So butanoic acid has four carbons and a –COOH group, and propan-1-ol and propan-2-ol are two alcohols with three carbons that differ only in which carbon carries the –OH group. Supplement: esters are named from the alcohol first, then the acid: ethanol plus ethanoic acid gives ethyl ethanoate, CH₃COOCH₂CH₃.

Notes · 3 of 5

Fossil fuels and fractional distillation

Petroleum is a mixture of hydrocarbons, and distillation sorts it into useful fractions.

The fossil fuels are coal, natural gas (mainly methane) and petroleum — a crude-oil mixture of hundreds of hydrocarbons. Fractional distillation separates petroleum in a tower that is hot at the bottom and cooler towards the top. Each fraction boils over a narrow range, so hydrocarbons of similar chain length collect together.

Fractions from bottom to top of the column
Fraction	Use
Bitumen	Road surfaces
Lubricating oil	Lubricants, waxes, polishes
Fuel oil	Ships, home heating
Diesel fraction	Diesel engines
Kerosene fraction	Jet fuel
Naphtha fraction	Chemical feedstock
Gasoline fraction	Fuel for cars
Refinery gas	Heating and cooking

Moving up the column the molecules get shorter, so the fractions become more volatile, boil at lower temperatures, and flow more easily (lower viscosity). Bitumen at the bottom is thick and hardly evaporates; refinery gas at the top is a gas at room temperature.

Notes · 4 of 5

Alkanes, alkenes and cracking

Saturated versus unsaturated — one boring family and one reactive family.

Alkanes are saturated: every bond is single, so they are generally unreactive apart from combustion and substitution by chlorine. Complete combustion burns them to carbon dioxide and water; a poor oxygen supply gives toxic carbon monoxide and soot instead. Supplement: in a substitution reaction one atom replaces another — methane with chlorine in bright sunlight (ultraviolet light supplies the activation energy) gives chloromethane and hydrogen chloride, CH₄ + Cl₂ → CH₃Cl + HCl, and the process can continue one substitution at a time.

Alkenes carry the reactive C=C double bond and are unsaturated. The aqueous bromine test tells the two families apart: the orange-brown bromine water is decolourised quickly by an alkene but stays coloured with an alkane. Supplement: alkenes undergo addition reactions, in which the double bond opens and exactly one product forms — with bromine to give a dibromo compound, with hydrogen over a nickel catalyst, or with steam over an acid catalyst to give an alcohol.

1Why crack?
Long-chain fractions exceed demand, while short-chain alkenes and hydrogen are in high demand for plastics and industry.
2How
Heat a long alkane to a high temperature over a catalyst such as silica or alumina.
3Example
C₁₀H₂₂ → C₈H₁₈ + C₂H₄ — a shorter alkane plus an alkene; hydrogen may also be a product.

Notes · 5 of 5

Alcohols, carboxylic acids and polymers

Ethanol can be brewed or synthesised; oxidise it further for vinegar; then string molecules into plastics.

Ethanol is made two ways. Fermentation uses yeast to convert aqueous glucose to ethanol and carbon dioxide at 25–35 °C with no oxygen — renewable and low-energy, but slow and batch-by-batch. The industrial route adds steam to ethene at 300 °C and 6000 kPa (60 atm) with an acid catalyst — fast and continuous, but it needs crude oil and high energy. Ethanol burns cleanly to carbon dioxide and water, and is widely used as a solvent and a fuel. Supplement: oxidation of ethanol with acidified aqueous potassium manganate(VII), or bacterial oxidation in the production of vinegar, forms ethanoic acid.

Ethanoic acid is a weak acid with the usual acid reactions: reactive metals give a salt and hydrogen, bases give a salt and water, and carbonates give a salt, water and carbon dioxide. Its salts are ethanoates — for example sodium ethanoate, CH₃COONa. Supplement: warming a carboxylic acid with an alcohol over an acid catalyst forms an ester plus water; ethanoic acid with ethanol gives ethyl ethanoate.

Polymers are giant molecules built from many small monomers. Poly(ethene) forms by addition polymerisation: thousands of ethene molecules open their double bonds and link into long saturated chains. Plastics bring disposal problems — crowded landfill, floating accumulation in oceans, and toxic gases when burnt.

Supplement: condensation polymerisation joins monomers that each carry two functional groups and releases a small molecule such as water. Nylon is a polyamide made from a dicarboxylic acid and a diamine; PET is a polyester from a dicarboxylic acid and a diol. Proteins are natural polyamides built from amino acid monomers. PET is unusual in that it can be depolymerised back to its monomers and made into fresh polymer again.

Revise

Key terms

The words examiners expect you to use precisely.

Homologous series: A family of organic compounds with the same functional group, the same general formula and similar chemical properties.
Functional group: The atom or group of atoms that determines the chemical properties of a homologous series.
Saturated: Containing only single carbon–carbon bonds, like the alkanes.
Unsaturated: Containing at least one carbon–carbon double or triple bond, like the alkenes.
Structural isomers: Compounds with the same molecular formula but different structural formulae (Supplement).
Fractional distillation: Separating petroleum into fractions using their different boiling points in a tall column.

Show 7 more key terms

Cracking: Splitting long-chain alkanes into shorter alkenes (and alkanes or hydrogen) using high temperature and a catalyst.
Substitution reaction: A reaction in which one atom or group replaces another, such as chlorine replacing hydrogen in methane (Supplement).
Addition reaction: A reaction in which atoms add across a double bond to give a single product (Supplement).
Fermentation: The conversion of aqueous glucose into ethanol and carbon dioxide by yeast at 25–35 °C without oxygen.
Ester: The compound formed when a carboxylic acid reacts with an alcohol over an acid catalyst, for example ethyl ethanoate (Supplement).
Monomer: The small molecule that joins with thousands of others to build a polymer.
Condensation polymerisation: Polymer formation from monomers with two functional groups, releasing a small molecule such as water (Supplement).

Revise

Common mistakes

Where students most often lose marks in this topic — and what to do instead.

Mistake: Writing the alkene general formula as CₙH₂ₙ₊₂.
That is the alkane formula; the double bond costs two hydrogens.
Instead: Alkanes CₙH₂ₙ₊₂; alkenes CₙH₂ₙ.
Mistake: Saying petroleum "contains" fractions.
Fractions are what we separate out; petroleum is a mixture of hydrocarbons.
Instead: Fractional distillation splits the hydrocarbon mixture into fractions by boiling point.

Show 8 more mistakes

Mistake: Testing saturated and unsaturated compounds with limewater.
Limewater tests for carbon dioxide; it says nothing about C=C bonds.
Instead: Aqueous bromine decolourises with unsaturated compounds only.
Mistake: Giving fermentation 300 °C conditions.
Yeast is killed by heat; 300 °C belongs to the steam–ethene route.
Instead: Fermentation: yeast, 25–35 °C, no oxygen. Hydration of ethene: 300 °C, 60 atm, acid catalyst.
Mistake: Calling ethanoic acid a strong acid.
All carboxylic acids are weak — only partially dissociated.
Instead: Ethanoic acid is a weak acid, but still reacts with metals, bases and carbonates like any acid.
Mistake: Saying condensation polymerisation "adds monomers without any by-product".
That describes addition polymerisation.
Instead: Condensation polymerisation releases a small molecule, usually water, alongside the polymer.
Mistake: Claiming cracking turns long alkanes into alkenes only.
The chains split in two, so a shorter alkane appears as well as the alkene (and sometimes hydrogen).
Instead: Cracking gives a shorter alkane plus an alkene, and hydrogen can also be a product.
Mistake: Calling the reaction between methane and chlorine an addition reaction.
Nothing adds across a double bond — a hydrogen atom is swapped for a chlorine atom instead.
Instead: Alkane plus chlorine is substitution (ultraviolet light needed); alkenes undergo addition.
Mistake: Drawing a polymer repeat unit with the double bond still inside it.
The double bond opened up to join the monomers together, so the repeat unit must show only single bonds at the linkages.
Instead: Open the C=C into two single bonds and draw the connecting bonds crossing the brackets of the repeat unit.
Mistake: Describing proteins as polyesters.
Proteins are built from amino acids through amide linkages, not ester linkages.
Instead: Proteins are natural polyamides; PET is the polyester to quote.

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How do I tell alkanes and alkenes apart?

Add orange-brown aqueous bromine. An alkene decolourises it quickly because bromine adds across the C=C double bond; an alkane, being saturated, leaves the colour unchanged.

What are the general formulae I must learn?

Alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH and carboxylic acids CₙH₂ₙ₊₁COOH. Check any molecular formula against these to name its family.

What are the two ways of making ethanol?

Fermentation of aqueous glucose with yeast at 25–35 °C without oxygen, or the catalytic addition of steam to ethene at 300 °C and 60 atmospheres with an acid catalyst. Fermentation is renewable and low-energy but slow; the ethene route is fast but depends on crude oil.

What is the difference between addition and condensation polymerisation?

Addition polymerisation joins one type of monomer by opening C=C bonds and produces only the polymer. Condensation polymerisation joins monomers with two functional groups each and releases a small molecule, usually water — nylon, PET and proteins form this way.

Why is cracking important?

Long-chain fractions of petroleum exceed demand, while short-chain alkenes and hydrogen are in high demand. Cracking converts the surplus long alkanes into the smaller, more useful molecules.

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