Fuels, plastics, medicines, food and every living cell are built on carbon. Learn the few simple rules that organise millions of carbon compounds.
Carbon is one atom with a remarkable talent: it can join to other carbon atoms and to a handful of other elements in almost endless combinations. Organic chemistry is the study of those carbon compounds. It explains petrol and plastic, vinegar and soap, sugar and DNA. The good news is that you do not need to memorise millions of compounds. A few families and a few rules organise nearly all of them.
Organic chemistry is the branch of chemistry that studies compounds of carbon, most of which also contain hydrogen and often oxygen, nitrogen, sulfur or halogens such as chlorine. The word “organic” has an old origin. Two centuries ago many scientists believed these compounds could only be made by living things. That belief ended in 1828, when Friedrich Wöhler made urea, a compound found in urine, in the laboratory from non-living starting materials. After that, chemists realised that carbon compounds follow the same rules as all other chemistry.
A few carbon-containing substances are still usually classed as inorganic in school chemistry: carbon dioxide (CO₂), carbon monoxide (CO), carbonates such as calcium carbonate (CaCO₃), and the pure element in its forms such as diamond and graphite. So “contains carbon” is not the full definition. Organic compounds are, in practice, the compounds built on chains and rings of carbon atoms, usually bonded to hydrogen.
Do not confuse this chemical meaning with the word “organic” on food labels, which is about how crops are farmed and has nothing to do with the chemistry of the molecules.
A carbon atom has four outer-shell electrons, so it forms four covalent bonds. This is called tetravalency (valency 4). In methane, CH₄, the four bonds point to the corners of a tetrahedron with angles of about 109.5°, so molecules are three-dimensional, not flat.
Carbon also shows strong catenation: its atoms bond to each other to form long chains, branched chains and rings. The carbon–carbon single bond is strong and stable, so these chains do not fall apart easily. Other elements such as silicon and sulfur can form short chains, but none matches carbon in the length, variety and stability of its structures. Carbon can also form double bonds (C=C) and triple bonds (C≡C), which adds even more variety.
With four bonds, one carbon atom can be at the centre of a molecule like methane, or it can be one link in a chain of thousands of carbon atoms. This one flexible building block is why carbon compounds outnumber the compounds of all other elements combined in everyday life.
A hydrocarbon contains only carbon and hydrogen. The main families are shown below. A saturated hydrocarbon has only single bonds between carbon atoms, so each carbon holds as many hydrogen atoms as it possibly can. An unsaturated hydrocarbon has at least one double or triple carbon–carbon bond, so it holds fewer hydrogen atoms.
| Family | Bonding | General formula | First members |
|---|---|---|---|
| Alkanes | Only C–C single bonds (saturated) | CₙH₂ₙ₊₂ | CH₄, C₂H₆, C₃H₈ |
| Alkenes | One C=C double bond (unsaturated) | CₙH₂ₙ | C₂H₄, C₃H₆, C₄H₈ |
| Alkynes | One C≡C triple bond (unsaturated) | CₙH₂ₙ₋₂ | C₂H₂, C₃H₄ |
Notice that “unsaturated” does not mean “has no hydrogen”. It means the molecule has room to take on extra atoms by opening up its double or triple bond. This is the key to alkene chemistry later in this article.
A homologous series is a family of compounds with the same functional group, the same general formula, and similar chemical properties. Each member differs from the next by one extra –CH₂– unit. Their physical properties change in a regular way, for example boiling point rises as the chain gets longer, because longer molecules have stronger forces between them.
The name of an organic compound tells you its structure. The first part (the stem) shows the number of carbon atoms in the longest chain, and the ending shows the family.
| Carbons | Stem | Alkane (–ane) | Alkene (–ene) | Boiling point of alkane (approx.) |
|---|---|---|---|---|
| 1 | meth- | methane CH₄ | none | −162 °C |
| 2 | eth- | ethane C₂H₆ | ethene C₂H₄ | −89 °C |
| 3 | prop- | propane C₃H₈ | propene C₃H₆ | −42 °C |
| 4 | but- | butane C₄H₁₀ | butene C₄H₈ | about −1 °C |
| 5 | pent- | pentane C₅H₁₂ | pentene C₅H₁₀ | 36 °C |
| 6 | hex- | hexane C₆H₁₄ | hexene C₆H₁₂ | 69 °C |
This is why methane, ethane, propane and butane are gases at room temperature, while pentane and hexane are liquids. Remember the first four stems (meth, eth, prop, but) in order; they are used in almost every organic question.
Question: Write the molecular formulae of the alkane and the alkene that each contain 7 carbon atoms.
Answer: heptane C₇H₁₆ and heptene C₇H₁₄.
Isomers are compounds with the same molecular formula but different structural formulae. The smallest alkane that has isomers is butane, C₄H₁₀. One isomer is a straight chain of four carbons (butane, CH₃CH₂CH₂CH₃). The other has a branch: a chain of three carbons with a CH₃ group on the middle one (2-methylpropane, CH₃CH(CH₃)CH₃). They have different boiling points and slightly different properties, even though both are C₄H₁₀.
The number of possible isomers grows quickly as chains get longer. This is one reason there are so many organic compounds: C₅H₁₂ has three isomers and C₈H₁₈ has eighteen.
Crude oil is a mixture of many hydrocarbons, formed over millions of years from the remains of tiny sea organisms. It is separated by fractional distillation. The oil is heated until most of it vaporises and is fed into a tall column that is hot at the bottom and cooler at the top. Each hydrocarbon rises until it reaches a height where it is cool enough to condense. Small molecules with low boiling points reach the top; large molecules with high boiling points condense near the bottom.
As chain length increases, hydrocarbons become less flammable, more viscous (thicker and more like treacle) and darker, and their boiling points rise. This pattern explains why bitumen is a thick black solid-like material while petrol flows easily.
The refinery produces more large molecules than society needs, and more small ones than it gets. Cracking fixes this by breaking long-chain alkanes into shorter, more useful molecules. It uses high temperature and a catalyst (or steam at very high temperature). The products include a shorter alkane and an alkene. For example, decane can crack to octane and ethene:
Check the balance: carbon 10 = 8 + 2 and hydrogen 22 = 18 + 4. Cracking is valuable because the alkenes it produces are the starting materials for plastics and many other chemicals.
Alkanes are fairly unreactive because their C–C and C–H bonds are strong and non-polar. Their two important reactions are burning and substitution.
With plenty of air, alkanes burn completely to give carbon dioxide and water, and release a lot of energy. This is why they are used as fuels.
If the air supply is limited, incomplete combustion occurs and the products can include carbon monoxide (CO) and carbon (soot), as well as water. The flame turns yellow and sooty.
Carbon monoxide is a colourless, odourless, poisonous gas. Never use a gas heater, charcoal burner or generator in a closed room. Make sure gas appliances are well ventilated and serviced.
Question: Write a balanced equation for the complete combustion of propane, C₃H₈.
In ultraviolet light, an alkane reacts with chlorine in a substitution reaction: a hydrogen atom is replaced by a chlorine atom.
Further substitution can continue to produce CH₂Cl₂, CHCl₃ and CCl₄, so a mixture of products is usually formed.
Alkenes are much more reactive than alkanes because the C=C double bond can open up. In an addition reaction two reactants combine to form a single product, and no atoms are lost. This is possible because the double bond becomes a single bond as new atoms attach to the two carbons.
| Reactant added | Conditions | Product from ethene |
|---|---|---|
| Hydrogen (H₂) | nickel catalyst, heat | ethane, C₂H₆ (hydrogenation) |
| Bromine (Br₂) | room temperature | 1,2-dibromoethane, C₂H₄Br₂ |
| Steam (H₂O) | phosphoric acid catalyst, high temperature and pressure | ethanol, C₂H₅OH (hydration) |
This is the standard test to tell alkenes from alkanes. Shake the substance with orange-brown bromine water. An alkene decolourises it (orange-brown to colourless) because the bromine adds across the double bond. An alkane causes no change, because it is saturated.
Bromine is toxic and corrosive. Bromine water tests are done by a teacher or in a properly equipped laboratory, never at home.
Question: Gas X and gas Y both burn. When each is bubbled through bromine water, X leaves the water orange-brown and Y turns it colourless. Which is the alkene?
Reasoning: A colour change to colourless means bromine has added across a C=C bond. So Y contains a double bond and is the alkene (unsaturated). X does not react, so it is consistent with a saturated alkane.
A functional group is the atom or group of atoms that gives a family its characteristic reactions. Two functional groups appear in almost every school syllabus.
| Family | Functional group | General formula | Common example |
|---|---|---|---|
| Alcohols | –OH (hydroxyl) | CₙH₂ₙ₊₁OH | ethanol, C₂H₅OH |
| Carboxylic acids | –COOH (carboxyl) | CₙH₂ₙ₊₁COOH | ethanoic acid, CH₃COOH |
Ethanol can be made by fermentation, in which yeast converts glucose to ethanol and carbon dioxide at about 25–35 °C without oxygen:
It is also made industrially from ethene and steam. Ethanol burns well, is used as a solvent, in disinfectant products and as a fuel additive. Drinking alcoholic beverages affects the brain and liver, which is why they are age-restricted.
Ethanoic acid (the acid in vinegar) is a weak acid. It reacts with metals, carbonates and bases like other acids, but only partly ionises in water. Ethanol and ethanoic acid react together, with a little concentrated sulfuric acid as catalyst, to form an ester and water. Ethyl ethanoate has a fruity smell, and many esters give fruits and perfumes their scents.
A polymer is a very long molecule made by joining many small molecules called monomers. In addition polymerisation, monomers that contain a C=C double bond open their double bonds and link together in a chain, with no other product. Ethene becomes poly(ethene), commonly called polythene; propene becomes poly(propene); chloroethene becomes PVC.
The simplest way to write it is: n CH₂=CH₂ → –(CH₂–CH₂)ₙ–, where n can be thousands.
Some polymers form by a different route called condensation polymerisation. Two different monomers join and a small molecule, usually water, is lost each time. Nylon and polyester (PET, used in drinks bottles) are made this way. Natural polymers include starch and cellulose (from glucose), proteins (from amino acids) and DNA.
Most common plastics are very durable and do not rot, which makes them useful but creates waste problems. Reducing use, reusing, recycling and proper disposal all help. Never burn plastics: many release harmful fumes, and PVC can produce toxic, corrosive gases.
Try each question first, then tap it to reveal the answer.
Answer: C₁₂H₂₆ (2 × 12 + 2 = 26).
Answer: CₙH₂ₙ; butene is C₄H₈.
Answer: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Answer: Longer molecules have stronger forces between them, so more energy is needed to separate them.
Answer: Orange-brown to colourless, because bromine adds across the C=C double bond.
Answer: Butane (straight chain) and 2-methylpropane (branched).
Answer: Ethene; addition polymerisation.
Suggested use: have students build models of C₄H₁₀ isomers with modelling kits, then draw them. The bromine-water test should only be demonstrated by a teacher with suitable ventilation and eye protection; cyclohexane versus cyclohexene is a typical alkane-versus-alkene pair for demonstration. Parents can help by connecting the topic to fuels, packaging and cooking at home, and by reinforcing the safety messages about carbon monoxide and burning plastics. Please follow your own syllabus for naming rules and required reactions.
These reputable sources were consulted to verify the facts in this article:
Copyright note: This article is original EDUSAMBAM educational writing. The diagrams are original inline SVG schematics created for this page and are not copied from any source. Links open external websites; the sources were used to check facts and are suggested for further study.
Last updated: 21 October 2026. Written for students up to age 18 and checked against standard school-level chemistry references. This is general education, not a substitute for your teacher, textbook or syllabus.
15 questions. Select an answer for each, then submit to see your score instantly.