This mini-lesson walks you through the whole of Edexcel iGCSE 4CH1 Section 4: the language of homologous series and functional groups, then crude oil, alkanes, alkenes, alcohols, carboxylic acids, esters and synthetic polymers.
Make a guess: how does a refinery sort crude oil?
Mission 1How do you sort a liquid of many molecules?
Mission 2What is the hydrogen-short molecule for?
Mission 3Which route to ethanol is better?
Mission 4Why do only some polymers give off water?
Work through each screen, answer the questions as you go (some are wordy, some need a structure) and collect ⭐ stars. Press Start when you're ready.
Introduction · (a)
The language of organic chemistry
A hydrocarbon is a compound of hydrogen and carbon only. Organic compounds are sorted into homologous series — families that:
share the same functional group (the bit of the molecule that decides how it reacts),
fit one general formula, and
have a steady trend in properties as the chain gets longer.
You represent molecules in several ways. For propane:
C3H8 · CH3CH2CH3molecular formula (left) · structural formula (right) · the displayed formula shows every bond
Empirical formula = simplest whole-number ratio of atoms (CH2 for C2H4). Isomerism = same molecular formula, different arrangement of atoms (e.g. butane vs methylpropane, both C4H10). You name compounds with up to six carbons using IUPAC rules.
Reactions of organic compounds are classified as substitution, addition or combustion — no mechanisms needed.
Quick check
What makes a homologous series?
?Which feature do all members of one homologous series, such as the alcohols, definitely share?
Crude oil · (b)
Crude oil & fractional distillation
Crude oil is a mixture of hydrocarbons (mostly alkanes). It is separated by fractional distillation in a column that is hot at the bottom, cool at the top. Each fraction is a group of hydrocarbons that condense over a similar boiling range:
The fractions, top to bottom: refinery gases, gasoline, kerosene, diesel, fuel oil, bitumen.
The trend down the column: longer chains → higher boiling point, more viscous (thicker), and darker in colour. A fuel is a substance that releases heat energy when burned.
Mission 1 check
How do you sort a liquid of many molecules?
?Heated crude oil enters a fractionating column. It contains C₅H₁₂ and a much longer alkane, C₂₀H₄₂. Which statement about where they leave the column is correct?
Crude oil · (b)
Burning fuels — and the pollutants
Complete combustion (plenty of oxygen) gives only carbon dioxide + water:
hydrocarbon + O2 → CO2 + H2O
Incomplete combustion (too little oxygen) also makes carbon monoxide (CO) and soot (carbon particulates):
CO is poisonous — it reduces the capacity of the blood to carry oxygen.
Soot blackens surfaces and causes breathing problems.
Sulfur impurities in the fuel burn to SO₂, which causes acid rain.
In hot car engines, nitrogen and oxygen from the air react to form oxides of nitrogen (NOₓ) — these also cause acid rain.
Watch out: CO₂ and water are the only products of complete combustion. CO and soot are signs of incomplete combustion. SO₂ comes from sulfur in the fuel; NOₓ forms from air in the engine.
Quick check
Reading a sooty flame
?A gas burner gives a yellow, smoky flame and deposits black soot on a beaker. What has happened, and which toxic gas is most likely also being produced?
Mission 2 of 4
What is the hydrogen-short molecule for?
Make a guess: what is the hydrogen-short molecule from cracking for?
Crude oil · (b)
Cracking — supply meets demand
Distillation makes too much of the long-chain fractions and not enough useful short-chain ones. Cracking breaks long-chain alkanes into shorter alkanes + alkenes:
C10H22 → C8H18 + C2H4a long alkane → a shorter alkane + an alkene (ethene)
Catalytic cracking conditions: a silica or alumina catalyst at 600–700 °C. The alkenes produced are valuable feedstock for making polymers.
Why crack? It balances supply and demand — turning surplus heavy fractions into the petrol-range alkanes and reactive alkenes that industry actually wants. Notice an alkene is always formed so the atoms balance.
Alkanes · (c)
Alkanes — saturated & unreactive
Alkanes are saturated hydrocarbons — only single C–C bonds, so they hold the maximum number of hydrogens. General formula:
1An alkane molecule contains 6 carbon atoms (hexane). Using CnH2n+2, how many hydrogen atoms does it have?
H atoms
Hint: 2n + 2 with n = 6.
Alkenes · (d)
Alkenes — the C=C double bond
Alkenes contain the functional group >C=C<. That double bond makes them unsaturated — they hold fewer hydrogens than the matching alkane. General formula:
CnH2nethene C₂H₄ · propene C₃H₆ · butene C₄H₈
Ethene (C₂H₄): a C=C double bond, only 4 hydrogens — the mark of an alkene.
Don't mix them up: an alkane (CₙH₂ₙ₊₂) is saturated with single bonds and does substitution; an alkene (CₙH₂ₙ) is unsaturated with a C=C and does addition, where the double bond opens up.
Alkenes · (d)
The bromine water test
Alkenes undergo addition with bromine — the C=C opens and a bromine atom adds to each carbon, making a colourless dibromoalkane:
This is the test that tells an alkane from an alkene. Shake each with orange bromine water:
With an alkene, orange bromine water turns colourless. With an alkane it stays orange.
Other additions: alkenes also add hydrogen (with a nickel catalyst → an alkane) and steam (with a phosphoric acid catalyst → an alcohol, e.g. ethene + steam → ethanol).
Predict it
Which tube decolourises?
?You add orange bromine water to two tubes: one holds ethane (C₂H₆), the other ethene (C₂H₄). Which result is correct?
Mission 2 check
What is the hydrogen-short molecule for?
?A refinery cracks a long alkane: C₁₂H₂₆ → C₁₀H₂₂ + X. What is X, and why do chemical companies want it?
Mission 3 of 4
Which route to ethanol is better?
Make a guess: which route to ethanol is better?
Alcohols · (e)
Alcohols — the –OH group
Alcohols contain the functional group –OH (hydroxyl). The first four are methanol, ethanol, propanol, butanol. Ethanol (in drinks, fuels and solvents) is the one you study in detail:
Ethanol, C₂H₅OH (CH₃CH₂OH): an ethyl chain ending in the –OH hydroxyl group.
Three ways ethanol is oxidised (4.31C):
Combustion — burns in air or oxygen: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Microbial oxidation — oxygen in the air, with microbes, oxidises it to ethanoic acid (as in vinegar).
Oxidising agent — heating with potassium dichromate(VI) in dilute sulfuric acid also oxidises it to ethanoic acid.
Alcohols · (e)
Making ethanol — two routes
Ethanol can be manufactured two different ways. You must compare the conditions:
Fermentation uses renewable sugar but is slow; hydration is fast and continuous but uses ethene from finite crude oil.
Why those fermentation conditions? No air stops the ethanol oxidising to vinegar; ~30 °C is the optimum for the yeast enzymes (too cold = slow, too hot = the enzymes denature).
Quick check
Which route is this?
?A factory makes ethanol continuously by reacting ethene with steam over a phosphoric acid catalyst at about 300 °C and 60–70 atm. Which process is this?
Mission 3 check
Which route to ethanol is better?
?A country grows lots of sugar cane but has no crude oil. Which route should it use to make ethanol, and why?
Carboxylic acids · (f)
Carboxylic acids — the –COOH group
Carboxylic acids contain the functional group –COOH (carboxyl). The first four are methanoic, ethanoic, propanoic, butanoic acid. Ethanoic acid is the acid in vinegar:
Ethanoic acid, CH₃COOH: a C=O double bond and an O–H, together the carboxyl group.
Carboxylic acids are weak acids (only partly ionise in water), but they still show typical acid reactions:
With metals (e.g. magnesium) → a salt + hydrogen gas.
With metal carbonates → a salt + water + carbon dioxide (fizzing).
Match it
Family ↔ functional group
Tap a family on the left, then its functional group on the right.
Esters · (g)
Esters — the sweet smells
An alcohol + a carboxylic acid react (with an acid catalyst, e.g. concentrated sulfuric acid) to make an ester + water. The ester functional group is –COO–:
Making an ester is a condensation: a small molecule (water) is released. The –OH of the acid and the H of the alcohol's –OH form the water.
Naming: the alcohol gives the first part (ethyl), the acid gives the second (ethanoate) → ethyl ethanoate. Esters are volatile with distinctive smells — used in food flavourings and perfumes.
Quick check
Name that ester
?You react propanol with ethanoic acid (with an acid catalyst). What is the ester product called?
Mission 4 of 4
Why do only some polymers give off water?
Make a guess: why do only some polymers give off water?
Synthetic polymers · (h)
Addition polymers
In addition polymerisation, many alkene monomers join — the C=C double bonds open up and link into one long chain, with no other product:
Poly(ethene): the repeat unit is –CH₂–CH₂– with open bonds at both ends, written inside brackets with n.
Other addition polymers: poly(propene), poly(chloroethene) (PVC) and poly(tetrafluoroethene) (PTFE). You can work a repeat unit ↔ its monomer either way.
Disposal problems: addition polymers are inert and do not biodegrade (they fill landfill for centuries), and burning them makes toxic gases (e.g. HCl from PVC, and carbon monoxide).
Quick check
Monomer → repeat unit
?The monomer is propene, CH₂=CH–CH₃. Which is the correct repeat unit of poly(propene)?
Synthetic polymers · (h)
Condensation polymers
In condensation polymerisation the monomers each have two functional groups. A dicarboxylic acid + a diol join to make a polyester — and a small molecule, water, is released each time a link forms:
Two different monomers, each with two groups, build a polyester and lose a small molecule (water) at every link.
Addition vs condensation: addition uses one monomer (an alkene) and makes nothing else; condensation uses two monomers (each with two groups) and releases a small molecule (water). Some polyesters, called biopolyesters, are biodegradable.
Quick check
Which polymerisation?
?A reaction joins a diol and a dicarboxylic acid into a long chain and releases water at every link. What type of polymerisation is it?
Mission 4 check
Why do only some polymers give off water?
?Poly(propene) is made from propene alone. A polyester is made from monomers with two –OH groups and monomers with two –COOH groups, and water is given off. Why does only the polyester give off water?
Sort it
Saturated or unsaturated?
Tap a molecule, then tap the box it belongs in. (Look for a C=C.)
🟦 Unsaturated (has C=C)
🟩 Saturated (single bonds)
Recap
Section 4 at a glance
Homologous series: same functional group, one general formula