Introduction: The Chemistry of Alkanes and Fuels
Welcome to the study of alkanes, combustion, and fuels! Alkanes might seem simple at first glance—they are just chains of carbon and hydrogen atoms joined by single bonds. Yet, these humble molecules power our vehicles, heat our homes, and provide the essential chemical feedstock for modern industry. In the OCR Chemistry B (Salters) course, this topic lies at the heart of the Developing fuels (DF) storyline.
In this chapter, you will learn how to name and draw alkanes, understand their structure and bonding, explore what happens when they burn, investigate how larger molecules are broken down through cracking, and evaluate alternative fuels like hydrogen and biofuels. Don't worry if organic chemistry has felt intimidating before—we will break down each concept step by step!
1. What are Alkanes? Structure and Bonding
An alkane is a saturated hydrocarbon:
• Hydrocarbon: A compound containing only carbon and hydrogen atoms.
• Saturated: Contains only single carbon–carbon (\(\text{C–C}\)) bonds, meaning no more hydrogen atoms can be added to the molecule.
General Formula and Homologous Series
Alkanes form a homologous series—a family of organic compounds with the same functional group, similar chemical properties, and the same general formula. For open-chain (aliphatic) alkanes, the general formula is:
\(\text{C}_n\text{H}_{2n+2}\)
For example, if an alkane has \(4\) carbon atoms (\(n = 4\)), it will have \((2 \times 4) + 2 = 10\) hydrogen atoms, giving the molecular formula \(\text{C}_4\text{H}_{10}\) (butane).
Bonding and Molecular Shape
Every carbon atom in an alkane forms four single covalent bonds. These single bonds are known as \(\sigma\)-bonds (sigma bonds). A \(\sigma\)-bond is formed by the direct head-on overlap of atomic orbitals between the bonding atoms.
Because each carbon atom is surrounded by four electron pairs (four bonding pairs and zero lone pairs), the electron pairs repel each other as far apart as possible to minimise repulsion (electron pair repulsion theory):
• Arrangement/Shape: Tetrahedral around each carbon atom.
• Bond Angle: Approximately \(109.5^\circ\).
• Free Rotation: The atoms can rotate freely around single \(\sigma\)-bonds, meaning carbon chains can twist into various shapes in three dimensions.
Representing Alkanes
You need to be confident working with different chemical representations:
• Molecular formula: Shows the actual number of atoms of each element (e.g., \(\text{C}_3\text{H}_8\)).
• Structural formula: Shows how atoms are arranged carbon by carbon without showing all the bonds (e.g., \(\text{CH}_3\text{CH}_2\text{CH}_3\)).
• Displayed formula: Shows all the atoms and all the bonds between them in full 2D.
• Skeletal formula: Shows only the carbon skeleton as a zig-zag line. Hydrogen atoms attached to carbons and \(\text{C–H}\) bonds are omitted; the ends and vertices represent carbon atoms with their associated hydrogens.
Quick Review: Alkanes are saturated hydrocarbons with general formula \(\text{C}_n\text{H}_{2n+2}\). Every carbon atom is tetrahedral with bond angles of \(109.5^\circ\), held by strong single \(\sigma\)-bonds that allow free rotation.
2. Nomenclature and Structural Isomerism
Naming Alkanes (IUPAC Rules)
To name any alkane, follow these simple steps:
1. Find the longest continuous carbon chain: This gives the stem name (e.g., \(1 = \text{meth-}\), \(2 = \text{eth-}\), \(3 = \text{prop-}\), \(4 = \text{but-}\), \(5 = \text{pent-}\), \(6 = \text{hex-}\)).
2. Number the chain: Start from the end that gives any attached alkyl side-chains the lowest possible numbers.
3. Name the side-chains: Alkyl groups end in -yl (e.g., \(-\text{CH}_3\) is methyl, \(-\text{C}_2\text{H}_5\) is ethyl).
4. Add prefixes for multiples: If there is more than one of the same side-chain, use di- (2), tri- (3), or tetra- (4).
5. Assemble the name alphabetically: List side-chains in alphabetical order (ignoring prefixes like di- and tri-), placing position numbers before each substituent separated by hyphens (e.g., 2,2-dimethylpropane).
Structural Isomerism in Alkanes
Structural isomers are molecules with the same molecular formula but different structural formulae.
Alkanes show chain isomerism. For example, \(\text{C}_4\text{H}_{10}\) can exist as:
• Butane: A straight chain of four carbons, \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\).
• 2-methylpropane: A branched chain with three carbons in the main chain and one methyl branch, \(\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_3\).
Common Mistake to Avoid: Drawing a straight chain bent around a corner does not create a new isomer! Because of free rotation around \(\sigma\)-bonds, a continuous chain of 4 carbons is still butane regardless of whether you draw it as a straight horizontal line or an L-shape.
3. Combustion of Alkanes
Alkanes are generally unreactive because they have strong, non-polar \(\text{C–C}\) and \(\text{C–H}\) bonds. However, their primary commercial use is as fuels because they react readily with oxygen in highly exothermic combustion reactions.
Complete Combustion
When alkanes burn in a plentiful supply of oxygen, complete combustion occurs. The only products are carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)).
General word equation:
alkane + oxygen \(\rightarrow\) carbon dioxide + water
Example: Complete combustion of propane (\(\text{C}_3\text{H}_8\)):
\(\text{C}_3\text{H}_8\text{(g)} + 5\text{O}_2\text{(g)} \rightarrow 3\text{CO}_2\text{(g)} + 4\text{H}_2\text{O}\text{(l)}\)
Top Tip for Balancing Combustion Equations: Balance carbon atoms first, then hydrogen atoms, and balance oxygen atoms last!
Incomplete Combustion
When the oxygen supply is limited, incomplete combustion takes place. In this case, there is not enough oxygen to convert all carbon into carbon dioxide. The products may include carbon monoxide (\(\text{CO}\)), solid carbon particles (soot / \(\text{C}\)), and water (\(\text{H}_2\text{O}\)).
Example: Incomplete combustion of propane forming carbon monoxide:
\(\text{C}_3\text{H}_8\text{(g)} + 3.5\text{O}_2\text{(g)} \rightarrow 3\text{CO}\text{(g)} + 4\text{H}_2\text{O}\text{(l)}\)
Or forming solid carbon (soot):
\(\text{C}_3\text{H}_8\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow 3\text{C}\text{(s)} + 4\text{H}_2\text{O}\text{(l)}\)
Hazards and Environmental Impacts
• Carbon monoxide (\(\text{CO}\)): A colourless, odourless, and highly toxic gas. It binds irreversibly to haemoglobin in red blood cells, preventing oxygen transport around the body, which can lead to asphyxiation and death.
• Carbon particulates (soot, \(\text{C}\)): Cause respiratory illnesses (such as asthma) when inhaled and contribute to global dimming.
• Carbon dioxide (\(\text{CO}_2\)): A greenhouse gas. Although essential for maintaining the Earth's natural temperature, excessive emissions from burning fossil fuels enhance the greenhouse effect, contributing to global warming and climate change.
Key Takeaway: Complete combustion yields \(\text{CO}_2\) and \(\text{H}_2\text{O}\). Incomplete combustion occurs in limited oxygen, producing toxic \(\text{CO}\) and soot alongside \(\text{H}_2\text{O}\).
4. Cracking of Hydrocarbons
Crude oil contains a high proportion of long-chain alkanes. However, consumer demand is much higher for short-chain alkanes (used directly as petrol/gasoline) and short-chain alkenes (used to manufacture polymers and other chemicals).
Cracking is the process of breaking down long-chain hydrocarbon molecules into smaller, more useful molecules by breaking covalent \(\text{C–C}\) bonds.
The Cracking Process
• Reaction Type: Thermal decomposition.
• Products: A mixture of shorter-chain alkanes and alkenes (unsaturated hydrocarbons containing \(\text{C=C}\) double bonds). In some conditions, hydrogen gas (\(\text{H}_2\)) is also produced.
• Heterogeneous Catalysis: Industrial cracking commonly uses a solid catalyst over which vaporised hydrocarbons pass. A heterogeneous catalyst is in a different physical state from the reactants (here, solid catalyst and gaseous reactants). Note: Specific examples of cracking catalysts are not required by the syllabus summary.
Example Equation for Cracking
Decane (\(\text{C}_{10}\text{H}_{22}\)) can be cracked into octane (\(\text{C}_8\text{H}_{18}\)) and ethene (\(\text{C}_2\text{H}_4\)):
\(\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4\)
Notice how the number of carbon atoms and hydrogen atoms must balance on both sides: \(10 = 8 + 2\) and \(22 = 18 + 4\). (For further details on alkene chemistry, see the chapter on Alkenes and electrophilic addition).
5. Alternative Fuels and Sustainability
Because fossil fuels are finite resources and their combustion releases large quantities of greenhouse gases, alternative fuels are increasingly important in modern society.
Biofuels
Biofuels are fuels derived from recently living biological material (biomass), such as crops, agricultural waste, or vegetable oils.
Common examples include:
• Bioethanol: Produced by the fermentation of sugars derived from crops such as sugar cane or maize.
• Biodiesel: Produced by chemically processing vegetable oils or animal fats.
Advantages and Sustainability of Biofuels
• Renewable: Derived from crops that can be regrown year after year.
• Concept of "Carbon Neutrality": Plants absorb \(\text{CO}_2\) during photosynthesis as they grow. When the resulting biofuel is burned, that same quantity of \(\text{CO}_2\) is released back into the atmosphere:
\(\text{CO}_2\text{ absorbed during growth} \approx \text{CO}_2\text{ released during combustion}\)
Limitations and Reality of Biofuels
In practice, biofuels are rarely 100% carbon neutral because fossil fuels are burned during farming machinery operation, the manufacture of fertilisers, transport, and industrial refining processes. Additionally, dedicating fertile land to fuel crops can displace food production, leading to food shortages and increased food prices (the "food vs. fuel" debate).
Hydrogen as a Fuel
Hydrogen gas (\(\text{H}_2\)) can be burned directly as a fuel in modified engines or used in fuel cells to generate electricity.
Equation for the combustion of hydrogen:
\(2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O}\text{(l)}\)
Advantages of Hydrogen
• Clean emissions at the point of use: The only chemical product of combustion is water (\(\text{H}_2\text{O}\)), with zero emissions of \(\text{CO}_2\), \(\text{CO}\), or soot particulates.
• High energy density: Releases a high amount of energy per gram of fuel.
Challenges of Hydrogen
• Storage and Transport: Hydrogen is a low-density gas requiring either high-pressure compression tanks or liquefaction at cryogenic temperatures, which poses safety and engineering hurdles.
• Production: Hydrogen does not occur naturally as an element on Earth in large quantities. It must be manufactured (e.g., by the electrolysis of water or reforming methane), both of which require significant energy inputs. Unless renewable electricity is used, the overall process still generates carbon emissions.
Key Takeaway: While traditional alkanes provide reliable and energy-dense fuel, concerns over depletion and climate change drive the development of biofuels and hydrogen. Evaluating alternative fuels requires assessing the whole lifecycle, from production and transport to combustion.
Summary Checklist
Before moving on to the next chapter, check that you can:
• Define hydrocarbon and saturated, and state the general formula for alkanes (\(\text{C}_n\text{H}_{2n+2}\)).
• Describe the shape around carbon atoms as tetrahedral (\(109.5^\circ\)) and explain bonding via \(\sigma\)-bonds.
• Name branched alkanes and draw their molecular, structural, displayed, and skeletal formulae.
• Identify and draw structural (chain) isomers.
• Write balanced equations for complete and incomplete combustion of alkanes and describe the hazards of \(\text{CO}\) and soot.
• Explain why cracking is carried out and balance cracking equations.
• Compare fossil fuels with biofuels and hydrogen in terms of benefits, limitations, and sustainability.