Alkanes

Structure of Alkanes

Alkanes are a family of hydrocarbons that contain only carbon (C) and hydrogen (H) atoms. They are saturated hydrocarbons, meaning all carbon atoms are connected by single bonds only. This saturation means each carbon atom forms four single covalent bonds, either with other carbons or hydrogens.

The general formula for alkanes is:

C_nH_2n+2

where n is the number of carbon atoms. For example, methane (CH₄) has one carbon, ethane (C₂H₆) has two carbons, and propane (C₃H₈) has three carbons.

For instance, pentane has 5 carbon atoms, so its molecular formula is C₅H₁₂ using the formula C_nH_2n+2.

Alkanes form a homologous series, where each successive member differs by a CH₂ group. This series starts with methane and continues with ethane, propane, butane, and so on.

Physical Properties

Alkanes are non-polar molecules because the bonds between carbon and hydrogen have very similar electronegativities, so electrons are shared fairly evenly. This means alkanes do not mix well with water (which is polar) but dissolve in non-polar solvents like oils.

They are relatively unreactive due to the strong C–C and C–H single bonds and lack of polarity. This makes them stable under normal conditions.

The boiling points of alkanes increase as the chain length increases. This is because longer chains have more surface area, leading to stronger London dispersion forces (a type of intermolecular force) between molecules, requiring more energy to separate them.

For example, methane boils at –161°C, propane at –42°C, and octane at about 125°C.

Alkanes with shorter chains are gases at room temperature, medium chains are liquids, and very long chains are solids.

Chemical Properties

Alkanes undergo two main types of chemical reactions:

• Combustion
• Substitution reactions with halogens

Combustion Reactions

Alkanes burn in oxygen to release energy, producing carbon dioxide and water if the combustion is complete:

$$\text{Alkane} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{energy}$$

For example, the complete combustion of methane is:

$$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Incomplete combustion happens when there is not enough oxygen, producing carbon monoxide (CO) or soot (carbon) instead of carbon dioxide. This is dangerous because CO is toxic.

Substitution Reactions with Halogens

Alkanes react slowly with halogens (like chlorine or bromine) in the presence of ultraviolet (UV) light. This is called a substitution reaction because one or more hydrogen atoms are replaced by halogen atoms.

For example, methane reacts with chlorine under UV light:

$$\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{CH}_3\text{Cl} + \text{HCl}$$

This reaction proceeds by a free radical mechanism, where UV light breaks the Cl-Cl bond to form chlorine radicals that replace hydrogen atoms on the alkane. This can continue to substitute more hydrogens, forming dichloromethane, trichloromethane, and so on.

Because alkanes have strong C–H bonds and are non-polar, they are generally unreactive except under special conditions like UV light for substitution or burning in combustion.

Uses of Alkanes

Alkanes are very important in everyday life and industry:

• Fuels: Many alkanes are found in fuels like petrol (gasoline), diesel, and liquefied petroleum gas (LPG). Their combustion releases energy used to power vehicles, heat homes, and generate electricity.
• Feedstocks for chemicals: Alkanes are starting materials (feedstocks) for making other chemicals, including plastics, solvents, and pharmaceuticals.
• Energy release: The energy released when alkanes burn comes from breaking and forming bonds. The strong C–H and C–C bonds release lots of energy when converted to CO₂ and H₂O.

For example, petrol is a mixture of alkanes with about 5 to 12 carbon atoms. When petrol burns in car engines, it releases energy that moves the car.

Examples of Calculations and Reactions

Example: Calculate the molecular formula of an alkane with 7 carbon atoms.

Using the general formula $C_nH_{2n+2}$:

$$n = 7 \implies H = 2 \times 7 + 2 = 16$$

So, the molecular formula is $C_7H_{16}$, which is heptane.

Example: Write the balanced equation for the complete combustion of propane (C₃H₈).

Step 1: Write unbalanced equation:

$$C_3H_8 + O_2 \rightarrow CO_2 + H_2O$$

Step 2: Balance carbon atoms:

$$C_3H_8 + O_2 \rightarrow 3CO_2 + H_2O$$

Step 3: Balance hydrogen atoms:

$$C_3H_8 + O_2 \rightarrow 3CO_2 + 4H_2O$$

Step 4: Balance oxygen atoms:

On the right, oxygen atoms = $3 \times 2 + 4 \times 1 = 6 + 4 = 10$

On the left, oxygen comes as O₂, so number of O₂ molecules = $10 \div 2 = 5$

Final balanced equation:

$$C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$$