Introduction to Organic Chemistry
3. Hydrocarbons
Learning outcomes
- I can define hydrocarbons as compounds containing only carbon and hydrogen.
- I can distinguish between saturated and unsaturated hydrocarbons.
- I can identify common hydrocarbon families.
- I can write simple molecular formulas for hydrocarbons.
- I can explain the importance of hydrocarbons as fuels and raw materials.
Introduction
Many of the fuels that power our cars, heat our homes, and generate electricity are made from a group of compounds called hydrocarbons. Natural gas, petrol (gasoline), diesel, and many plastics all begin with these simple molecules.
Hydrocarbons are the simplest organic compounds because they contain only carbon and hydrogen atoms. Despite their simple composition, hydrocarbons form millions of different compounds with a wide range of properties and uses. They are the foundation of the petroleum and chemical industries and are among the most important substances in modern society.
What Are Hydrocarbons?
A hydrocarbon is a compound that contains only carbon (C) and hydrogen (H) atoms.
Hydrocarbons do not contain:
- Oxygen.
- Nitrogen.
- Sulfur.
- Chlorine.
- Any other elements.
Examples include:
- Methane (CH₄)
- Ethane (C₂H₆)
- Propane (C₃H₈)
- Butane (C₄H₁₀)
Hydrocarbons are the simplest members of organic chemistry.
Figure 1. Hydrocarbons are compounds made only of carbon and hydrogen atoms.
Carbon Bonding in Hydrocarbons
Carbon atoms form four covalent bonds.
These bonds may connect carbon atoms to:
- Hydrogen atoms.
- Other carbon atoms.
Carbon atoms can join together to form:
- Straight chains.
- Branched chains.
- Rings.
This ability allows hydrocarbons to exist in many different forms.
Saturated Hydrocarbons
Saturated hydrocarbons contain only single covalent bonds between carbon atoms.
Because each carbon atom has the maximum possible number of hydrogen atoms, these compounds are said to be saturated with hydrogen.
General formula:
CnH2n+2Examples:
| Name | Formula |
|---|---|
| Methane. | CH₄ |
| Ethane | C₂H₆ |
| Propane | C₃H₈ |
| Butane | C₄H₁₀ |
These compounds belong to the alkane family.
Figure 2. Alkanes are saturated hydrocarbons containing only single carbon-carbon bonds.
Unsaturated Hydrocarbons
Unsaturated hydrocarbons contain at least one multiple bond between carbon atoms.
These may include:
- Double bonds.
- Triple bonds.
Because they contain fewer hydrogen atoms than saturated hydrocarbons with the same number of carbon atoms, they are described as unsaturated.
Alkenes
Alkenes contain:
- At least one double bond (C=C).
General formula:
Examples:
| Name | Formula |
|---|---|
| Ethene | C₂H₄ |
| Propene. | C₃H₆ |
| Butene | C₄H₈ |
Alkynes
Alkynes contain:
- At least one triple bond (C≡C).
General formula:
CnH2n−2Examples:
| Name | Formula |
|---|---|
| Ethyne | C₂H₂ |
| Propyne. | C₃H₄ |
Figure 3. Saturated hydrocarbons contain single bonds, while unsaturated hydrocarbons contain double or triple bonds.
The Main Hydrocarbon Families
Hydrocarbons are grouped into families based on the types of bonds they contain.
| Family | Bond Type | General Formula |
|---|---|---|
| Alkanes | Single bonds only | CnH2n+2 |
| Alkenes | At least one double bond. | CnH2n |
| Alkynes. | At least one triple bond | CnH2n−2 |
Members of each family have similar chemical properties.
Writing Molecular Formulae
A molecular formula shows the number of each type of atom in a molecule.
Examples:
Methane: CH4
Ethane: C2H6
Propene: C3H6
Ethyne: C2H2
The molecular formula does not show how the atoms are connected—it only gives the numbers of each atom.
Hydrocarbons as Fuels
Hydrocarbons are widely used as fuels because they contain large amounts of chemical energy.
Common hydrocarbon fuels include:
- Natural gas.
- Liquefied petroleum gas (LPG).
- Petrol (gasoline).
- Diesel.
- Kerosene.
- Jet fuel.
When hydrocarbons burn in oxygen, they release energy.
A complete combustion reaction produces:
- Carbon dioxide (CO₂).
- Water (H₂O).
- Energy.
Example: CH4 + 2O2 → CO2 + 2H2O
Figure 4. Hydrocarbons are widely used as fuels because they release large amounts of energy when burned.
Hydrocarbons as Raw Materials
Hydrocarbons are also important raw materials for manufacturing.
They are used to make:
- Plastics.
- Synthetic fibres.
- Rubber.
- Solvents.
- Paints.
- Detergents.
- Medicines.
- Cosmetics.
Many everyday products begin as hydrocarbons extracted from crude oil or natural gas.
Environmental Considerations
Although hydrocarbons are valuable fuels, burning them produces carbon dioxide.
Large amounts of carbon dioxide contribute to:
- The greenhouse effect.
- Climate change.
Incomplete combustion can also produce:
- Carbon monoxide.
- Soot (carbon particles).
Scientists are developing cleaner fuels and renewable alternatives to reduce these environmental impacts.
Figure 5. Hydrocarbons are important both as fuels and as raw materials for manufacturing.
Why Hydrocarbons Are Important
Hydrocarbons are essential because they:
- Provide energy.
- Supply raw materials for industry.
- Support transportation.
- Help manufacture thousands of products.
They form the foundation of the modern petrochemical industry.
Worked Example
Question
Classify each hydrocarbon as saturated or unsaturated.
- CH₄
- C₂H₄
- C₃H₈
- C₂H₂
Solution
| Formula. | Classification |
|---|---|
| CH₄ | Saturated (alkane) |
| C₂H₄ | Unsaturated (alkene) |
| C₃H₈ | Saturated (alkane) |
| C₂H₂ | Unsaturated (alkyne) |
Real-World Connection
Every day, millions of vehicles rely on hydrocarbon fuels such as petrol and diesel. At the same time, the plastics used in water bottles, food packaging, mobile phones, and medical equipment are manufactured from hydrocarbons obtained from crude oil or natural gas. This makes hydrocarbons essential not only for energy production but also for producing many of the materials used in modern life.
Did You Know?
Methane (CH₄), the simplest hydrocarbon, is the main component of natural gas. It burns with a clean blue flame and produces more energy per unit of carbon dioxide released than coal, making it one of the cleaner fossil fuels for electricity generation and heating.
Key Terms
Alkane – A saturated hydrocarbon containing only single carbon-carbon bonds.
Alkene – An unsaturated hydrocarbon containing at least one carbon-carbon double bond.
Alkyne – An unsaturated hydrocarbon containing at least one carbon-carbon triple bond.
Combustion – A chemical reaction in which a substance burns in oxygen, releasing energy.
Hydrocarbon – A compound containing only carbon and hydrogen atoms.
Molecular formula – A formula showing the actual number of each type of atom in a molecule.
Saturated hydrocarbon – A hydrocarbon containing only single carbon-carbon bonds.
Unsaturated hydrocarbon – A hydrocarbon containing one or more carbon-carbon double or triple bonds.
Key Takeaways
- Hydrocarbons are compounds made only of carbon and hydrogen atoms.
- Saturated hydrocarbons (alkanes) contain only single carbon-carbon bonds, while unsaturated hydrocarbons contain double (alkenes) or triple (alkynes) bonds.
- The main hydrocarbon families are alkanes, alkenes, and alkynes.
- Molecular formulae show the numbers of carbon and hydrogen atoms in a molecule.
- Hydrocarbons are important as fuels because they release energy during combustion.
- They are also valuable raw materials used to manufacture plastics, medicines, synthetic fibres, and many other everyday products.