Alkanes Revision Notes

Alkanes are organic compounds composed exclusively of carbon and hydrogen atoms. Each carbon atom in an alkane forms four single covalent bonds, either with other carbon atoms or with hydrogen atoms. These single bonds involve the sharing of one pair of electrons between two atoms, resulting in a stable bond.

For example, in methane (CH₄), the simplest alkane, the carbon atom forms four single covalent bonds with four hydrogen atoms. In ethane (C₂H₆), two carbon atoms are connected by a single bond, and each carbon is bonded to three hydrogen atoms.

• Each carbon atom forms four single covalent bonds.
• Bonds can be C–C or C–H single bonds.
• Single covalent bonds involve sharing one pair of electrons.

Alkanes are classified as saturated hydrocarbons because all carbon-carbon bonds are single bonds. Saturation means that the carbon atoms have formed the maximum possible number of single bonds with hydrogen atoms, leaving no room for additional atoms to bond without breaking existing bonds.

This contrasts with unsaturated hydrocarbons, such as alkenes and alkynes, which contain double or triple bonds and therefore fewer hydrogen atoms per carbon atom.

• Saturated hydrocarbons have only single bonds between carbon atoms.
• Maximum hydrogen atoms are bonded to carbon atoms.
• No double or triple bonds are present in alkanes.

Alkanes follow a general molecular formula of CₙH₂ₙ₊₂, where n represents the number of carbon atoms. This formula reflects the saturation and bonding pattern in alkanes.

For example, if n = 3 (propane), the formula is C₃H₈, which fits the pattern: 2×3 + 2 = 8 hydrogen atoms. This formula helps identify whether a hydrocarbon is an alkane by comparing its molecular formula.

• General formula: CₙH₂ₙ₊₂.
• n = number of carbon atoms.
• Used to confirm alkane identity.

The presence of only single covalent bonds makes alkanes relatively stable and less reactive compared to unsaturated hydrocarbons. Single bonds are strong and require significant energy to break.

Because alkanes are saturated, they do not readily undergo addition reactions that break double or triple bonds. Instead, they mainly undergo substitution reactions and combustion.

For example, methane burns in oxygen to produce carbon dioxide and water, but it does not easily react with bromine under normal conditions without UV light.

• Single bonds confer chemical stability.
• Alkanes are less reactive than unsaturated hydrocarbons.
• Main reactions include combustion and substitution.

Methane (CH₄): The simplest alkane with one carbon atom bonded to four hydrogens.

Ethane (C₂H₆): Two carbon atoms connected by a single bond, each bonded to three hydrogens.

Propane (C₃H₈): Three carbon atoms in a chain with eight hydrogens total.

• Methane: CH₄
• Ethane: C₂H₆
• Propane: C₃H₈

Alkanes are hydrocarbons containing only single covalent bonds between carbon atoms, making them saturated. The C–C and C–H bonds in alkanes are relatively strong and non-polar, which contributes to their chemical stability. Because these bonds require significant energy to break, alkanes do not readily participate in many chemical reactions under normal laboratory conditions.

For example, methane (CH₄) and ethane (C₂H₆) do not react with acids, bases, or most oxidising agents at room temperature. This low reactivity is why alkanes are often described as chemically inert or unreactive in everyday conditions.

• Alkanes have strong single C–C and C–H bonds.
• These bonds are non-polar and require high energy to break.
• Alkanes do not react easily with many common reagents at room temperature.

The main reaction alkanes undergo is combustion, which occurs when they burn in the presence of oxygen. During combustion, the alkane molecules react with oxygen (O₂) to produce carbon dioxide (CO₂) and water (H₂O). This reaction releases energy in the form of heat and light, making alkanes useful as fuels.

For example, the combustion of methane can be represented by the balanced chemical equation: CH₄ + 2O₂ → CO₂ + 2H₂O. This reaction is exothermic, meaning it releases energy to the surroundings.

• Combustion requires oxygen and a source of ignition (heat).
• Products are carbon dioxide and water if combustion is complete.
• Energy is released, which can be harnessed for heating or power.

Alkanes do not spontaneously combust at room temperature; they require an initial energy input, such as a spark or flame, to start the reaction. Once ignited, the combustion reaction is self-sustaining because it releases heat.

Complete combustion occurs when there is sufficient oxygen, producing only CO₂ and H₂O. In limited oxygen, incomplete combustion occurs, producing carbon monoxide (CO) or soot (carbon particles), which are harmful pollutants.

For example, a gas stove uses methane combustion with controlled oxygen supply to provide heat efficiently.

• Ignition energy is needed to start combustion.
• Complete combustion requires excess oxygen.
• Incomplete combustion produces toxic gases and soot.

The combustion of alkanes is highly exothermic because the bonds formed in the products (C=O in CO₂ and O–H in H₂O) are stronger and more stable than the bonds broken in the reactants (C–H and O=O).

This net release of energy is why alkanes are valuable fuels. For example, burning 1 mole of methane releases about 890 kJ of energy.

This energy release can be calculated using bond enthalpies or measured experimentally in calorimetry.

• Energy released comes from forming stronger bonds in products.
• Alkane combustion is a major energy source worldwide.
• Energy calculations involve comparing bond energies of reactants and products.

In summary, alkanes are mostly unreactive due to their strong, non-polar single bonds. Their main chemical reaction is combustion, which requires oxygen and ignition energy, producing carbon dioxide and water while releasing heat.

This limited reactivity explains why alkanes are stable fuels but do not easily undergo other chemical transformations without special conditions such as high temperature or catalysts.

• Alkanes are chemically stable and unreactive under normal conditions.
• Combustion is their primary and significant reaction.
• Understanding combustion is essential for applications in energy and environmental science.