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.

How do you sort a liquid of many molecules?

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 (general formula CₙH₂ₙ₊₁OH)
• Carboxylic acids — the –COOH group (general formula CₙH₂ₙ₊₁COOH)
• Esters — the –COO– group SUPPLEMENT

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?

What is the hydrogen-short molecule for?

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). It is done because there is more demand for the smaller, more useful molecules (such as petrol) than petroleum supplies, and alkenes are needed to make polymers and other chemicals:

SUPPLEMENT Because of the reactive C=C, alkenes undergo addition reactions — only one product forms. With bromine: ethene + Br₂ → C₂H₄Br₂ (1,2-dibromoethane); with hydrogen (nickel catalyst) → ethane; with steam (acid catalyst) → ethanol.

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

What is the hydrogen-short molecule for?

Which route to ethanol is better?

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). SUPPLEMENT 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. SUPPLEMENT 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.

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

Manufacturing ethanol

Which route to ethanol is better?

Carboxylic acids & ethanoic acid

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

• SUPPLEMENT 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, e.g. sodium ethanoate, CH₃COONa). 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

Why do only some polymers give off water?

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. PET can be converted back into its monomers and re-polymerised.

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).

Why do only some polymers give off 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 key ideas in Cambridge IGCSE Topic 11 — Organic chemistry. Press Finish to see your score.