Alkanes

Q: What is the strict chemical definition of a hydrocarbon?

A compound consisting of carbon and hydrogen atoms only .

Q: What is the molecular geometry and bond angle around each carbon atom in an alkane?

Tetrahedral shape with a bond angle of approximately \(109.5^\circ\) .

Q: How and why does boiling point change with increasing carbon chain length in alkanes?

Boiling point increases because longer molecules have more electrons and greater surface area, leading to stronger van der Waals forces .

Q: On what basis does fractional distillation separate crude oil into fractions?

It separates hydrocarbons based on differences in their boiling points along a temperature gradient.

Q: What conditions and catalyst are used in catalytic cracking, and what are its main products?

High temperature and a zeolite / aluminosilicate catalyst ; produces branched alkanes , cycloalkanes , and aromatic compounds .

Q: What are the products of the complete combustion of an alkane?

Carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)) only.

Q: What harmful products form during the incomplete combustion of alkanes?

Carbon monoxide (\(\text{CO}\)) , carbon particulates / soot (\(\text{C}\)) , and water (\(\text{H}_2\text{O}\)) .

Chemistry 1110 · CCEA AS Level · 8 min read · Updated: 31 Aug 2026

Welcome to Topic 2.3: Alkanes

Welcome to one of the most fundamental chapters in your AS Chemistry journey! Alkanes form the backbone of organic chemistry. They power our vehicles, heat our homes, and provide the raw starting materials to make everything from everyday plastics to medicines. Don't worry if organic chemistry feels a bit overwhelming at first — we will break down each concept step by step so you can ace your CCEA AS 2 exam.

What you will master in this chapter:
- The structure, bonding, and shape of alkane molecules
- Physical trends such as boiling points and solubility
- How crude oil is separated and cracked into useful fuels
- Combustion reactions, flame tests, and environmental impacts
- The step-by-step free radical substitution mechanism

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1. Classification, Bonding, and Structure

What is a Hydrocarbon?

A hydrocarbon is defined strictly as a compound consisting of carbon and hydrogen atoms only. (Exam Tip: Always remember to include the word "only" in your exam definitions, or you risk losing the mark!)

Saturated Nature & General Formulae

Alkanes are described as saturated hydrocarbons. This means they contain only single covalent bonds between atoms; there are no double or triple bonds (\(\text{C}=\text{C}\) or \(\text{C}\equiv\text{C}\)) present.

- Acyclic (open-chain) alkanes follow the general formula: \(\text{C}_n\text{H}_{2n+2}\)
- Cycloalkanes (ring-shaped alkanes) follow the general formula: \(\text{C}_n\text{H}_{2n}\)

Bonding and Molecular Geometry

Every carbon atom in an alkane forms four single covalent bonds (also called \(\sigma\)-bonds). Because electron pairs repel each other equally to get as far apart as possible:

- Each carbon atom has a tetrahedral geometry.
- The bond angle around each carbon atom is approximately \(109.5^\circ\).
- There is free rotation around every single \(\text{C}-\text{C}\) bond, allowing these molecules to twist and bend into different conformations.

Quick Review Box: Key Takeaway
Alkanes = Carbon + Hydrogen only. Saturated = single \(\sigma\)-bonds only. Shape = tetrahedral with bond angles of \(109.5^\circ\).

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2. Physical Properties & Intermolecular Forces

Boiling Point Trends

Alkanes are simple molecular substances held together internally by strong covalent bonds, but between separate molecules there are only weak van der Waals forces (also called London dispersion forces).

1. Effect of Carbon Chain Length:
As the carbon chain gets longer, the boiling point increases.
Why? Longer alkanes have a larger molecular surface area and more total electrons per molecule. This creates stronger van der Waals forces between molecules, requiring more thermal energy to separate them.

2. Effect of Branching (Structural Isomers):
Branched alkanes have lower boiling points than straight-chain isomers with the identical molecular formula.
Why? Think of straight-chain alkanes like flat sheets of paper that can stack closely together, creating lots of surface contact. Branched alkanes are shaped more like compact balls (spheres); they cannot pack as closely together, which reduces the surface area of contact. Less surface contact leads to weaker van der Waals forces, so less energy is needed to boil them.

Solubility

Alkanes are non-polar molecules because the electronegativities of carbon and hydrogen are very similar. As a result:

- Alkanes are insoluble and immiscible in water (they cannot form hydrogen bonds with water molecules).
- Alkanes are soluble in non-polar organic solvents.

Quick Review Box: Key Takeaway
Longer chain \(\implies\) more electrons & greater surface area \(\implies\) stronger van der Waals forces \(\implies\) higher boiling point.
More branching \(\implies\) less surface contact \(\implies\) weaker van der Waals forces \(\implies\) lower boiling point.

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3. Petroleum Processing & Cracking

Fractional Distillation of Crude Oil

Crude oil is a complex mixture consisting predominantly of alkanes. It is separated into useful fractions using fractional distillation:

- The column is hotter at the bottom and cooler at the top (a temperature gradient).
- Crude oil is vaporised before entering the fractionating column.
- Vapours rise up the column, cool, and condense when the temperature drops below their specific boiling point.
- Short-chain alkanes (low boiling points) condense near the top, while long-chain alkanes (high boiling points) condense near the bottom.

Cracking: Making Long Molecules Useful

Economic demand for short-chain alkanes (like petrol) and alkenes (for making plastics) is much higher than the supply found in crude oil. Conversely, there is a surplus of long-chain alkanes. Cracking is the process of breaking down long-chain alkanes into shorter, more valuable alkanes and alkenes via thermal decomposition.

There are two primary methods of cracking tested by CCEA:

1. Thermal Cracking:
- Conditions: High temperature and high pressure.
- Main Products: A high proportion of alkenes (used to manufacture polymers).

2. Catalytic Cracking:
- Conditions: High temperature and a zeolite / aluminosilicate catalyst.
- Main Products: Branched alkanes, cycloalkanes, and aromatic compounds (ideal for clean-burning motor fuels).

Quick Review Box: Key Takeaway
Fractional distillation separates by boiling point; cracking breaks \(\text{C}-\text{C}\) bonds. Catalytic cracking uses a zeolite catalyst to produce branched and cyclic fuels.

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4. Reactions of Alkanes: Combustion & Environment

1. Complete Combustion

When alkanes burn in an excess of oxygen, they react fully to produce only carbon dioxide and water:

\(\text{C}_n\text{H}_{2n+2} + \left(\frac{3n+1}{2}\right)\text{O}_2 \longrightarrow n\text{CO}_2 + (n+1)\text{H}_2\text{O}\)

Example (Propane):
\(\text{C}_3\text{H}_8 + 5\text{O}_2 \longrightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}\)

2. Incomplete Combustion

When oxygen is limited, incomplete combustion takes place. The products include carbon monoxide (\(\text{CO}\)), carbon particulates / soot (\(\text{C}\)), and water (\(\text{H}_2\text{O}\)).

Example Equations:
\(\text{CH}_4 + \frac{3}{2}\text{O}_2 \longrightarrow \text{CO} + 2\text{H}_2\text{O}\)
\(\text{CH}_4 + \text{O}_2 \longrightarrow \text{C} + 2\text{H}_2\text{O}\)

CCEA Flame Observations

In your practical assessments and written papers, alkanes burn with a smoky / sooty yellow flame when combustion is incomplete or when burning alkanes with longer carbon chains (due to high carbon-to-hydrogen ratio).

Pollutants and Environmental Impact

- Carbon monoxide (\(\text{CO}\)): A colourless, odourless, toxic gas that binds preferentially and irreversibly to haemoglobin in red blood cells, preventing oxygen transport.
- Carbon particulates (\(\text{C}\) / Soot): Exacerbates asthma, causes respiratory problems, and contributes to global dimming.
- Carbon dioxide (\(\text{CO}_2\)): A greenhouse gas that absorbs infrared radiation, contributing to enhanced global warming and climate change.
- Sulfur dioxide (\(\text{SO}_2\)): Formed when sulfur impurities in hydrocarbon fuels react with oxygen (\(\text{S} + \text{O}_2 \longrightarrow \text{SO}_2\)). Dissolves in atmospheric moisture causing acid rain.
- Nitrogen oxides (\(\text{NO}_x\), e.g., \(\text{NO}\), \(\text{NO}_2\)): Formed in internal combustion engines where extreme temperatures cause atmospheric nitrogen and oxygen to react (\(\text{N}_2 + \text{O}_2 \longrightarrow 2\text{NO}\)). Contributes to acid rain and photochemical smog.

Catalytic Converters: Fitted to car exhausts to convert hazardous gases (\(\text{CO}\), unburnt hydrocarbons, and \(\text{NO}_x\)) into less harmful gases (\(\text{CO}_2\), \(\text{N}_2\), and \(\text{H}_2\text{O}\)) using precious metal catalysts such as platinum (Pt), palladium (Pd), and rhodium (Rh).

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5. Free Radical Substitution (Halogenation)

Alkanes are generally unreactive because their single \(\text{C}-\text{C}\) and \(\text{C}-\text{H}\) bonds are strong and non-polar. However, they react with halogens (\(\text{Cl}_2\) or \(\text{Br}_2\)) in the presence of ultraviolet (UV) light or sunlight. This reaction is called free radical substitution.

A free radical is a species with an unpaired electron, denoted by a single dot (\(\bullet\)).

Mechanism 1: Chlorination of Methane (\(\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV}} \text{CH}_3\text{Cl} + \text{HCl}\))

Step 1: Initiation
UV light provides the energy to break the \(\text{Cl}-\text{Cl}\) covalent bond symmetrically, so each chlorine atom receives one electron. This is called homolytic fission.
\(\text{Cl}_2 \xrightarrow{\text{UV}} 2\text{Cl}^\bullet\)

Step 2: Propagation (Chain Steps)
Radicals are highly reactive. In this two-step cyclic process, radicals are continuously consumed and regenerated:
1. \(\text{CH}_4 + \text{Cl}^\bullet \longrightarrow {}^\bullet\text{CH}_3 + \text{HCl}\)
2. \({}^\bullet\text{CH}_3 + \text{Cl}_2 \longrightarrow \text{CH}_3\text{Cl} + \text{Cl}^\bullet\)

Step 3: Termination (Removing Radicals)
Two free radicals collide and combine their unpaired electrons to form a stable covalent bond. This stops the chain reaction:
- \(\text{Cl}^\bullet + \text{Cl}^\bullet \longrightarrow \text{Cl}_2\)
- \({}^\bullet\text{CH}_3 + \text{Cl}^\bullet \longrightarrow \text{CH}_3\text{Cl}\)
- \({}^\bullet\text{CH}_3 + {}^\bullet\text{CH}_3 \longrightarrow \text{CH}_3\text{CH}_3\) (ethane)

Did you know? The formation of trace amounts of ethane in this reaction provides direct experimental evidence that methyl radicals (\({}^\bullet\text{CH}_3\)) are formed during the process!

Mechanism 2: Monochlorination of Ethane (\(\text{C}_2\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{UV}} \text{C}_2\text{H}_5\text{Cl} + \text{HCl}\))

Initiation:
\(\text{Cl}_2 \xrightarrow{\text{UV}} 2\text{Cl}^\bullet\)

Propagation:
1. \(\text{CH}_3\text{CH}_3 + \text{Cl}^\bullet \longrightarrow {}^\bullet\text{CH}_2\text{CH}_3 + \text{HCl}\)
2. \({}^\bullet\text{CH}_2\text{CH}_3 + \text{Cl}_2 \longrightarrow \text{CH}_3\text{CH}_2\text{Cl} + \text{Cl}^\bullet\)

Termination:
- \(\text{Cl}^\bullet + \text{Cl}^\bullet \longrightarrow \text{Cl}_2\)
- \({}^\bullet\text{CH}_2\text{CH}_3 + \text{Cl}^\bullet \longrightarrow \text{CH}_3\text{CH}_2\text{Cl}\)
- \({}^\bullet\text{CH}_2\text{CH}_3 + {}^\bullet\text{CH}_2\text{CH}_3 \longrightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3\) (butane)

Controlling Further Substitution

If chlorine is in excess, the halogenated products will continue to react, leading to a mixture of multiple-substituted products: \(\text{CH}_2\text{Cl}_2\) (dichloromethane), \(\text{CHCl}_3\) (trichloromethane), and \(\text{CCl}_4\) (tetrachloromethane).

- To maximise the yield of monochloromethane (\(\text{CH}_3\text{Cl}\)), use an excess of methane (\(\text{CH}_4\)).
- To maximise the yield of tetrachloromethane (\(\text{CCl}_4\)), use an excess of chlorine (\(\text{Cl}_2\)).

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6. Common Pitfalls & Examiner Traps

Be sure to avoid these frequent errors highlighted in CCEA Chief Examiner reports:

1. Missing or Misplaced Radical Dots:
Always show the radical dot clearly on the atom holding the unpaired electron. In the ethyl radical, place the dot on the carbon: \({}^\bullet\text{CH}_2\text{CH}_3\) or \(\text{CH}_3\text{CH}_2^\bullet\), never on hydrogen.

2. Inventing a Hydrogen Radical (\(\text{H}^\bullet\)):
In propagation step 1, students often incorrectly write: \(\text{CH}_4 + \text{Cl}^\bullet \rightarrow \text{CH}_3\text{Cl} + \text{H}^\bullet\). This is wrong! Chlorine radicals abstract a hydrogen atom to form stable \(\text{HCl}\), generating a carbon radical: \(\text{CH}_4 + \text{Cl}^\bullet \rightarrow {}^\bullet\text{CH}_3 + \text{HCl}\).

3. Boiling Point Explanations:
Do not simply say "longer chains have higher boiling points because they have more mass." You must state that longer chains have more electrons and a larger surface area, which leads to stronger van der Waals (intermolecular) forces.

4. Incomplete Combustion Products:
Never forget that \(\text{H}_2\text{O}\) is always produced during incomplete combustion along with \(\text{CO}\) and/or soot (\(\text{C}\)).

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the strict chemical definition of a hydrocarbon?

A compound consisting of carbon and hydrogen atoms only.

What are the general formulae for acyclic alkanes and cycloalkanes?

Acyclic alkanes: \(\text{C}_n\text{H}_{2n+2}\); Cycloalkanes: \(\text{C}_n\text{H}_{2n}\).

What is the molecular geometry and bond angle around each carbon atom in an alkane?

Tetrahedral shape with a bond angle of approximately \(109.5^\circ\).

How and why does boiling point change with increasing carbon chain length in alkanes?

Boiling point increases because longer molecules have more electrons and greater surface area, leading to stronger van der Waals forces.

Why do branched alkanes have lower boiling points than their straight-chain structural isomers?

Branching makes molecules more spherical/compact, reducing surface contact and weakening van der Waals forces between molecules.

Why are alkanes insoluble in water but soluble in non-polar organic solvents?

Alkanes are non-polar and cannot form hydrogen bonds with water molecules.

On what basis does fractional distillation separate crude oil into fractions?

It separates hydrocarbons based on differences in their boiling points along a temperature gradient.

What conditions and catalyst are used in catalytic cracking, and what are its main products?

High temperature and a zeolite / aluminosilicate catalyst; produces branched alkanes, cycloalkanes, and aromatic compounds.

What are the products of the complete combustion of an alkane?

Carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)) only.

What harmful products form during the incomplete combustion of alkanes?

Carbon monoxide (\(\text{CO}\)), carbon particulates / soot (\(\text{C}\)), and water (\(\text{H}_2\text{O}\)).

What are the three steps of the free radical substitution mechanism for chlorinating methane?

1. Initiation (homolytic fission by UV light), 2. Propagation (chain reaction steps), and 3. Termination (combination of two radicals).

How can reaction conditions be adjusted to favour monochloromethane over multiple-substituted chloromethanes?

Use a large excess of methane (\(\text{CH}_4\)).

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