Understanding Organic chemistry: IGCSE Chemistry Explained

Why organic chemistry matters more than you think

When you hear "organic chemistry," you might picture complicated university textbooks, but here is the thing: you already interact with organic compounds every single day. The gas that heats your home, the plastic bottle you drink from, the vinegar on your chips, the fabric of your clothes - all organic. Once you see that connection, the IGCSE Chemistry organic chemistry unit stops feeling abstract and starts making sense.

At its core, organic chemistry is the study of carbon-based compounds. Carbon is special because each atom can form four bonds, which means it can build chains, branches, and rings in ways no other element can. Think of carbon atoms like LEGO bricks with four connectors - you can snap them together in an enormous number of arrangements, and each arrangement gives you a different molecule with different properties.

Homologous series: families that share the same rules

A homologous series is a family of compounds that share the same functional group and follow the same general formula. Each member differs from the next by a CH₂ unit - like adding one more carriage to a train. Because they share a functional group, members of the same series react in similar ways, which is fantastic news for revision: learn the pattern once, apply it to every member.

A functional group is the atom or group of atoms responsible for a compound's characteristic reactions. Alkanes have none (just single C-C and C-H bonds), alkenes have C=C, alcohols have -OH, and carboxylic acids have -COOH. Spot the functional group and you can predict behaviour - it is like recognising a team's jersey and knowing their playing style.

The four series you must know inside out

Naming organic compounds: a system, not a guessing game

Naming in organic chemistry follows a recipe. Count the longest carbon chain to get the root name, identify any functional groups to get the suffix, and note any branches. It is the same logic as a postal address - country, city, street.

Then add the suffix that matches the homologous series: -ane for alkanes, -ene for alkenes, -ol for alcohols, -oic acid for carboxylic acids.

Displayed formulae and structural formulae

A displayed formula shows every atom and every bond in the molecule. Think of it as the fully expanded, no-shortcuts version - every hydrogen, every single and double bond drawn out. Examiners love asking you to draw these because they test whether you truly understand the bonding.

A structural formula is a shorthand that still tells you exactly how atoms are arranged, but without drawing every bond. For ethanol, the structural formula is CH₃CH₂OH. It is like writing a recipe as a list of steps rather than filming every hand movement - less detail, but no ambiguity.

Structural isomers: same ingredients, different arrangement

Imagine you have the same set of LEGO bricks. You can build a straight tower or a squat house - same pieces, different structure. Structural isomers are compounds with the same molecular formula but different structural formulae.

Take C₄H₁₀. You can arrange those atoms as a straight chain (butane: CH₃CH₂CH₂CH₃) or as a branched chain (methylpropane: CH₃CH(CH₃)CH₃). Both have four carbons and ten hydrogens, but the molecules look and behave slightly differently. The IGCSE syllabus expects you to recognise and draw isomers for simple alkanes and alkenes.

Alkanes in detail

Alkanes are the simplest organic family. Every bond is a single bond, which makes them saturated hydrocarbons - they are holding as many hydrogen atoms as they possibly can, like a sponge that cannot soak up any more water.

Key properties and trends

• As the chain length increases, boiling point rises (bigger molecules have stronger intermolecular forces).
• Short-chain alkanes (methane, ethane, propane, butane) are gases at room temperature; longer chains are liquids, then waxy solids.
• They are relatively unreactive because C-C and C-H single bonds are strong and hard to break.

Combustion

Alkanes burn in oxygen. Complete combustion (plenty of oxygen) produces carbon dioxide and water. Incomplete combustion (limited oxygen) produces carbon monoxide and/or soot (carbon) alongside water - a serious health and environment concern.

Substitution with halogens

In ultraviolet light, alkanes react with halogens by substitution: a hydrogen atom is replaced by a halogen atom. Methane plus chlorine in UV light gives chloromethane and hydrogen chloride.

Alkenes in detail

Alkenes contain a C=C double bond, which makes them unsaturated. That double bond is their signature - it is the reactive hotspot where addition reactions happen.

Testing for alkenes

Add bromine water (orange-brown) to the substance. If the bromine water decolourises, an alkene is present. The bromine adds across the double bond. This test appears in exam questions regularly, so commit it to memory.

Addition reactions

• Hydrogenation: alkene + hydrogen (with a nickel catalyst) produces an alkane. The double bond opens and hydrogen atoms add on.
• Hydration: alkene + steam (with an acid catalyst) produces an alcohol. This is how industrial ethanol is made from ethene.
• Halogenation: alkene + halogen produces a dihalogenoalkane. Both halogen atoms add across the double bond.

Alcohols in detail

The -OH group gives alcohols their characteristic properties. Ethanol is the most famous member - it is in alcoholic drinks, hand sanitiser, and is used as a fuel and solvent.

Two ways to make ethanol

Reactions of alcohols

• Combustion: alcohols burn cleanly, making them useful as fuels.
• Oxidation: gentle oxidation (using acidified potassium dichromate) converts an alcohol to a carboxylic acid. You will see the colour change from orange to green.
• Esterification: alcohol + carboxylic acid (with an acid catalyst) produces an ester + water. Esters have fruity smells and are used in flavourings and perfumes.

Carboxylic acids in detail

Carboxylic acids carry the -COOH group. Ethanoic acid (the acid in vinegar) is the one you will meet most often. They are weak acids - they do not fully ionise in water - which means they react more gently than strong acids like hydrochloric acid.

Typical reactions

• With metals: carboxylic acid + reactive metal produces a salt + hydrogen.
• With bases: carboxylic acid + base produces a salt + water (neutralisation).
• With carbonates: carboxylic acid + carbonate produces a salt + water + carbon dioxide. The fizzing (CO₂) is a classic test.
• With alcohols: esterification (covered above).

Polymers: long chains from small repeating units

A polymer is a very long molecule built by joining many small molecules called monomers. Think of it like making a paper chain - each paper loop is a monomer, and the finished chain is the polymer.

Addition polymerisation

Alkene monomers join together when the C=C double bond opens up. Many ethene molecules become poly(ethene), commonly known as polythene. The process needs high pressure and a catalyst. To draw the repeating unit, take the monomer, open the double bond to single bonds, and place brackets with extended bonds at each end.

Condensation polymerisation (Supplement)

Two different types of monomer join together, and a small molecule (usually water) is released each time a link forms. Nylon and polyesters are made this way. The key difference from addition polymerisation: a by-product is produced.

Problems with polymers

Most addition polymers are not biodegradable. They persist in landfill for hundreds of years, which creates significant environmental problems. Approaches to managing polymer waste include recycling, developing biodegradable alternatives, and incineration (though this can release toxic gases if not carefully controlled).

Fuels and crude oil

Crude oil is a mixture of hydrocarbons, mostly alkanes of different chain lengths. Fractional distillation separates this mixture based on boiling point. Short-chain fractions (like petroleum gas and petrol) collect near the top of the column where it is cooler; long-chain fractions (like bitumen) collect near the bottom.

There is greater demand for short-chain hydrocarbons than the fraction naturally provides. Cracking solves this by breaking long-chain alkanes into shorter, more useful alkanes and alkenes. Thermal cracking uses high temperatures; catalytic cracking uses a catalyst at a lower temperature.

Common mistakes to watch out for

Self-check questions

1. Write the general formula for alkenes and give the molecular formula of the third member.
2. Draw the displayed formula of propanol.
3. What do you observe when bromine water is added to ethene?
4. Name the two products of the complete combustion of methane.
5. What is the difference between addition polymerisation and condensation polymerisation?
6. Give two structural isomers of C₄H₁₀, showing their structural formulae.
7. Describe how ethanol can be made from ethene.
8. What type of reaction occurs when ethanoic acid reacts with sodium carbonate? Name the products.

Organic chemistry can feel like a lot of content at first glance, but the beauty of the IGCSE syllabus is that it is built on patterns. Master the four homologous series, understand the functional groups, and practise drawing displayed formulae until it feels automatic. Once those foundations are solid, the reactions and exam questions will click into place far more quickly than you expect.