Hydrocarbons (Grade 12 NSC Matric Physical Sciences): Revision Notes
Hydrocarbons
What are hydrocarbons?
Hydrocarbons are organic molecules that contain only carbon and hydrogen atoms, with no other functional groups besides single, double, or triple carbon-carbon bonds. These compounds form the foundation of organic chemistry and are essential to many industrial processes and biological systems.
Definition: A hydrocarbon is an organic molecule which contains only carbon and hydrogen atoms with no other functional groups besides single, double or triple carbon-carbon bonds.
Hydrocarbons can be divided into two main categories: aliphatic compounds (which have chain structures) and cyclic compounds (which have ring structures). In this note, we will focus on the aliphatic hydrocarbons, which are further classified into three important groups based on the types of bonds between carbon atoms.
Classification of aliphatic hydrocarbons
Aliphatic hydrocarbons are organised into three main groups based on their bonding patterns:
- Alkanes - contain only single bonds between carbon atoms
- Alkenes - contain at least one double bond between carbon atoms
- Alkynes - contain at least one triple bond between carbon atoms
The presence of double or triple bonds determines whether a hydrocarbon is saturated (alkanes) or unsaturated (alkenes and alkynes). This difference in bonding significantly affects their chemical properties and reactivity.
Alkanes - the saturated hydrocarbons
Alkanes are hydrocarbons that contain only single covalent bonds between their carbon atoms. This means they are saturated compounds and are relatively unreactive compared to other hydrocarbon groups.
Key characteristics of alkanes
The defining features of alkanes include:
- All carbon-carbon bonds are single bonds
- They follow the general formula: (where n = number of carbon atoms)
- They are the least reactive of the hydrocarbon groups
- They form a homologous series
Homologous Series: A series of compounds with the same general formula. All molecules in this series will contain the same functional groups. In alkanes, each successive member differs from the previous one by a unit.
Examples of alkanes
Worked Example: Methane Structure
Methane () - the simplest alkane with one carbon atom
This shows how methane has four single bonds connecting the central carbon to four hydrogen atoms.
Worked Example: Ethane Structure
Ethane () - contains two carbon atoms
Notice how ethane fits the general formula:
Worked Example: Propane Structure
Propane () - contains three carbon atoms
Again, this follows the pattern:
Properties and uses of alkanes
Alkanes serve as the most important source of fuel worldwide and are extensively used in the chemical industry. Their key properties include:
- Low reactivity due to saturation
- Compounds with four or fewer carbon atoms exist as gases at room temperature
- Larger alkanes (five or more carbon atoms) exist as liquids
- They are major components of natural gas and petroleum products
Common applications include heating fuels, cooking gas, and as raw materials for producing other chemicals.
Alkenes - the unsaturated hydrocarbons
Alkenes are hydrocarbons that contain at least one double bond between two carbon atoms. This double bond makes them unsaturated and significantly more reactive than alkanes.
Key characteristics of alkenes
The distinguishing features of alkenes are:
- Contain at least one carbon-carbon double bond
- Follow the general formula: (where n = number of carbon atoms)
- They are highly reactive due to the presence of double bonds
- Form their own homologous series
Examples of alkenes
Worked Example: Ethene Structure
Ethene () - also known as ethylene, the simplest alkene
Note how ethene follows the alkene formula:
Worked Example: Propene Structure
Propene () - contains three carbon atoms with one double bond
This also fits the general formula:
Alkenes can also contain multiple double bonds in the same molecule, as shown in more complex structures:
Properties and uses of alkenes
Alkenes are more reactive than alkanes because they are unsaturated. Key properties include:
- Higher reactivity compared to alkanes
- Used extensively in the chemical industry
- Can undergo addition reactions due to double bonds
- Important starting materials for polymer production
Ethene is particularly important in industry for making plastics and other synthetic materials. Propene is used to manufacture polypropylene and serves as a fuel gas in industrial processes.
Alkynes - the triple-bonded hydrocarbons
Alkynes are hydrocarbons that contain at least one triple bond between two carbon atoms. They are unsaturated compounds and are even more reactive than alkenes.
Key characteristics of alkynes
The main features of alkynes include:
- Contain at least one carbon-carbon triple bond
- Follow the general formula: (where n = number of carbon atoms)
- They are highly reactive due to triple bonds
- Less common than alkanes and alkenes in everyday applications
Worked Example: Ethyne Structure
Ethyne () - also known as acetylene, the simplest alkyne:
The structure shows: H-C≡C-H
Following the formula:
This compound is widely used in industrial welding due to the extremely high temperatures produced when it burns with oxygen.
Summary and comparison of hydrocarbon types
The following table summarises the key differences between the three main hydrocarbon families:
Key Comparison of Hydrocarbon Types:
| Hydrocarbon type | General formula | Bond type | Reactivity |
|---|---|---|---|
| Alkanes | Single bonds only | Low reactivity | |
| Alkenes | At least one double bond | High reactivity | |
| Alkynes | At least one triple bond | High reactivity |
Testing for saturation - laboratory methods
Chemists can distinguish between saturated (alkanes) and unsaturated (alkenes/alkynes) hydrocarbons using simple chemical tests.
Laboratory Tests for Saturation
Bromine water test:
- Saturated compounds (alkanes): No reaction occurs with bromine water - the orange colour remains unchanged
- Unsaturated compounds (alkenes/alkynes): React with bromine water, causing the orange colour to disappear (decolourisation occurs)
Potassium permanganate test:
- Saturated compounds (alkanes): No reaction with potassium permanganate solution
- Unsaturated compounds (alkenes/alkynes): React with potassium permanganate, causing the purple colour to fade
These tests work because unsaturated compounds can undergo addition reactions with these reagents, while saturated compounds cannot react under normal conditions.
Exam Tips
- Always check the general formulas when identifying hydrocarbon types: (alkanes), (alkenes), (alkynes)
- Remember that alkanes are "saturated" (no double/triple bonds) while alkenes and alkynes are "unsaturated"
- In chemical tests, look for colour changes - unsaturated compounds will decolourise bromine water and potassium permanganate
- When drawing structures, ensure you show the correct number of bonds for each carbon atom (carbon always forms four bonds)
Key Points to Remember:
- Hydrocarbons contain only carbon and hydrogen atoms with single, double, or triple bonds between carbons
- Alkanes () are saturated and have low reactivity due to single bonds only
- Alkenes () and alkynes () are unsaturated and highly reactive due to multiple bonds
- Chemical tests using bromine water or potassium permanganate can distinguish between saturated and unsaturated hydrocarbons
- Each type forms a homologous series where members differ by units and have similar chemical properties