Last updated: 12 October 2026
Petrol, bottled gas, plastics and many industrial chemicals all begin with hydrocarbons. Although a hydrocarbon molecule may contain only carbon and hydrogen, changing the length of its carbon chain or the type of bond between its carbon atoms can completely change how it behaves. This makes hydrocarbons an ideal introduction to organic chemistry: once you learn to read the structure, many of the reactions become predictable.
The central idea is simple. Alkanes contain only carbon–carbon single bonds, whereas alkenes contain a carbon–carbon double bond. That one structural difference explains why the two families have different general formulae, why aqueous bromine can distinguish them and why alkenes undergo addition reactions.
Try these questions before reading the chapter. They check the basic ideas that will make alkane and alkene chemistry much easier to understand.
- Identify what you already understand.
- Spot common misconceptions early.
- Read the chapter with a clearer purpose.
Hydrocarbons and Homologous Series
A hydrocarbon is a compound containing carbon and hydrogen only. Methane, CH₄, ethane, C₂H₆, and ethene, C₂H₄, are all hydrocarbons. Ethanol, C₂H₅OH, is not because it also contains oxygen.
There are an enormous number of possible hydrocarbons, so chemists organise them into families called homologous series.
What is a homologous series?
A homologous series is a family of organic compounds with the same functional group and general formula. Members have similar chemical properties and show a gradual change in physical properties as molecular size increases.
Members follow the same general formula
A general formula shows the relationship between the numbers of atoms in every member of a homologous series. For example, the general formula of an alkane is:
Here, n represents the number of carbon atoms. If n = 3:
so the molecular formula is C₃H₈.
Successive members differ by –CH₂–
Consider the first few alkanes:
Each time one carbon atom is added, two hydrogen atoms are also added. Consecutive members therefore differ by a –CH₂– unit.
Members have similar chemical properties
Members of the same homologous series contain the same characteristic functional group or type of bonding and therefore undergo similar reactions. For example, the alkenes studied here contain a C=C bond and undergo addition reactions.
Physical properties change gradually
As a hydrocarbon chain becomes longer, its molecules become larger. The attractive forces between neighbouring molecules become stronger, so more energy is needed to separate the molecules.
As the number of carbon atoms increases:
- boiling point generally increases;
- viscosity increases; and
- volatility generally decreases.
Chemical similarity does not mean identical physical properties
Members of a homologous series undergo similar types of chemical reaction, but properties such as boiling point and viscosity change as molecular size increases.
Alkanes: Saturated Hydrocarbons
Alkanes are hydrocarbons containing only carbon–carbon single bonds. They are described as saturated.
What does saturated mean?
A saturated hydrocarbon contains only carbon–carbon single bonds. It contains no carbon–carbon double bond.
The general formula of the alkanes is:
The first four alkanes
| Number of carbon atoms | Name | Molecular formula | Condensed structural formula |
|---|---|---|---|
| 1 | Methane | CH₄ | CH₄ |
| 2 | Ethane | C₂H₆ | CH₃–CH₃ |
| 3 | Propane | C₃H₈ | CH₃–CH₂–CH₃ |
| 4 | Butane | C₄H₁₀ | CH₃–CH₂–CH₂–CH₃ |
The beginnings of these names — meth-, eth-, prop- and but- — tell you the number of carbon atoms. The ending -ane tells you that the compound is an alkane.
Checking an alkane formula
Suppose an alkane contains four carbon atoms. Using CnH2n+2:
so the molecular formula is C₄H₁₀.
Reactions of Alkanes
Alkanes are generally less reactive than alkenes because they contain only C–C and C–H single bonds. Two important alkane reactions at O-Level are combustion and substitution.
Combustion
In a plentiful supply of oxygen, an alkane undergoes complete combustion to form carbon dioxide and water.
This reaction releases energy, which explains why many hydrocarbons are useful fuels.
Substitution with chlorine
Methane reacts with chlorine in the presence of ultraviolet light. One hydrogen atom in methane is replaced by a chlorine atom:
CH₃Cl is chloromethane. The reaction is called substitution because one atom has taken the place of another.
Substitution does not mean simply adding chlorine
In methane, one H atom is replaced by Cl. The carbon atom still forms four covalent bonds. Remember the essential condition: ultraviolet light.
Isomerism: Same Molecular Formula, Different Structure
A molecular formula tells you how many atoms are present, but it does not always tell you exactly how those atoms are connected. Carbon atoms can sometimes be arranged in more than one way.
Definition of structural isomerism
Structural isomers are compounds with the same molecular formula but different structural formulae.
The two structural isomers of C₄H₁₀
Butane has a continuous chain of four carbon atoms:
2-Methylpropane has a branched carbon arrangement:
Both contain four carbon atoms and ten hydrogen atoms, so both have molecular formula C₄H₁₀. However, their atoms are connected differently. They are therefore structural isomers.
A rotated molecule is not a new isomer
If the same atoms are still connected to the same atoms, you have only redrawn the original molecule. A genuine structural isomer requires a different arrangement of bonds.
Alkenes: Unsaturated Hydrocarbons
Alkenes are hydrocarbons containing a carbon–carbon double bond, C=C. They are described as unsaturated.
Why are alkenes called unsaturated?
The C=C bond can take part in addition reactions in which new atoms become attached to the two carbon atoms. An alkene can therefore be converted into a more saturated product.
The general formula of the alkenes studied here is:
The first few alkenes
| Name | Molecular formula | Condensed structural formula |
|---|---|---|
| Ethene | C₂H₄ | CH₂=CH₂ |
| Propene | C₃H₆ | CH₂=CH–CH₃ |
| But-1-ene | C₄H₈ | CH₂=CH–CH₂–CH₃ |
| But-2-ene | C₄H₈ | CH₃–CH=CH–CH₃ |
There is no alkene called methene because a C=C bond requires two carbon atoms. Ethene is therefore the smallest alkene.
Why does an alkene contain fewer hydrogen atoms than the corresponding alkane?
Carbon forms four covalent bonds. In ethane, CH₃–CH₃, each carbon uses one bond to join the other carbon, leaving three bonds for hydrogen.
In ethene, CH₂=CH₂, each carbon uses two bonds to join the other carbon. Each carbon therefore has only two remaining bonds available for hydrogen.
This gives C₂H₆ for ethane but C₂H₄ for ethene.
Testing for Unsaturation with Aqueous Bromine
The C=C bond gives alkenes a characteristic reaction with aqueous bromine. This provides a convenient chemical test for distinguishing an alkene from an alkane.
| Substance tested | Observation with aqueous bromine | Conclusion |
|---|---|---|
| Alkane | Orange-brown colour remains | No C=C bond detected |
| Alkene | Orange-brown solution is decolourised | C=C bond present |
For example, ethene reacts with bromine:
The two carbon atoms change from being joined by a double bond to being joined by a single bond. One bromine atom becomes attached to each carbon atom.
Use precise observation language
In an examination answer, write that orange-brown aqueous bromine is decolourised. The important observation is the disappearance of bromine's colour.
Addition Reactions of Alkenes
Alkenes undergo addition reactions. In each reaction, atoms are added across the C=C bond. The double bond becomes a carbon–carbon single bond, while the carbon atoms form new bonds to the incoming atoms.
The pattern to recognise
C=C becomes C–C, and new atoms or groups become attached to the two carbon atoms. Once you recognise this pattern, several apparently different alkene reactions become much easier to understand.
Addition of bromine
This reaction explains the aqueous bromine test for unsaturation.
Addition of hydrogen: hydrogenation
Hydrogen adds across the C=C bond, converting an alkene into an alkane:
A nickel catalyst and heating are used.
Addition of steam: hydration
Ethene reacts with steam to produce ethanol:
This reaction requires a catalyst together with high temperature and pressure. The H and OH from water are added across the C=C bond.
Addition polymerisation
Many alkene molecules can join together to form a long-chain molecule called a polymer. The small molecules that join together are called monomers.
For ethene:
During the reaction, the C=C double bond in each ethene molecule becomes single bonds that connect neighbouring monomers together. The product is poly(ethene).
Do not leave the C=C bond in the polymer repeating unit
During addition polymerisation, the C=C bonds are used to link the monomers. The carbon atoms in the polymer backbone are therefore joined by single bonds.
Cracking Large Hydrocarbon Molecules
Fractional distillation separates crude oil into fractions, but it does not necessarily produce the quantities of hydrocarbons that society needs. There may be too much of some large hydrocarbons and greater demand for smaller molecules.
Cracking converts large hydrocarbon molecules into smaller molecules that are often more useful.
Cracking
Cracking is the breakdown of large hydrocarbon molecules into smaller alkanes, alkenes and sometimes hydrogen using high temperature, usually in the presence of a catalyst.
One possible cracking equation is:
The products are octane, which is an alkane, and ethene, which is an alkene. Notice that the numbers of carbon and hydrogen atoms are conserved.
Why is cracking useful?
- It increases the supply of smaller hydrocarbons that can be used as fuels.
- It produces alkenes used as raw materials for polymers and other chemicals.
- It helps the petroleum industry match the supply of different hydrocarbons to demand.
Fractional distillation and cracking are not the same process
Fractional distillation separates molecules that already exist in crude oil. Cracking breaks molecules and produces new substances, so cracking is a chemical reaction.
Polyunsaturated Oils and Margarine
The idea of unsaturation also appears in vegetable oils. A molecule described as polyunsaturated contains more than one carbon–carbon double bond.
Breaking down the word
Poly- means many. A polyunsaturated molecule therefore contains several sites of unsaturation — in this context, several C=C bonds.
Vegetable oils containing C=C bonds can react with hydrogen in the presence of a nickel catalyst. Hydrogen is added across some of the double bonds.
The product becomes more saturated and has a higher melting point, so it is more solid at room temperature. This principle is used in the manufacture of margarine.
The oil does not necessarily need to have every C=C bond converted. Partial hydrogenation can give a product with the required consistency.
Alkanes and Alkenes Compared
| Feature | Alkanes | Alkenes |
|---|---|---|
| Saturation | Saturated | Unsaturated |
| Carbon–carbon bonding | C–C single bonds only | Contains C=C |
| General formula | CnH2n+2 | CnH2n |
| Name ending | -ane | -ene |
| Characteristic reaction | Substitution | Addition |
| Aqueous bromine | Orange-brown colour remains | Orange-brown solution is decolourised |
| Example | Ethane, C₂H₆ | Ethene, C₂H₄ |
Common Mistakes in Hydrocarbon Questions
| Common mistake | Why it is wrong | Correct idea |
|---|---|---|
| Calling every compound containing carbon a hydrocarbon | A hydrocarbon must contain only carbon and hydrogen. | C₂H₅OH is not a hydrocarbon because it also contains oxygen. |
| Using CnH2n for an alkane | CnH2n is the alkene general formula studied here. | Use CnH2n+2 for alkanes. |
| Defining saturated as “containing lots of hydrogen” | Saturation is determined by the carbon–carbon bonding. | A saturated hydrocarbon contains only C–C single bonds. |
| Forgetting ultraviolet light in alkane substitution | Methane and chlorine require the correct reaction condition. | State ultraviolet light. |
| Calling methane + chlorine an addition reaction | A hydrogen atom is replaced by chlorine. | The reaction is substitution. |
| Saying aqueous bromine “turns clear” | This does not clearly describe the colour change being observed. | Orange-brown aqueous bromine is decolourised. |
| Leaving the C=C bond in an addition product | The double bond is used during the addition reaction. | C=C becomes C–C and new atoms attach to the carbon atoms. |
| Calling two rotated drawings structural isomers | The atoms may still be connected in exactly the same way. | Structural isomers require different structural formulae. |
| Describing cracking as separation | Cracking breaks covalent bonds and forms new molecules. | Cracking is chemical; fractional distillation is physical separation. |
Hydrocarbons Checklist
Before moving on, make sure you can do each of the following without relying on memorised sentences alone.
Homologous series
- Define a hydrocarbon.
- Describe the characteristics of a homologous series.
- Recognise that successive members differ by –CH₂–.
- Explain why physical properties change gradually.
Alkanes
- Use CnH2n+2.
- Name methane, ethane, propane and butane.
- Explain what saturated means.
- Write equations for combustion and substitution.
- State ultraviolet light for reaction with chlorine.
Alkenes
- Use CnH2n.
- Recognise the C=C bond.
- Explain what unsaturated means.
- Use aqueous bromine to test for unsaturation.
Addition reactions
- Show bromine adding across C=C.
- Describe hydrogenation.
- Describe hydration using steam.
- Explain addition polymerisation.
Structures and isomers
- Draw structural formulae of simple hydrocarbons.
- Recognise compounds with the same molecular formula.
- Check whether the atoms are connected differently.
- Distinguish a genuine isomer from a rotated drawing.
Cracking and oils
- Explain why cracking is useful.
- State high temperature and a catalyst.
- Recognise smaller alkanes and alkenes as products.
- Explain hydrogenation in margarine manufacture.
Now apply the ideas from the chapter. These questions focus on structures, reactions, observations and chemical reasoning rather than simple definition recall.
- Apply general formulae and structural ideas.
- Interpret reactions and experimental observations.
- Check the reasoning behind every answer choice.
Questions Students Often Ask About Hydrocarbons
These questions connect the main ideas and clear up some of the points that often cause confusion.
Why does ethene have fewer hydrogen atoms than ethane?
Carbon forms four covalent bonds. In ethane, the two carbon atoms are joined by one bond, leaving three bonds on each carbon for hydrogen. In ethene, the carbon atoms are joined by a double bond, leaving only two bonds on each carbon for hydrogen. Ethane is therefore C₂H₆, while ethene is C₂H₄.
Does saturated mean the same thing as a saturated solution?
No. In hydrocarbon chemistry, saturated describes carbon–carbon bonding: a saturated hydrocarbon contains only C–C single bonds. A saturated solution is a completely different idea involving the maximum amount of solute that can dissolve under particular conditions.
Why are alkenes more reactive than alkanes?
Alkenes contain a C=C bond that can participate in addition reactions. During these reactions, the C=C bond becomes a C–C bond and new atoms become attached to the two carbon atoms. Alkanes do not contain this reactive C=C bond.
Why does aqueous bromine lose its colour when an alkene is present?
Bromine reacts chemically with the alkene by adding across the C=C bond. The coloured bromine is consumed, so the orange-brown colour disappears. The colour change is therefore caused by a chemical reaction, not simply by dilution.
Why does an alkane not decolourise aqueous bromine?
An alkane contains no C=C bond, so it does not undergo the rapid addition reaction shown by an alkene under the conditions of the bromine test. The orange-brown colour therefore remains.
Why is methane reacting with chlorine called substitution rather than addition?
One hydrogen atom in methane is replaced by a chlorine atom. Because one atom takes the place of another, the reaction is substitution. In an alkene addition reaction, atoms are added across the C=C bond instead.
How can I tell whether two structures are really isomers?
First check that they have exactly the same molecular formula. Then examine which atoms are bonded to which. If the connections are genuinely different, they are structural isomers. If one drawing is merely a rotated or flipped version of the other, they are the same molecule.
Does cracking always produce the same products?
No. A large hydrocarbon can break in different places, so different combinations of smaller molecules may form. Any proposed cracking equation must still conserve the total numbers of carbon and hydrogen atoms.
Why is ethene useful if it is not mainly used as a fuel?
The C=C bond makes ethene a useful chemical feedstock. It can undergo addition polymerisation to form poly(ethene), hydration to form ethanol and other addition reactions used in chemical manufacture.
Why does hydrogenation make vegetable oil more solid?
Hydrogenation converts some C=C bonds into C–C single bonds, making the material more saturated. The resulting substances have higher melting points, so the product is more solid at room temperature.
Hydrocarbon chemistry becomes much easier when you stop treating each reaction as an isolated fact and instead look at the structure of the molecule. Alkanes contain only C–C single bonds and are saturated; alkenes contain C=C bonds and are unsaturated. From that distinction follow the general formulae, the aqueous bromine test, substitution and addition reactions, hydrogenation, polymerisation and even the industrial importance of cracking. Learn to recognise the bond first, and much of the chemistry can then be reasoned out rather than memorised.