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IGCSE Chemistry 0620 · Chapter 11 notes · ~14 min read

Organic chemistry

The chemistry of carbon compounds made clear: how to name and draw them, crude oil and cracking, alkanes and alkenes, alcohols, acids, esters and polymers.

Homologous seriesAlkanesAlkenesIsomersAlcoholsCarboxylic acidsEstersPolymers
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Organic compounds are built on carbon chains. A homologous series is a family with the same functional group and similar reactions.
  • Alkanes (C–C single bonds) are saturated and burn. Alkenes (C=C double bond) are unsaturated and react by addition. Bromine water turns colourless with alkenes.
  • Crude oil is separated by fractional distillation. Cracking turns long alkanes into short alkanes and alkenes.
  • Alcohols end in -ol, carboxylic acids in -oic acid. Polymers are made from many monomers.

By the end you can

  • Draw and name the first four members of the alkane, alkene, alcohol and carboxylic acid series.
  • Describe fractional distillation of crude oil and the uses of the fractions.
  • Describe cracking, and the reactions and tests of alkanes and alkenes.
  • Describe how ethanol is made and its uses, and the reactions of ethanoic acid.
  • Describe polymerisation and the environmental problems of plastics.
  • Draw structural isomers, name esters and describe condensation polymers. EXT

1Naming and drawing carbon compounds

The key words

Hydrocarbon — a compound containing only hydrogen and carbon.

Homologous series — a family of compounds with the same functional group, the same general formula and similar chemical properties. Each member differs from the next by CH₂. Physical properties change gradually (e.g. boiling point rises).

Functional group — the atom or group of atoms that gives a compound its characteristic reactions.

Saturated — contains only single C–C bonds. Unsaturated

The four families

alkanes: single bonds only (saturated), general formula CₙH₂ₙ₊₂CHHHHmethane CH₄CHHCHHHHethane C₂H₆CHHCHHCHHHHpropane C₃H₈CHHCHHCHHCHHHHbutane C₄H₁₀each carbon forms 4 bonds · each hydrogen forms 1 bond
The first four alkanes. Each carbon forms four bonds and each hydrogen forms one. These are displayed formulae — every atom and bond is drawn.
SeriesFunctional groupGeneral formulaFirst member
alkanesC–C single bonds onlyCₙH₂ₙ₊₂methane CH₄
alkenesC=C double bondCₙH₂ₙethene C₂H₄
alcohols–OHCₙH₂ₙ₊₁OHmethanol CH₃OH
carboxylic acids–COOHCₙH₂ₙ₊₁COOHmethanoic acid HCOOH
CCHHHHALKENEC=C double bondCCOHHHHHHALCOHOL–OH groupCH₃COOHCARBOXYLIC ACID–COOH groupCH₃COOCH₃ESTER–COO– link
The functional groups on a displayed formula: the C=C of an alkene, the –OH of an alcohol, the –COOH of a carboxylic acid and the –COO– link of an ester.
  • A displayed formula shows every atom and every bond. A structural formula groups the atoms: ethanol is CH₃CH₂OH. A molecular formula just gives the totals: C₂H₆O.
Worked example

What is the formula of the alkane with 6 carbon atoms, and the alkene with 5 carbon atoms?

  1. alkane CₙH₂ₙ₊₂ with n = 6: C₆H₁₄
  2. alkene CₙH₂ₙ with n = 5: C₅H₁₀
Worked example

What type of compound is each: C₃H₈, C₃H₆, C₂H₅OH?

  1. C₃H₈ fits CₙH₂ₙ₊₂ → alkane (propane)
  2. C₃H₆ fits CₙH₂ₙ → alkene (propene)
  3. C₂H₅OH contains –OH → alcohol (ethanol)

Naming organic compoundsEXTENDED

how organic names are built1 carbonsmeth = 1eth = 2prop = 3but = 42 the bondsan = all singleen = one C=C3 the familye : alkane/alkeneol : alcoholoic acid: acideth + an + e = ethaneprop + en + e = propenebut + an + ol = butanoleth + an + oic acid = ethanoic acidesters: alcohol part first, then acid part:ethyl ethanoate = made from ethanol + ethanoic acid
Building a name: carbon count + bond type + family. Esters are named alcohol part first, then acid part.
  • Stem (number of carbons): meth = 1, eth = 2, prop = 3, but = 4.
  • Ending: -ane (alkane), -ene (alkene), -ol (alcohol), -oic acid (carboxylic acid).
  • Examples: eth + an + e = ethane; prop + en + e = propene; but + an + ol = butanol; eth + an + oic acid = ethanoic acid.
  • Esters are named alcohol part first, then the acid: ethanol + ethanoic acid → ethyl ethanoate.
Common mistakeCalling CH₃COOH "ethanol". The –COOH group makes it an acid: ethanoic acid. Ethanol is C₂H₅OH.

Structural isomersEXTENDED

two different molecules with the same formula, C₄H₁₀CH₃CH₂CH₂CH₃butane(straight chain)CHCH₃CH₃CH₃2-methylpropane(branched chain)isomers: same molecular formula, different structural formulathey have different physical properties (e.g. boiling point)molecular formula: how many atoms · structural formula: how they are joined
Two isomers of C₄H₁₀: butane (a straight chain) and 2-methylpropane (a branched chain). Same molecular formula, different structure.

Structural isomers — compounds with the same molecular formula but different structural formulae.

  • Isomers have different physical properties, such as boiling points. Branched isomers usually boil at a lower temperature.
Exam tipDraw isomers by changing the carbon skeleton. First draw the longest chain, then move carbons into branches. Check that each carbon still has four bonds.

2Fuels and crude oil

Fossil fuels

  • Fossil fuels are coal, oil (petroleum) and natural gas. They formed over millions of years from the remains of dead organisms. They are non-renewable.
  • Natural gas is mostly methane (CH₄). Petroleum (crude oil) is a mixture of hydrocarbons.
  • When a hydrocarbon burns completely: hydrocarbon + oxygen → carbon dioxide + water. CH₄ + 2O₂ → CO₂ + 2H₂O. See incomplete combustion for what happens if oxygen is limited.

Fractional distillation

refinery gasesC₁–C₄: bottled gas fuelpetrolC₅–C₁₀: car fuelnaphthachemicals feedstockkerosenejet fueldiesel oillorries, buses, trainsfuel oilships and heatingbitumenroad surfacesheated crude oiltop: coolersmall moleculeslow boiling pointruns easilyignites easilybottom: hotterlarge moleculeshigh boiling pointthick (viscous)hard to ignitethe fractions are separated because their boiling points are different
The fractionating column. Crude oil is heated and the vapours rise. Each fraction condenses at a different height, where the temperature falls below its boiling point.
  • Crude oil is heated and the vapour enters a tall column that is hot at the bottom and cool at the top.
  • Each fraction is a mixture of hydrocarbons with similar boiling points. Each one condenses and is collected where the temperature is just below its boiling point.
FractionUse
refinery gases (smallest molecules)bottled gas for heating and cooking
petrol (gasoline)fuel for cars
naphthachemicals feedstock (making plastics)
kerosene (paraffin)jet fuel
diesel oillorries, buses and trains
fuel oilships and heating
bitumen (largest molecules)road surfaces and roofing
  • Going down the column: molecules get bigger, the boiling point rises, the liquid is more viscous (thicker), less flammable (harder to ignite) and darker.
Picture itShort-chain molecules are like marbles — they roll easily and escape as a gas. Long chains are like tangled spaghetti — thick, slow, and sticking together.
Common mistakeSaying that crude oil is "boiled" in a chemical reaction. Fractional distillation is a physical separation. No new substances are made.

3Alkanes

Saturated and unreactive

  • Alkanes have only single bonds — they are saturated. They are generally unreactive, apart from combustion and reaction with chlorine.
  • Combustion: they burn well, so they are used as fuels.
  • EXT Substitution with chlorine: in UV light, a hydrogen atom is replaced by a chlorine atom. CH₄ + Cl₂ → CH₃Cl + HCl (chloromethane and hydrogen chloride).
  • UV light supplies the energy to start the reaction — it is called a photochemical reaction.
Worked example

EXT Write the equation for the substitution of ethane by chlorine, making one product with a single chlorine.

  1. replace one H by Cl
  2. C₂H₆ + Cl₂ → C₂H₅Cl + HCl (chloroethane and hydrogen chloride)
Common mistakeDescribing the chlorine reaction as "addition". It is substitution — two products are formed.

4Alkenes

Cracking

cracking: breaking long alkane molecules into shorter onesdecane C₁₀H₂₂(long, from fuel oil)heat +catalystoctane C₈H₁₈(petrol: a fuel)ethene C₂H₄(for plastics)+cracking needs a high temperature (about 600–700 °C) and a catalyst (such asaluminium oxide). Why? There is more demand for petrol and alkenes than for large alkanes.C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Cracking breaks long alkane molecules into shorter ones. Decane is cracked into octane (for petrol) and ethene (for plastics).

Cracking — breaking down long-chain alkanes into shorter alkanes and alkenes using heat and a catalyst.

  • Conditions: high temperature (about 600–700 °C) and a catalyst (aluminium oxide or silica).
  • Why: there is more demand for petrol (shorter chains) than the crude oil can supply, and alkenes are needed to make plastics (polymers) and other chemicals.
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
decane → octane + ethene
Worked example

Complete the cracking equation: C₁₅H₃₂ → C₈H₁₈ + ?

  1. carbon atoms: 15 − 8 = 7
  2. hydrogen atoms: 32 − 18 = 14
  3. the other product is C₇H₁₄, which fits CₙH₂ₙ, so it is an alkene (heptene)
  • Every cracking reaction makes at least one alkene.

The alkene test and reactions

  • Alkenes have a C=C double bond — they are unsaturated. This makes them much more reactive than alkanes.
  • Test for an alkene: shake with bromine water. An alkene turns it from orange-brown to colourless. An alkane has no effect.
CCHHHHethene+ bromine, Br₂room temperature+ hydrogen, H₂nickel catalyst, heat+ steam, H₂OH₃PO₄, 300 °C, 60 atmCCBrBrHHHH1,2-dibromo-ethaneCCHHHHHHethaneCCOHHHHHHethanoltest: alkenes turn orange-brown bromine water colourless; alkanes do notaddition reactions: the C=C opens and only one product forms
Addition reactions of ethene. The double bond opens and the other molecule joins on, so only one product forms.
  • EXT Addition reaction: the double bond opens and atoms add on. Only one product forms.
ReagentConditionsProduct
bromine, Br₂room temperature1,2-dibromoethane
hydrogen, H₂nickel catalyst, heatethane
steam, H₂Ophosphoric acid catalyst, 300 °C, 60–70 atmethanol
  • EXT C₂H₄ + Br₂ → C₂H₄Br₂; C₂H₄ + H₂ → C₂H₆; C₂H₄ + H₂O → C₂H₅OH.
Exam tipAlkene tests: "turns orange-brown bromine water colourless" — never "clear" and never "decolourises" without saying what it was before. The reason is the C=C double bond.
Common mistakeSaying "the bromine water turns clear". Say colourless: it was coloured before.

5Alcohols and carboxylic acids

Making ethanol

FERMENTATIONglucose → ethanol + carbon dioxideC₆H₁₂O₆ → 2C₂H₅OH + 2CO₂yeast, 25–35 °C, no oxygen+ renewable (sugar crops)+ low temperature, cheap equipment− slow batch process− impure: needs distillingSTEAM HYDRATIONethene + steam ⇌ ethanolC₂H₄ + H₂O ⇌ C₂H₅OH300 °C, 60–70 atm,phosphoric acid catalyst+ fast, continuous, pure− uses non-renewable crude oil− high temperature and pressuretwo ways to make ethanoluses: a solvent and a fuel (ethanol burns: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O)the fermentation mixture is about 15% ethanol; fractional distillation makes it purer
Two ways to make ethanol. Fermentation uses yeast and glucose. Steam hydration uses ethene from crude oil.
  • Fermentation: glucose is broken down by yeast in warm (25–35 °C), oxygen-free conditions. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
  • Hydration of ethene: ethene reacts with steam, with a phosphoric acid catalyst, at 300 °C and 60–70 atm. C₂H₄ + H₂O ⇌ C₂H₅OH.
FermentationSteam hydration
raw materialsugar (renewable)ethene from crude oil (non-renewable)
conditionslow temperaturehigh temperature and pressure
speedslow batch processfast, continuous
purityimpure, needs distillingpure
  • Uses of ethanol: a solvent (perfumes, paints) and a fuel. C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

Ethanoic acid

  • Ethanoic acid (CH₃COOH) is a weak acid. It reacts like other acids:
  • with metals: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
  • with bases: CH₃COOH + NaOH → CH₃COONa + H₂O
  • with carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
  • The salts are called ethanoates (magnesium ethanoate, sodium ethanoate).
  • EXT Making it: ethanol is oxidised to ethanoic acid by acidified potassium manganate(VII) (purple → colourless), or by bacteria in air (wine goes sour to make vinegar). C₂H₅OH + O₂ → CH₃COOH + H₂O.

EstersEXTENDED

CH₃COOH+CH₃CH₂OHethanoic acidethanolconcentrated H₂SO₄catalyst, heatCH₃COOCH₂CH₃+H₂Oethyl ethanoate (an ester)waterethanoic acid + ethanol ⇌ ethyl ethanoate + wateresters smell sweet and fruity: used in flavourings and perfumes–OH of the acid and –H ofthe alcohol leave as water
Ethanoic acid + ethanol ⇌ ethyl ethanoate + water. The –OH of the acid and the –H of the alcohol leave as water.
  • Esterification: carboxylic acid + alcohol ⇌ ester + water, using concentrated sulfuric acid as catalyst and heat.
  • Esters smell sweet and fruity. They are used in flavourings and perfumes.
Worked example

EXT Name the ester made from propanol and ethanoic acid, and give the other product.

  1. alcohol part first: propyl
  2. acid part second: ethanoate
  3. propyl ethanoate
  4. the other product is water

6Polymers

Long molecules from small ones

Polymer — a very large molecule made of many repeating units joined together. The small molecules that join are monomers.

addition polymerisationCCHHHHn × ethenehigh pressure,catalystCCHHHHnpoly(ethene)the C=C opensone product onlycondensation polymerisation (Extended)HOdiolOH+HOOCdiacidCOOH…● = ester link+ H₂O lost at every linkPET is a polyester; nylon uses a diamine and a diacid (amide links);proteins are natural polyamides
Top: addition polymerisation. Many ethene molecules join to make poly(ethene); the C=C opens. Bottom: condensation polymerisation (Extended). A small molecule, water, is lost at every link.
  • Addition polymerisation: many unsaturated monomers (alkenes) join by opening their C=C bonds. Only a polymer forms.
  • Example: n C₂H₄ → –(CH₂–CH₂)ₙ– poly(ethene). The repeat unit is –CH₂–CH₂–.
  • EXT To find the repeat unit from a monomer: open the double bond and show a bond going out of each end (propene → –CH₂–CH(CH₃)–). To find the monomer from a polymer: take one repeat unit and put the double bond back between the two carbons in the chain.
  • EXT Condensation polymerisation: monomers with two functional groups join and a small molecule (usually water) is lost at each link.
PolymerMonomersLink
polyester (PET)a diol + a dicarboxylic acidester
nylon (a polyamide)a diamine + a dicarboxylic acidamide
protein (natural polyamide)amino acidsamide
  • EXT PET can be recycled by melting and reshaping, or by breaking it down chemically into monomers to make new PET.

Plastics and the environment

  • Most plastics are non-biodegradable — they do not rot, so they stay in the environment for hundreds of years.
  • Landfill fills up. Litter harms wildlife: animals get tangled or swallow plastic. Plastic in the sea breaks into microplastics.
  • Burning plastics releases CO₂ (global warming), and some release toxic gases (for example HCl from PVC).
  • Solutions: reduce use, reuse, recycle (sorting is needed), and develop biodegradable plastics.
Common mistakeWriting that all plastics are made by addition polymerisation. Many (PET, nylon) are condensation polymers.

7Test yourself

Test yourself — cover the answers first

Quick check

  1. What is a homologous series?
  2. What is the general formula of the alkanes?
  3. What is the functional group of the alcohols?
  4. What is the test for an alkene?
  5. Name two uses of ethanol.
  6. What is the repeat unit of poly(ethene)?
  7. What does the fractional distillation of crude oil separate it into?
  8. EXT What are structural isomers?

Exam-style

  1. Describe how petrol is obtained from crude oil. Include fractional distillation. [4]
  2. Explain why cracking is carried out in an oil refinery. [3]
  3. Compare the two methods of making ethanol. [4]
  4. EXT Draw the structures of the two isomers of C₄H₁₀ and name them. [4]
  5. EXT A student mixes ethanoic acid and ethanol with a few drops of concentrated sulfuric acid and warms them. Name the product and the smell. [2]
Show answers
  1. a family of compounds with the same functional group and similar chemical properties, differing by CH₂
  2. CₙH₂ₙ₊₂
  3. –OH
  4. bromine water goes from orange-brown to colourless
  5. solvent, fuel
  6. –CH₂–CH₂–
  7. fractions with different boiling points
  8. same molecular formula, different structural formula
  9. crude oil is heated and the vapour enters a fractionating column
    the column is hotter at the bottom and cooler at the top
    petrol condenses at its boiling point range, near the top
    it is collected as a liquid
  10. there is a higher demand for shorter alkanes (petrol) than the supply
    large alkanes are cracked into shorter alkanes
    and alkenes, which are used to make plastics
  11. fermentation: sugar, yeast, 25–35 °C, slow, impure, renewable
    steam hydration: ethene and steam, 300 °C and 60–70 atm, fast, pure, non-renewable
  12. CH₃CH₂CH₂CH₃ (butane)
    CH₃CH(CH₃)CH₃ (2-methylpropane)
    each carbon has four bonds
    different structural formulae with the same molecular formula
  13. ethyl ethanoate
    sweet fruity smell
Practise this chapter
Formulae, functional groups & namingAlkanes & alkenesAlcohols & carboxylic acidsPolymers
Next chapterExperimental techniques & chemical analysis →
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