Fossil fuels & hydrocarbons

The three fossil fuels you must name are coal, natural gas and petroleum (crude oil). The main constituent of natural gas is methane, CH₄.

• A hydrocarbon is a compound containing only hydrogen and carbon.
• Petroleum is a mixture of hydrocarbons.
• It is separated into useful fractions by fractional distillation.

Watch out: petroleum is a mixture, not a compound — that's exactly why a physical method (distillation by boiling point) can separate it.

The fractionating column

Petroleum is heated and fed into a fractionating column that is hot at the bottom, cool at the top. Each fraction condenses where the temperature matches its boiling point.

Trend to learn: as chain length increases (down the column) boiling point and viscosity rise, while volatility and flammability fall.

Reading the column

Match the fraction to its use

Tap a fraction on the left, then tap its main use on the right.

Functional groups & homologous series

A functional group is the atom or group of atoms that gives a family its characteristic reactions. A homologous series is a family of compounds that share the same functional group:

• Alkanes — only C–C single bonds (no special group)
• Alkenes — the C=C double bond
• Alcohols — the –OH group
• Carboxylic acids — the –COOH group
• Esters — the –COO– group SUPPLEMENT

Members share these general characteristics of a homologous series:

• the same general formula
• each member differs from the next by –CH₂–
• the same functional group, so similar chemical properties
• a gradual change in physical properties (e.g. boiling point rises with chain length)

Naming: the stem gives the carbon count — meth- (1), eth- (2), prop- (3), but- (4) — and the ending gives the family (-ane, -ene, -ol, -oic acid).

Saturated vs unsaturated

A saturated hydrocarbon has only single C–C bonds (the maximum number of hydrogens). An unsaturated hydrocarbon has a C=C double bond, so it holds fewer hydrogens.

Structural isomerism SUPPLEMENT: compounds with the same molecular formula but different structural formula — e.g. butane and methylpropane both share C₄H₁₀.

Alkane or alkene?

Alkanes — the saturated family

Alkanes (methane, ethane, propane, butane…) are saturated hydrocarbons with the general formula:

They are generally unreactive (apart from combustion), but they do undergo a substitution reaction with chlorine:

SUPPLEMENT This is a photochemical reaction — ultraviolet light supplies the activation energy. In a substitution reaction, one atom (here an H) is replaced by another (a Cl). Products are limited to monosubstitution.

Reaction of methane with chlorine

Alkenes — the unsaturated family

Alkenes (ethene, propene, butene…) contain a C=C double bond and have the general formula:

They are made by cracking large, less-useful alkanes from petroleum, using a high temperature and a catalyst. Cracking produces smaller alkanes and alkenes (and hydrogen):

Because of the reactive C=C, alkenes undergo addition reactions — only one product forms. With bromine: ethene + Br₂ → C₂H₄Br₂ (1,2-dibromoethane).

The bromine-water test

To tell a saturated from an unsaturated hydrocarbon, shake it with aqueous bromine (bromine water):

Watch out: it is the alkene (unsaturated) that removes the colour — the C=C adds bromine across it. A saturated alkane has no C=C, so it can't, and the orange remains.

Predict the bromine test

Alcohols & making ethanol

Alcohols (methanol, ethanol, propanol, butanol) contain the –OH functional group. Ethanol, C₂H₅OH, can be manufactured two ways:

• Fermentation — aqueous glucose + yeast, at 25–35 °C, in the absence of oxygen (anaerobic). Renewable raw material; slow, batch process; gives dilute, impure ethanol.
• Catalytic addition (hydration) of steam to ethene — at 300 °C, 60 atm, with an acid catalyst. Uses ethene from crude oil (finite); fast, continuous process; gives pure ethanol.

Alcohols are good fuels — ethanol burns (combusts) in air: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

SUPPLEMENT Uses of ethanol: as a solvent and as a fuel (e.g. blended with petrol).

Manufacturing ethanol

Carboxylic acids & ethanoic acid

Carboxylic acids contain the –COOH functional group (methanoic, ethanoic, propanoic, butanoic acid). Ethanoic acid, CH₃COOH, is the key example.

• It is formed by the oxidation of ethanol — e.g. by warming with acidified potassium manganate(VII), or by bacterial oxidation in air (which sours wine to vinegar).
• It reacts like a typical acid: with metals → salt + hydrogen; with bases / carbonates → a salt (an ethanoate). With carbonates it also gives CO₂ + water.

SUPPLEMENT Carboxylic acids are weak acids — only partially ionised in water, so they release relatively few H⁺ ions.

From ethanol to acid

Esters SUPPLEMENT

An ester is made when an alcohol reacts with a carboxylic acid, with an acid catalyst. Water is also produced (a condensation), and the ester contains the –COO– group:

Naming: the first part (e.g. "ethyl") comes from the alcohol; the second part (e.g. "ethanoate") comes from the acid. So methanol + ethanoic acid → methyl ethanoate.

Order matters in the name: alcohol-part first, acid-part second. Propanol + ethanoic acid → propyl ethanoate.

Name the ester SUPPLEMENT

Addition polymerisation

In addition polymerisation, many small monomers with a C=C double bond join into one long chain — with no other product. Many ethene molecules give poly(ethene):

Watch out: only molecules with a C=C double bond can form addition polymers. Put the bonds through the brackets and write n outside — that shows the unit repeating.

Monomer ↔ repeat unit

Condensation polymers SUPPLEMENT

In condensation polymerisation, monomers each have two functional groups, and as they join a small molecule (usually water) is lost each time. Two important examples:

• Nylon — a polyamide, from a diamine + a dicarboxylic acid; contains the amide linkage.
• PET (Terylene) — a polyester, from a diol + a dicarboxylic acid; contains the ester linkage.

Given the monomers, you can deduce the repeat unit by joining them and removing water at each new linkage.

Key difference: addition polymers come from one C=C monomer and lose nothing; condensation polymers come from two monomers (two functional groups) and lose a small molecule (water).

Natural polymers & the plastics problem

SUPPLEMENT Proteins are natural polymers — long chains of amino acids joined by amide linkages (like nylon). They can be hydrolysed back to amino acids.

Synthetic plastics bring serious pollution problems:

• Many are non-biodegradable — microbes can't break them down, so they persist for centuries.
• They litter land and oceans, harming wildlife, and fragment into microplastics.
• Disposal is hard: landfill fills up; burning them can release toxic gases (e.g. HCl, CO).

Tackling it: reduce, re-use and recycle plastics, and develop biodegradable alternatives.

Name the family

Tap the homologous series each compound belongs to, using its formula and functional group.

Saturated or unsaturated?

Tap a compound, then tap the box it belongs in.

The key facts to know

You've covered the whole of Cambridge IGCSE Topic 11 — Organic chemistry. Press Finish to see your score.