5

What Are Alkanes?

Alkanes are a family of organic compounds made entirely from carbon and hydrogen.

A compound containing only carbon and hydrogen is called a hydrocarbon.

Alkanes contain only single covalent bonds between their carbon atoms.

Because they contain only single carbon-carbon bonds, alkanes are described as saturated hydrocarbons.

The simplest alkane is methane, CH₄.

Other common alkanes include:

• ethane
• propane
• butane
• pentane
• hexane

Alkanes are important because they form a major part of fuels such as natural gas, gasoline, diesel, and liquefied petroleum gas.

Hydrocarbons

A hydrocarbon is a compound containing only:

carbon (C)

and:

hydrogen (H)

 

 

 

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For example:

CH₄

contains only carbon and hydrogen, so it is a hydrocarbon.

C₂H₆

also contains only carbon and hydrogen.

However:

C₂H₅OH

contains oxygen as well as carbon and hydrogen, so it is not a hydrocarbon.

What Does Saturated Mean?

Alkanes are described as saturated.

A saturated hydrocarbon contains only single covalent bonds between carbon atoms.

For example, ethane has the structure:

CH₃–CH₃

The two carbon atoms are connected by a single bond.

There are no:

C=C double bonds

or:

C≡C triple bonds

 

 

 

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This is the key feature of an alkane.

Why Are They Called Saturated?

Carbon normally forms:

4 covalent bonds

In an alkane, each carbon atom forms as many bonds with hydrogen atoms as possible while maintaining only single bonds between carbon atoms.

For example, methane is:

CH₄

One carbon atom forms four single bonds with four hydrogen atoms.

Ethane is:

C₂H₆

Each carbon forms one bond with the other carbon and three bonds with hydrogen.

The molecule is therefore "saturated" with hydrogen for that carbon skeleton.

Carbon Forms Four Bonds

Carbon has four electrons available for bonding in its outer shell and normally forms four covalent bonds in stable organic molecules.

Consider methane:

CH₄

The carbon forms:

4 C–H bonds

In ethane:

CH₃–CH₃

each carbon forms:

• 1 C–C bond
• 3 C–H bonds

Total for each carbon:

4 bonds

 

 

 

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Hydrogen Forms One Bond

Hydrogen normally forms:

1 covalent bond

Therefore, each hydrogen atom in an alkane is connected to one carbon atom.

For example, in methane:

CH₄

four hydrogen atoms each form one bond with the central carbon atom.

Covalent Bonding in Alkanes

The bonds within alkane molecules are covalent bonds.

A covalent bond forms when atoms share pairs of electrons.

Alkanes contain:

C–C single covalent bonds

and:

C–H single covalent bonds

 

 

 

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There are no ionic bonds within an alkane molecule.

Methane

The simplest alkane is:

methane

Molecular formula:

CH₄

Methane contains:

• 1 carbon atom
• 4 hydrogen atoms

The carbon forms four single covalent bonds.

 

 

 

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Methane is the main component of natural gas and is commonly used as a fuel.

The Shape of Methane

Methane is sometimes drawn as a flat structure on paper.

However, the actual molecule is three-dimensional.

The four C–H bonds point toward the corners of a tetrahedral arrangement.

The bond angle is approximately:

109.5°

This arrangement keeps the bonding electron pairs as far apart as possible.

Ethane

The second member of the alkane family is:

ethane

Molecular formula:

C₂H₆

Structural formula:

CH₃–CH₃

 

 

 

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Ethane contains:

• 2 carbon atoms
• 6 hydrogen atoms
• 1 C–C single bond
• 6 C–H single bonds

Both carbon atoms form four covalent bonds.

Propane

The third alkane is:

propane

Molecular formula:

C₃H₈

Structural formula:

CH₃–CH₂–CH₃

Propane contains:

• 3 carbon atoms
• 8 hydrogen atoms

It is commonly stored as a fuel under pressure.

 

 

 

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Butane

The fourth member is:

butane

Molecular formula:

C₄H₁₀

One possible structural formula is:

CH₃–CH₂–CH₂–CH₃

Butane is commonly used as a fuel in products such as portable gas canisters.

The First Members of the Alkane Family

 

 

 

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Notice that each new member adds:

CH₂

to the previous member.

A Homologous Series

Alkanes form a homologous series.

A homologous series is a family of organic compounds that have:

• the same general formula
• similar chemical properties
• the same type of functional pattern
• a gradual change in physical properties
• successive members differing by CH₂

For example:

Ethane:

C₂H₆

Propane:

C₃H₈

Difference:

CH₂

Naming Alkanes

The beginning of an alkane's name tells us how many carbon atoms are present.

meth- = 1 carbon

eth- = 2 carbons

prop- = 3 carbons

but- = 4 carbons

pent- = 5 carbons

hex- = 6 carbons

hept- = 7 carbons

oct- = 8 carbons

non- = 9 carbons

dec- = 10 carbons

The ending:

-ane

indicates that the compound is an alkane.

 

 

 

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The General Formula of Alkanes

All simple acyclic alkanes follow the general formula:

CₙH₂ₙ₊₂

where:

n = number of carbon atoms

This formula allows us to predict the molecular formula of an alkane.

Using the General Formula

Suppose an alkane contains:

5 carbon atoms

Then:

n = 5

General formula:

CₙH₂ₙ₊₂

Number of hydrogen atoms:

2(5) + 2

= 12

Therefore:

C₅H₁₂

This is pentane.

Another General Formula Example

Suppose:

n = 8

Hydrogen atoms:

2(8) + 2

= 18

Formula:

C₈H₁₈

This is:

octane

 

 

 

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Why Is the Formula CₙH₂ₙ₊₂?

Consider a straight chain containing several carbon atoms.

Each carbon needs:

4 bonds

The carbon atoms inside the chain are bonded to two other carbon atoms.

The carbon atoms at the ends are bonded to only one other carbon atom.

The remaining bonds are filled by hydrogen.

This produces the relationship:

number of H atoms = 2n + 2

Therefore:

CₙH₂ₙ₊₂

Identifying an Alkane from Its Formula

To determine whether a molecular formula could represent an acyclic alkane:

Step 1: Identify the number of carbon atoms.

Step 2: Substitute that number into:

H = 2n + 2

Step 3: Compare the predicted number of hydrogens with the formula.

Example: Is C₆H₁₄ an Alkane?

Number of carbon atoms:

n = 6

Calculate:

2(6) + 2 = 14

The formula contains:

14 hydrogen atoms

Therefore:

C₆H₁₄ fits the general formula for an alkane.

Example: Is C₅H₁₀ an Alkane?

Number of carbon atoms:

n = 5

An acyclic alkane would require:

2(5) + 2 = 12

So its formula would be:

C₅H₁₂

But the given formula is:

C₅H₁₀

Therefore, C₅H₁₀ does not fit the general formula for an acyclic alkane.

 

 

 

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A molecular formula that does not fit the alkane formula may belong to another type of compound.

Important Limitation of the General Formula

The formula:

CₙH₂ₙ₊₂

applies to acyclic alkanes, meaning saturated hydrocarbons without carbon rings.

Saturated hydrocarbons containing rings are called cycloalkanes.

For example, cyclohexane has the formula:

C₆H₁₂

but it still contains only single carbon-carbon bonds.

So when using molecular formulas to identify alkanes, it is important to know whether the question is referring specifically to the ordinary acyclic alkane homologous series.

Molecular Formula and Structural Formula

A molecular formula tells us the number of each type of atom.

Example:

C₄H₁₀

A structural formula gives additional information about how atoms are connected.

For example:

CH₃–CH₂–CH₂–CH₃

 

 

 

Both represent butane, but the structural formula provides more information about bonding.

Displayed Formulas

A displayed formula shows the individual bonds between atoms.

For example, methane can be represented as:

    H
    |
H - C - H
    |
    H

Each line represents a:

single covalent bond

Displayed formulas are useful because they make it easy to count the bonds formed by each atom.

Counting Bonds in an Alkane

Consider propane:

CH₃–CH₂–CH₃

Carbon 1:

3 C–H bonds + 1 C–C bond = 4 bonds

Carbon 2:

2 C–H bonds + 2 C–C bonds = 4 bonds

Carbon 3:

3 C–H bonds + 1 C–C bond = 4 bonds

Every carbon forms:

4 covalent bonds

Every hydrogen forms:

1 covalent bond

Single Bonds and Saturation

Compare these carbon-carbon bonds:

C–C

single bond

C=C

double bond

C≡C

triple bond

Alkanes contain only:

C–C

single bonds.

 

 

 

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Compounds containing carbon-carbon double or triple bonds are described as unsaturated.

Alkanes Compared with Alkenes

Alkanes:

• saturated
• only C–C single bonds
• general formula CₙH₂ₙ₊₂ for acyclic alkanes

Alkenes:

• unsaturated
• contain at least one C=C double bond
• simple acyclic alkenes containing one double bond have the general formula CₙH₂ₙ

For example:

Ethane:

C₂H₆

Ethene:

C₂H₄

The alkene contains fewer hydrogen atoms because of the carbon-carbon double bond.

Why Alkanes Are Molecular Substances

Alkanes exist as individual molecules.

Strong covalent bonds hold the atoms within each molecule together.

However, the forces between separate alkane molecules are much weaker than the covalent bonds within the molecules.

This distinction helps explain many physical properties of hydrocarbons.

Structure of Larger Alkanes

As more carbon atoms are added, the carbon chain becomes longer.

For example:

Pentane:

CH₃–CH₂–CH₂–CH₂–CH₃

Hexane:

CH₃–CH₂–CH₂–CH₂–CH₂–CH₃

 

 

 

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Each additional carbon in the homologous series effectively adds:

CH₂

to the molecular formula.

Straight-Chain and Branched Alkanes

Alkanes do not always form straight carbon chains.

For example, the molecular formula:

C₄H₁₀

can represent different arrangements of atoms.

One arrangement is straight-chain butane.

Another is a branched structure called 2-methylpropane.

 

 

 

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These compounds have the same molecular formula but different structural arrangements.

They are examples of structural isomers.

Both are still alkanes because they contain only single carbon-carbon bonds.

Physical Trends in the Alkane Family

As the carbon chain becomes longer, several physical properties change gradually.

Generally, larger alkanes have:

• higher boiling points
• higher melting points overall, although the pattern is not perfectly smooth
• greater viscosity
• lower volatility

Small alkanes are often gases at room conditions.

Medium-sized alkanes are commonly liquids.

Very large hydrocarbons may be thick liquids or solids.

 

 

 

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This gradual change in physical properties is characteristic of a homologous series.

Alkanes as Fuels

Many alkanes are useful fuels because they release energy when they undergo combustion.

Examples include:

• methane in natural gas
• propane in fuel cylinders
• butane in portable fuel canisters
• hydrocarbons in gasoline
• hydrocarbons in kerosene and diesel

 

 

 

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The properties of different hydrocarbons make them suitable for different applications.

Complete Combustion

When an alkane burns in sufficient oxygen, complete combustion produces:

carbon dioxide + water

For example, methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

Energy is released during the reaction.

This is why methane can be used as a fuel.

Carbon Chains in Everyday Fuels

Different fuels contain hydrocarbons with different ranges of carbon-chain lengths.

Shorter hydrocarbons tend to be more volatile.

Longer hydrocarbons tend to have higher boiling points and greater viscosity.

This helps explain why hydrocarbon mixtures can be separated and used for different purposes.

Worked Example 1: Identify the Alkane

Is:

C₄H₁₀

an alkane?

Use:

CₙH₂ₙ₊₂

For:

n = 4

Hydrogen:

2(4) + 2 = 10

Therefore:

C₄H₁₀ fits the general formula.

It is an alkane.

Worked Example 2: Predict a Formula

Find the molecular formula of an alkane containing:

7 carbon atoms

Use:

CₙH₂ₙ₊₂

n = 7

Hydrogen:

2(7) + 2 = 16

Formula:

C₇H₁₆

This is heptane.

Worked Example 3: Identify a Non-Alkane Formula

Does:

C₃H₆

fit the acyclic alkane general formula?

For:

n = 3

Expected hydrogen:

2(3) + 2 = 8

Expected alkane:

C₃H₈

Therefore:

C₃H₆ does not fit the acyclic alkane general formula.

Worked Example 4: Find the Number of Carbons

An alkane has the formula:

C₉H₂₀

Number of carbon atoms:

9

Check:

2(9) + 2 = 20

Therefore, the formula follows:

CₙH₂ₙ₊₂

The alkane is:

nonane

Worked Example 5: Find the Number of Hydrogens

An alkane contains:

12 carbon atoms

Use:

H = 2n + 2

H = 2(12) + 2

H = 26

Formula:

C₁₂H₂₆

Worked Example 6: Comparing Two Hydrocarbons

Consider:

C₅H₁₂

and:

C₅H₁₀

For five carbon atoms, the acyclic alkane formula predicts:

H = 2(5) + 2 = 12

Therefore:

C₅H₁₂

fits the alkane formula.

C₅H₁₀

does not fit the acyclic alkane formula.

Worked Example 7: Bonding

Consider:

CH₃–CH₂–CH₃

This molecule contains:

• 3 carbon atoms
• 8 hydrogen atoms
• only single covalent bonds

Its molecular formula is:

C₃H₈

Therefore, it is:

propane

and belongs to the alkane family.

Worked Example 8: Following the Pattern

Complete the sequence:

CH₄

C₂H₆

C₃H₈

C₄H₁₀

Next member:

Add:

CH₂

Therefore:

C₅H₁₂

The next alkane is:

pentane

Recognizing Alkane Formulas Quickly

A useful pattern is:

Carbon atoms:

1, 2, 3, 4, 5, 6...

Hydrogen atoms:

4, 6, 8, 10, 12, 14...

Each time carbon increases by 1:

hydrogen increases by 2

This follows directly from:

CₙH₂ₙ₊₂

 

 

 

A Reliable Alkane Identification Strategy

Step 1: Check the elements.

Does the formula contain only carbon and hydrogen?

If not, it is not a hydrocarbon and therefore cannot be an alkane.

Step 2: Count the carbon atoms.

This gives:

n

Step 3: Calculate:

2n + 2

Step 4: Compare with the number of hydrogen atoms.

Step 5: If the formula matches CₙH₂ₙ₊₂, it can represent an acyclic alkane.

Step 6: If a structure is provided, check that all carbon-carbon bonds are single bonds.

Common Mistakes

Mistake 1: Saying every hydrocarbon is an alkane

Not all hydrocarbons are alkanes.

Some hydrocarbons contain double or triple bonds.

Mistake 2: Saying saturated means "full of carbon"

Saturated means the carbon framework contains only single carbon-carbon bonds, allowing the maximum number of hydrogen atoms for that acyclic carbon skeleton.

Mistake 3: Forgetting the +2

The alkane formula is:

CₙH₂ₙ₊₂

not:

CₙH₂ₙ

Mistake 4: Thinking alkanes contain ionic bonds

The atoms within alkane molecules are held together by:

covalent bonds

Mistake 5: Thinking carbon forms only one or two bonds

Carbon normally forms:

4 covalent bonds

in alkanes.

Mistake 6: Thinking hydrogen forms four bonds

Hydrogen normally forms:

1 covalent bond

Mistake 7: Identifying a compound from the number of carbons alone

For example:

C₄H₈

contains four carbon atoms, but it does not match the acyclic alkane formula:

C₄H₁₀

Real-World Connections

Alkanes are some of the most important compounds used as fuels and chemical raw materials.

 

 

 

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Examples include:

Methane

Major component of natural gas.

Propane

Used in heating, cooking, and portable fuel systems.

Butane

Used in some portable fuel canisters.

Larger alkanes

Found in petroleum-derived fuels and other hydrocarbon products.

Understanding the structure of alkanes provides a foundation for studying fuels, petroleum chemistry, combustion, and other families of organic compounds.

Did You Know?

Methane contains only five atoms:

1 carbon + 4 hydrogen

but some hydrocarbons contain hundreds or even thousands of carbon atoms.

Organic chemistry contains an enormous variety of compounds partly because carbon atoms can form strong covalent bonds with one another, creating:

• chains
• branches
• rings

This ability of carbon to bond to itself is one reason carbon chemistry is so diverse.

Key Terms

• Alkane: Saturated hydrocarbon containing only single carbon-carbon bonds.
• Hydrocarbon: Compound containing only carbon and hydrogen.
• Saturated: Containing only single carbon-carbon bonds.
• Covalent bond: Bond formed through the sharing of electron pairs.
• Single bond: Covalent bond involving one shared pair of electrons.
• Molecular formula: Shows the number of each type of atom in a molecule.
• Structural formula: Shows how atoms are connected.
• Displayed formula: Representation showing individual bonds between atoms.
• General formula: Formula representing an entire family of related compounds.
• Homologous series: Family of organic compounds with the same general formula and similar chemical properties.
• Carbon chain: Sequence of carbon atoms bonded together.
• Structural isomer: Compound with the same molecular formula as another compound but a different arrangement of atoms.
• Combustion: Reaction with oxygen that releases energy.
• Acyclic: Structure that does not contain a ring.

Key Formula

For the ordinary acyclic alkane homologous series:

CₙH₂ₙ₊₂

where:

n = number of carbon atoms

Examples:

n = 1 → CH₄

n = 2 → C₂H₆

n = 3 → C₃H₈

n = 4 → C₄H₁₀

n = 5 → C₅H₁₂

n = 6 → C₆H₁₄

Key Takeaways

• Alkanes are hydrocarbons, meaning they contain only carbon and hydrogen.
• Alkanes are saturated hydrocarbons.
• Saturated hydrocarbons contain only single carbon-carbon bonds.
• The atoms within alkane molecules are joined by covalent bonds.
• Carbon normally forms four covalent bonds.
• Hydrogen normally forms one covalent bond.
• Alkanes contain C–C and C–H single covalent bonds.
• The general formula for acyclic alkanes is CₙH₂ₙ₊₂.
• The first four alkanes are methane, ethane, propane, and butane.
• Successive members of the alkane homologous series differ by CH₂.
• Molecular formulas can be checked against CₙH₂ₙ₊₂ to determine whether they fit the acyclic alkane series.
• Molecular formulas show which atoms are present, while structural and displayed formulas provide information about how the atoms are connected.
• Alkanes can have straight or branched carbon chains.
• Different structural arrangements can produce structural isomers.
• Alkanes are an important foundation for understanding organic chemistry, hydrocarbons, petroleum, and fuels.

 

 

 

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Why Do Chemical Compounds Need Names?

Millions of organic compounds are known. Chemists therefore need a systematic way of naming them.

The International Union of Pure and Applied Chemistry (IUPAC) provides internationally accepted rules for naming chemical compounds.

A systematic name gives information about the compound's structure.

For simple alkanes, the name tells us:

• how many carbon atoms are present
• which carbon chain is the longest
• whether branches are attached
• where those branches are located

Review: What Is an Alkane?

An alkane is a saturated hydrocarbon.

This means it contains:

• only carbon and hydrogen
• only single carbon-carbon bonds

For acyclic alkanes, the general formula is:

CₙH₂ₙ₊₂

where n is the number of carbon atoms.

Examples:

CH₄

C₂H₆

C₃H₈

C₄H₁₀

 

 

 

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The Alkane Naming Pattern

The names of alkanes have two important parts:

prefix + ane

The prefix tells us the number of carbon atoms.

The ending:

-ane

tells us that the compound is an alkane.

For example:

prop + ane = propane

The prefix prop- indicates three carbon atoms.

The First Ten Alkanes

These names are important to learn.

 

 

 

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Learning the Prefixes

The first four prefixes are especially important because they are not as obvious:

meth- = 1

eth- = 2

prop- = 3

but- = 4

From five onward, the prefixes are more closely related to familiar numerical prefixes:

pent- = 5

hex- = 6

hept- = 7

oct- = 8

non- = 9

dec- = 10

Determining Carbon Atoms from the Name

Once the prefixes are known, the number of carbon atoms can be identified immediately.

For example:

hexane

The prefix is:

hex-

Hex means:

6

Therefore, hexane contains:

6 carbon atoms

Its molecular formula is:

C₆H₁₄

Another Example

Consider:

octane

The prefix:

oct-

means:

8 carbon atoms

Use the alkane general formula:

CₙH₂ₙ₊₂

For:

n = 8

Hydrogen:

2(8) + 2 = 18

Therefore:

octane = C₈H₁₈

 

 

 

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Writing a Molecular Formula from a Name

Suppose you are asked:

What is the molecular formula of heptane?

Step 1: Identify the prefix.

hept- = 7 carbons

Step 2: Use:

CₙH₂ₙ₊₂

Step 3: Substitute:

n = 7

Step 4: Calculate hydrogen:

2(7) + 2 = 16

Therefore:

heptane = C₇H₁₆

Writing a Name from a Molecular Formula

Suppose you are given:

C₅H₁₂

First check whether it follows the alkane formula.

For:

n = 5

2(5) + 2 = 12

Yes.

Five carbon atoms use the prefix:

pent-

Add:

-ane

Therefore:

C₅H₁₂ = pentane

Straight-Chain Alkanes

The simplest alkanes have carbon atoms arranged in a continuous chain.

For example:

propane

CH₃–CH₂–CH₃

contains three carbon atoms.

butane

CH₃–CH₂–CH₂–CH₃

contains four carbon atoms.

pentane

CH₃–CH₂–CH₂–CH₂–CH₃

contains five carbon atoms.

 

 

 

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Count the carbon atoms in the continuous chain to determine the basic name.

Condensed Structural Formulas

Organic structures can be written in a compact form called a condensed structural formula.

Examples:

Methane:

CH₄

Ethane:

CH₃CH₃

Propane:

CH₃CH₂CH₃

Butane:

CH₃CH₂CH₂CH₃

Pentane:

CH₃CH₂CH₂CH₂CH₃

Each C represents one carbon atom.

Counting the carbon atoms allows us to determine the alkane name.

Naming a Straight-Chain Structure

Consider:

CH₃–CH₂–CH₂–CH₂–CH₂–CH₃

Count the carbon atoms:

6

Six carbons correspond to:

hex-

Because the molecule is an alkane:

hex + ane = hexane

Therefore, the name is:

hexane

Why Naming Becomes More Complicated

Not every alkane forms one straight chain.

Carbon atoms can also form branches.

For example, the molecular formula:

C₄H₁₀

can represent more than one structure.

 

 

 

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One structure is:

butane

Another is:

2-methylpropane

These compounds have the same molecular formula but different arrangements of atoms.

They are structural isomers.

Naming Branched Alkanes

For a simple branched alkane, use this basic procedure:

1. Find the longest continuous carbon chain.

2. Name that chain.

3. Identify any branches.

4. Number the main chain.

5. Give each branch the lowest possible position number.

6. Combine the numbers, branch names, and parent-chain name.

Step 1: Find the Longest Carbon Chain

The longest continuous chain determines the parent name.

 

 

 

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For example, if the longest continuous chain contains:

5 carbon atoms

the parent alkane is:

pentane

Even if the entire molecule contains more than five carbon atoms, the longest continuous chain determines the parent name.

The Longest Chain Does Not Have to Look Straight

This is an important rule.

When a structure is drawn on paper, the longest carbon chain may bend or change direction.

You are looking for the longest continuous sequence of connected carbon atoms, not necessarily the longest horizontal line on the page.

Step 2: Identify the Branches

A branch attached to the main chain is called a substituent.

Simple alkane branches are called alkyl groups.

They are named by changing:

-ane

to:

-yl

For example:

Methane → methyl

Ethane → ethyl

Propane → propyl

 

 

 

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For introductory naming, methyl and ethyl branches are especially common.

The Methyl Group

A methyl group is:

–CH₃

It contains one carbon atom.

If a methyl group is attached to a propane chain, the molecule might be named:

2-methylpropane

The name tells us:

2- → branch is attached to carbon 2

methyl → branch contains one carbon

propane → longest chain contains three carbon atoms

Numbering the Main Chain

The main carbon chain must be numbered.

Number from the end that gives the first branch the lowest possible number.

Consider a five-carbon chain with a methyl branch.

If numbering from the left places the branch on carbon:

2

but numbering from the right places it on carbon:

4

choose:

2

 

 

 

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The correct name would be:

2-methylpentane

not:

4-methylpentane

Example: 2-Methylpropane

Consider:

    CH3
     |
CH3-CH-CH3

The longest continuous chain contains:

3 carbons

Parent:

propane

There is one:

methyl group

attached to carbon:

2

Therefore:

2-methylpropane

Example: 2-Methylbutane

Consider:

    CH3
     |
CH3-CH-CH2-CH3

Longest chain:

4 carbon atoms

Parent:

butane

Branch:

methyl

Position:

2

Name:

2-methylbutane

 

 

 

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Why It Is Not 3-Methylbutane

The same structure could be numbered from the opposite end.

That would place the methyl group at:

carbon 3

But IUPAC naming requires the lower number.

Compare:

2-methylbutane

and:

3-methylbutane

Choose:

2-methylbutane

More Than One Branch

A molecule can contain more than one identical branch.

Prefixes are used to indicate how many identical substituents are present.

di- = 2

tri- = 3

tetra- = 4

For example:

2,3-dimethylbutane

contains:

• a four-carbon parent chain
• one methyl group on carbon 2
• one methyl group on carbon 3

 

 

 

Example: 2,2-Dimethylpropane

Consider:

      CH3
       |
CH3 - C - CH3
       |
      CH3

The longest continuous chain contains:

3 carbons

Parent:

propane

Two methyl groups are attached to:

carbon 2

Therefore:

2,2-dimethylpropane

Writing Position Numbers

Position numbers are called locants.

When more than one number is used, separate the numbers with commas.

Correct:

2,3-dimethylbutane

Not:

2-3-dimethylbutane

Use a hyphen between numbers and words.

Correct:

2,3-dimethylbutane

This punctuation is part of the systematic name.

Different Types of Branches

Some alkanes contain different substituents.

For example, a molecule might contain:

• an ethyl group
• a methyl group

When different substituent names appear together, they are normally listed in alphabetical order.

For example:

ethyl

comes before:

methyl

because E comes before M.

 

 

 

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Prefixes such as di-, tri-, and tetra- are not normally considered when alphabetizing substituent names.

The Lowest Set of Locants

With more complicated branched structures, choose the numbering direction that gives the substituents the lowest set of locants according to IUPAC rules.

For introductory examples, this often means:

start numbering from the end nearest the first branch.

For example:

2,3-dimethylpentane

is preferred over:

3,4-dimethylpentane

because:

2,3

is the lower set of position numbers.

Naming from a Displayed Structure

When given a structure:

Step 1: Find the longest continuous carbon chain.

Step 2: Count its carbon atoms.

Step 3: Determine the parent alkane.

Step 4: Number the chain.

Step 5: Identify each branch.

Step 6: Record each branch position.

Step 7: Use di-, tri-, or tetra- if identical branches repeat.

Step 8: Arrange different substituents alphabetically.

Step 9: Assemble the complete name.

 

 

 

5

Example: Name This Structure

Consider:

      CH3
       |
CH3-CH-CH2-CH2-CH3

Step 1: Longest chain:

5 carbons

Parent:

pentane

Step 2: Branch:

methyl

Step 3: Number from the nearest end.

Branch position:

2

Name:

2-methylpentane

Example: Two Methyl Branches

Consider:

     CH3   CH3
      |     |
CH3 - CH - CH - CH3

Longest chain:

4 carbons

Parent:

butane

Branches:

two methyl groups

Positions:

2 and 3

Use:

di-

Name:

2,3-dimethylbutane

Molecular Formula from a Branched Alkane Name

Branching changes the arrangement of atoms but does not change the general formula for an acyclic saturated alkane.

Consider:

2-methylbutane

Count all carbon atoms.

Parent chain:

4 carbons

Methyl branch:

1 carbon

Total:

5 carbon atoms

Therefore:

n = 5

Use:

CₙH₂ₙ₊₂

C₅H₁₂

So:

2-methylbutane = C₅H₁₂

Another Formula Example

Find the molecular formula of:

2,3-dimethylbutane

Parent chain:

4 carbon atoms

Two methyl groups:

2 additional carbon atoms

Total:

6 carbon atoms

Use:

CₙH₂ₙ₊₂

H = 2(6) + 2

= 14

Therefore:

C₆H₁₄

 

 

 

Structural Isomers

Compounds with the same molecular formula but different structural formulas are called structural isomers.

For example:

C₅H₁₂

has several possible structures.

These include:

pentane

2-methylbutane

2,2-dimethylpropane

All three contain:

5 carbon atoms

and:

12 hydrogen atoms

but their carbon atoms are arranged differently.

Why Names Matter

If we simply said:

C₅H₁₂

we would know the molecular formula but not necessarily the exact structure.

The systematic name:

2-methylbutane

provides structural information.

It tells us:

• the parent chain has four carbons
• there is a one-carbon methyl branch
• the branch is attached to carbon 2

This is why systematic naming becomes increasingly useful as molecules become more complicated.

From Name to Structure

Suppose you are given:

3-methylhexane

Break the name apart.

hexane

means the main chain contains:

6 carbons

Draw:

C–C–C–C–C–C

Then:

3-methyl

means place a:

CH₃

group on carbon 3.

 

 

 

4

A condensed formula can be written as:

CH₃CH₂CH(CH₃)CH₂CH₂CH₃

From Name to Formula

Find the formula of:

3-methylhexane

Parent chain:

6 carbons

Methyl branch:

1 carbon

Total:

7 carbons

For an acyclic alkane:

CₙH₂ₙ₊₂

Therefore:

C₇H₁₆

Worked Example 1: Methane

Name:

methane

Prefix:

meth-

Carbon atoms:

1

Formula:

CH₄

Worked Example 2: Butane

Name:

butane

Prefix:

but-

Carbon atoms:

4

Formula:

C₄H₁₀

Condensed structure:

CH₃CH₂CH₂CH₃

Worked Example 3: Nonane

Name:

nonane

Carbon atoms:

9

Calculate hydrogen:

2(9) + 2 = 20

Formula:

C₉H₂₀

Worked Example 4: Name C₈H₁₈

Carbon atoms:

8

Prefix:

oct-

Add:

-ane

Name:

octane

Worked Example 5: Name a Straight Chain

Structure:

CH₃–CH₂–CH₂–CH₂–CH₃

Count carbons:

5

Parent:

pentane

Answer:

pentane

Worked Example 6: Name a Branched Structure

Structure:

    CH3
     |
CH3-CH-CH2-CH3

Longest chain:

4 carbons

Parent:

butane

Branch:

methyl

Position:

2

Answer:

2-methylbutane

Worked Example 7: Formula from a Branched Name

Name:

2-methylpentane

Parent chain:

5 C

Methyl branch:

1 C

Total:

6 C

Use:

CₙH₂ₙ₊₂

Formula:

C₆H₁₄

Worked Example 8: Two Branches

Name:

2,3-dimethylpentane

Parent chain:

5 C

Two methyl branches:

2 C

Total:

7 C

Formula:

C₇H₁₆

Worked Example 9: Correct the Name

A student names a structure:

4-methylpentane

But numbering from the other end places the branch at:

carbon 2

Therefore, the correct name is:

2-methylpentane

Always number from the end that gives the substituent the appropriate lowest locant.

Worked Example 10: Find the Structure from the Name

Name:

2,2-dimethylbutane

Parent:

butane = 4-carbon chain

Branches:

two methyl groups

Both are attached to:

carbon 2

A condensed structural formula is:

CH₃C(CH₃)₂CH₂CH₃

Total carbon atoms:

6

Molecular formula:

C₆H₁₄

A Useful Naming Checklist

When naming a straight-chain alkane:

Count the carbons → choose the prefix → add -ane

When naming a branched alkane:

Find longest chain → name parent → number chain → identify branches → locate branches → assemble name

When finding a formula:

Count all carbon atoms → use CₙH₂ₙ₊₂

 

 

 

5

Common Mistakes

Mistake 1: Counting only the carbon atoms drawn horizontally

The longest chain may bend.

Look for the longest continuous carbon chain.

Mistake 2: Counting the branch as part of the parent automatically

The parent is determined by the longest continuous chain, not by how the structure is positioned on the page.

Mistake 3: Numbering from the wrong end

Choose the direction that gives the branches the appropriate lowest locants.

Mistake 4: Forgetting the branch position

Write:

2-methylbutane

not simply:

methylbutane

when a locant is needed.

Mistake 5: Forgetting di- for two identical branches

Two methyl groups require:

dimethyl

Mistake 6: Using hyphens between numbers

Correct:

2,3-dimethylbutane

Numbers are separated by commas.

Mistake 7: Forgetting hyphens between numbers and words

Correct:

2-methylpentane

Mistake 8: Counting only the parent chain when finding the molecular formula

For:

2-methylpentane

there are:

5 + 1 = 6 carbons

not five.

Mistake 9: Changing the molecular formula because the chain is branched

Straight-chain and branched acyclic alkanes still follow:

CₙH₂ₙ₊₂

Real-World Connection: Octane

The name octane tells us that the molecule contains:

8 carbon atoms

Its molecular formula is:

C₈H₁₈

 

 

 

4

The word "octane" is also familiar from gasoline octane ratings.

However, an octane rating does not simply tell us how much octane is present in gasoline. It describes the fuel's resistance to engine knocking using standardized reference fuels.

One important reference compound is:

2,2,4-trimethylpentane

commonly called isooctane.

Its molecular formula is also:

C₈H₁₈

This provides a real-world example of why different structural arrangements of the same molecular formula matter.

Real-World Connection: Hydrocarbon Fuels

Alkanes occur in many fuels and petroleum-derived products.

Examples include:

Methane — natural gas

Propane — LPG and heating fuels

Butane — portable fuel canisters

Octane-related hydrocarbons — gasoline

 

 

 

5

Knowing the names and structures of alkanes provides a foundation for understanding petroleum chemistry, combustion, fuels, polymers, and many other areas of organic chemistry.

Did You Know?

Carbon's ability to form long chains and branches means that the number of possible alkane structures increases rapidly as the number of carbon atoms increases.

Methane has only one possible carbon arrangement.

Ethane and propane also have only one.

But larger alkanes can have several different structural isomers.

As molecules become larger, systematic IUPAC naming becomes essential because a molecular formula alone may not identify a unique structure.

Key Terms

• IUPAC: International Union of Pure and Applied Chemistry; organization responsible for internationally standardized chemical naming systems.
• Alkane: Saturated hydrocarbon containing only single carbon-carbon bonds.
• Homologous series: Family of compounds with the same general formula and similar chemical properties.
• Parent chain: Longest continuous carbon chain used as the basis of an alkane name.
• Prefix: Part of the name indicating the number of carbon atoms.
• Substituent: Atom or group attached to the parent chain.
• Alkyl group: Hydrocarbon substituent formed conceptually by removing one hydrogen from an alkane.
• Methyl: One-carbon alkyl group, –CH₃.
• Ethyl: Two-carbon alkyl group, –CH₂CH₃.
• Locant: Number indicating the position of a substituent.
• Molecular formula: Formula showing the number of each type of atom.
• Structural formula: Formula showing how atoms are connected.
• Structural isomer: Compound with the same molecular formula as another compound but a different arrangement of atoms.
• Straight-chain alkane: Alkane with an unbranched carbon chain.
• Branched alkane: Alkane containing one or more carbon branches.

Key Naming Rules

For a simple straight-chain alkane:

1. Count the carbon atoms.

2. Choose the correct prefix.

3. Add -ane.

For a branched alkane:

1. Find the longest continuous carbon chain.

2. Name the parent chain.

3. Number the parent chain appropriately.

4. Identify the substituents.

5. Determine their positions.

6. Use di-, tri-, tetra-, etc. for repeated identical substituents.

7. List different substituent types alphabetically.

8. Use commas between numbers and hyphens between numbers and words.

9. Combine everything into one systematic name.

Key Takeaways

• Alkane names provide information about molecular structure.
• The ending -ane identifies an alkane.
• meth-, eth-, prop-, and but- represent one to four carbon atoms.
• pent-, hex-, hept-, oct-, non-, and dec- represent five to ten carbon atoms.
• Methane through decane are the first ten members of the alkane homologous series.
• Straight-chain alkanes can be named by counting the carbon atoms and selecting the correct prefix.
• Acyclic alkanes follow the general formula CₙH₂ₙ₊₂.
• A molecular formula can be predicted from the number of carbon atoms.
• Branched alkanes are named using a parent chain and substituents.
• The longest continuous carbon chain determines the parent name.
• The longest chain does not necessarily appear as a straight horizontal line in a drawing.
• The parent chain is numbered to give substituents the appropriate lowest locants.
• Methyl and ethyl are common alkyl substituents.
• di-, tri-, and tetra- indicate repeated identical substituents.
• Different substituent types are listed alphabetically.
• Commas separate numbers, while hyphens separate numbers from words.
• Branched and straight-chain alkanes can have the same molecular formula but different structures.
• Systematic IUPAC naming allows chemists to communicate molecular structures clearly and consistently.

 

 

 

6

What Is Combustion?

Combustion is a chemical reaction in which a substance reacts with oxygen and releases energy.

The substance being burned is called the fuel.

In a combustion reaction:

fuel + oxygen → products + energy

Energy is usually transferred to the surroundings as:

• heat
• light

Because combustion releases energy, it is an exothermic reaction.

Combustion reactions are important in transportation, electricity generation, heating, cooking, industry, and many other areas of everyday life.

Fuels

A fuel is a substance that can release useful energy when it undergoes a chemical reaction.

Common fuels include:

• methane
• propane
• butane
• gasoline
• kerosene
• diesel
• coal
• biomass

 

 

 

6

Many common fuels contain hydrocarbons.

A hydrocarbon contains only:

carbon and hydrogen

Alkanes are an important family of hydrocarbon fuels.

Why Alkanes Burn

Alkanes react with oxygen when sufficient activation energy is supplied.

A flame or spark can provide the initial energy needed to start the reaction.

During combustion:

• bonds in the fuel are broken
• bonds in oxygen molecules are broken
• new bonds form in the products

Overall, forming the new bonds releases more energy than is required to break the original bonds.

Therefore:

energy is released overall

and the reaction is:

exothermic

Complete Combustion

Complete combustion occurs when a fuel burns with a sufficient supply of oxygen.

When a hydrocarbon undergoes complete combustion, the products are:

carbon dioxide + water

Therefore:

hydrocarbon + oxygen → carbon dioxide + water

 

 

 

5

For an alkane:

alkane + O₂ → CO₂ + H₂O

Complete Combustion of Methane

Methane has the formula:

CH₄

The balanced equation for complete combustion is:

CH₄ + 2O₂ → CO₂ + 2H₂O

Check the atoms.

Left:

C = 1

H = 4

O = 4

Right:

C = 1

H = 4

O = 4

The equation is balanced.

What Happens to the Atoms?

During combustion, atoms are rearranged.

They are not created or destroyed.

In methane combustion:

Carbon from methane becomes part of:

CO₂

Hydrogen from methane becomes part of:

H₂O

Oxygen from the air becomes part of both products.

 

 

 

This demonstrates the law of conservation of mass.

Balancing Combustion Equations

A chemical equation must contain the same number of each type of atom on both sides.

A useful order for balancing alkane combustion equations is:

1. Balance carbon.

2. Balance hydrogen.

3. Balance oxygen last.

This method works well because oxygen appears in both products.

Example: Combustion of Ethane

Start with:

C₂H₆ + O₂ → CO₂ + H₂O

Step 1: Balance carbon

There are 2 carbon atoms.

C₂H₆ + O₂ → 2CO₂ + H₂O

Step 2: Balance hydrogen

There are 6 hydrogen atoms.

We need:

3H₂O

So:

C₂H₆ + O₂ → 2CO₂ + 3H₂O

Step 3: Balance oxygen

Right side:

2CO₂ = 4 O atoms

3H₂O = 3 O atoms

Total:

7 O atoms

This requires:

7/2 O₂

So:

C₂H₆ + 7/2O₂ → 2CO₂ + 3H₂O

Multiply the entire equation by 2:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

 

 

 

Example: Combustion of Propane

Start:

C₃H₈ + O₂ → CO₂ + H₂O

Balance carbon:

C₃H₈ + O₂ → 3CO₂ + H₂O

Balance hydrogen:

C₃H₈ + O₂ → 3CO₂ + 4H₂O

Count oxygen on the right:

3CO₂ gives:

6 O atoms

4H₂O gives:

4 O atoms

Total:

10 O atoms

Therefore we need:

5O₂

Balanced equation:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Example: Combustion of Butane

Start:

C₄H₁₀ + O₂ → CO₂ + H₂O

Balance carbon:

C₄H₁₀ + O₂ → 4CO₂ + H₂O

Balance hydrogen:

C₄H₁₀ + O₂ → 4CO₂ + 5H₂O

Oxygen atoms required:

From CO₂:

4 × 2 = 8

From H₂O:

5 × 1 = 5

Total:

13 oxygen atoms

So:

C₄H₁₀ + 13/2O₂ → 4CO₂ + 5H₂O

Multiply everything by 2:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

 

 

 

5

General Complete Combustion Pattern

For an alkane:

CₙH₂ₙ₊₂

complete combustion produces:

CO₂

and:

H₂O

The general pattern can be written as:

CₙH₂ₙ₊₂ + (3n + 1)/2 O₂ → nCO₂ + (n + 1)H₂O

For many introductory problems, however, it is usually easier and safer to balance each equation using:

carbon → hydrogen → oxygen

Complete Combustion and Flames

Complete combustion is associated with efficient burning when sufficient oxygen is available.

For many gaseous hydrocarbon burners, a well-adjusted flame with good oxygen mixing is often:

blue

 

 

 

5

A blue Bunsen burner flame, for example, usually indicates relatively complete combustion.

However, flame appearance depends on the fuel and burner conditions, so colour alone should not be treated as a universal test for every combustion system.

Incomplete Combustion

Incomplete combustion occurs when there is not enough oxygen for the fuel to burn completely.

Instead of all carbon atoms becoming carbon dioxide, some may form:

carbon monoxide (CO)

or:

carbon (C)

Water is still produced from the hydrogen in the hydrocarbon.

Therefore, possible products include:

carbon monoxide + water

and/or:

carbon + water

Some carbon dioxide may also form.

 

 

 

5

Real incomplete combustion can produce a mixture of products rather than only one carbon-containing product.

Why Incomplete Combustion Happens

Incomplete combustion can occur when:

• oxygen supply is limited
• air and fuel do not mix properly
• a burner is poorly adjusted
• combustion occurs in a poorly ventilated space
• combustion conditions do not allow the fuel to burn completely

Less complete oxidation of the carbon occurs.

Incomplete Combustion Producing Carbon Monoxide

Methane can undergo incomplete combustion to produce carbon monoxide.

One balanced equation is:

2CH₄ + 3O₂ → 2CO + 4H₂O

Check:

Left:

C = 2

H = 8

O = 6

Right:

C = 2

H = 8

O = 6

The equation is balanced.

Incomplete Combustion Producing Carbon

With even more limited oxygen, solid carbon may form.

For methane:

CH₄ + O₂ → C + 2H₂O

This carbon can appear as:

soot

 

 

 

5

Soot is evidence that carbon-containing fuel has not been completely oxidized.

Complete vs Incomplete Combustion

Carbon Monoxide

Carbon monoxide, CO, is a colourless and odourless gas.

It is dangerous because it interferes with the blood's ability to transport oxygen.

 

 

 

6

Carbon monoxide can be produced by incomplete combustion in equipment such as:

• faulty heaters
• poorly maintained furnaces
• engines
• generators
• poorly ventilated combustion appliances

This is why proper ventilation and correctly maintained combustion equipment are important.

Why Carbon Monoxide Is Dangerous

Red blood cells contain hemoglobin, which normally transports oxygen.

Carbon monoxide binds strongly to hemoglobin.

This reduces the amount of oxygen that the blood can transport effectively.

Because carbon monoxide has no colour or smell, people may not notice its presence without appropriate detection equipment.

Soot

Soot consists largely of tiny carbon-rich particles formed during incomplete combustion.

It can:

• blacken surfaces
• reduce air quality
• enter the respiratory system
• contribute to atmospheric particulate pollution

 

 

 

5

Black carbon particles can also absorb sunlight and contribute to atmospheric warming.

Energy from Combustion

Combustion reactions are exothermic.

Energy is released because the products are at a lower chemical energy than the reactants.

This released energy can be used for:

• heating
• cooking
• transportation
• generating electricity
• industrial processes

Complete Combustion Releases More Useful Energy

Incomplete combustion does not fully oxidize the carbon in the fuel.

Products such as:

CO

and:

C

can still undergo further oxidation.

Therefore, incomplete combustion generally releases less energy from a given amount of fuel than complete combustion.

This makes incomplete combustion:

less efficient

as a way of obtaining energy from the fuel.

Carbon Dioxide and the Environment

Complete combustion of hydrocarbon fuels produces:

carbon dioxide, CO₂

Carbon dioxide is a greenhouse gas.

 

 

 

5

Greenhouse gases absorb and re-emit infrared radiation, helping keep Earth's surface and lower atmosphere warmer than they would otherwise be.

Increasing atmospheric carbon dioxide concentrations strengthen this greenhouse effect and contribute to climate change.

The Carbon Cycle and Fossil Fuels

Carbon naturally moves between:

• atmosphere
• oceans
• living organisms
• soils
• rocks

This movement is part of the carbon cycle.

Fossil fuels contain carbon that has been stored underground for very long periods.

When fossil fuels are burned, carbon is transferred into the atmosphere mainly as:

CO₂

 

 

 

4

Large-scale fossil-fuel combustion therefore changes the amount of carbon dioxide in the atmosphere.

Air Pollution from Combustion

Combustion can produce several pollutants depending on the fuel and conditions.

These can include:

• carbon monoxide
• particulate matter
• nitrogen oxides
• sulfur dioxide if sulfur-containing fuels are burned
• unburned hydrocarbons

Not all of these pollutants come directly from the alkane itself.

For example, nitrogen oxides can form when nitrogen and oxygen from the air react at the high temperatures found in engines and other combustion systems.

Nitrogen Oxides

Air contains large amounts of:

nitrogen, N₂

and:

oxygen, O₂

At very high combustion temperatures, these gases can react and form nitrogen oxides, often written collectively as:

NOₓ

 

 

 

5

Nitrogen oxides contribute to problems including:

• poor air quality
• photochemical smog
• acid deposition

Sulfur Dioxide

Some fossil fuels contain sulfur-containing impurities or compounds.

When sulfur is burned, sulfur dioxide can form:

S + O₂ → SO₂

Sulfur dioxide can contribute to:

• respiratory problems
• acid deposition
• environmental damage

Pure alkanes themselves contain only carbon and hydrogen, so sulfur dioxide is not a product of burning a pure alkane.

It results from sulfur-containing material in the fuel.

Particulate Matter

Incomplete combustion can release very small solid or liquid particles into the air.

These are called:

particulate matter

Fine particles can penetrate deeply into the respiratory system.

 

 

 

5

Reducing incomplete combustion can therefore improve both fuel efficiency and air quality.

Complete Combustion Is Not Pollution-Free

A common misconception is:

"Complete combustion is clean, so it has no environmental impact."

Complete combustion reduces products such as carbon monoxide and soot.

However, hydrocarbon complete combustion still produces:

carbon dioxide

Therefore, complete combustion can still contribute to greenhouse-gas emissions.

Combustion in Vehicle Engines

Vehicle engines burn hydrocarbon-based fuels to release energy.

Ideally:

hydrocarbon + oxygen → carbon dioxide + water

Real engines may also produce:

• CO
• NOₓ
• unburned hydrocarbons
• particulate matter

 

 

 

4

Modern engine and exhaust-control technologies attempt to reduce some of these pollutants.

Catalytic Converters

Many vehicles use a catalytic converter to reduce harmful exhaust emissions.

Catalytic converters can help convert:

carbon monoxide → carbon dioxide

nitrogen oxides → nitrogen

unburned hydrocarbons → carbon dioxide + water

They do not eliminate all environmental effects of fuel combustion, particularly the carbon dioxide produced from fossil carbon.

Combustion in Power Generation

Fuels may also be burned to generate electricity.

Chemical energy in the fuel is converted through several stages:

chemical energy → thermal energy → mechanical energy → electrical energy

 

 

 

5

Some energy is transferred to the surroundings at each stage, so the entire process is not 100% efficient.

Testing the Products of Complete Combustion

The products of complete hydrocarbon combustion are:

carbon dioxide

and:

water

Carbon dioxide can be tested using limewater.

Carbon dioxide causes limewater to turn:

milky/cloudy

Water can be detected using suitable chemical tests, such as cobalt chloride paper or anhydrous copper(II) sulfate in laboratory contexts.

 

 

 

5

Balancing Strategy for Complete Combustion

Consider:

C₅H₁₂ + O₂ → CO₂ + H₂O

Step 1: Carbon

There are 5 carbon atoms.

Write:

5CO₂

So:

C₅H₁₂ + O₂ → 5CO₂ + H₂O

Step 2: Hydrogen

There are 12 hydrogen atoms.

Write:

6H₂O

So:

C₅H₁₂ + O₂ → 5CO₂ + 6H₂O

Step 3: Oxygen

Products contain:

5CO₂ → 10 O atoms

6H₂O → 6 O atoms

Total:

16 O atoms

Therefore:

8O₂

Final equation:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Another Example: Hexane

Start:

C₆H₁₄ + O₂ → CO₂ + H₂O

Balance carbon:

C₆H₁₄ + O₂ → 6CO₂ + H₂O

Balance hydrogen:

C₆H₁₄ + O₂ → 6CO₂ + 7H₂O

Oxygen required:

6CO₂ → 12 O

7H₂O → 7 O

Total:

19 O atoms

So:

C₆H₁₄ + 19/2O₂ → 6CO₂ + 7H₂O

Multiply by 2:

2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O

 

 

 

Worked Example 1: Identify the Reaction

Methane reacts with oxygen and releases heat.

What type of reaction is this?

Combustion

If sufficient oxygen is available and the products are CO₂ and H₂O, it is:

complete combustion

Worked Example 2: Identify the Products

What are the products of complete combustion of propane?

Propane is a hydrocarbon.

Complete hydrocarbon combustion produces:

carbon dioxide + water

Therefore:

C₃H₈ + O₂ → CO₂ + H₂O

Balanced:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Worked Example 3: Balance Methane Combustion

Unbalanced:

CH₄ + O₂ → CO₂ + H₂O

Balanced:

CH₄ + 2O₂ → CO₂ + 2H₂O

Worked Example 4: Balance Ethane Combustion

Unbalanced:

C₂H₆ + O₂ → CO₂ + H₂O

Balanced:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Worked Example 5: Complete or Incomplete?

A hydrocarbon flame produces visible soot.

This suggests:

incomplete combustion

because some carbon has not been completely oxidized to carbon dioxide.

Worked Example 6: Carbon Monoxide

Why might a poorly ventilated fuel-burning appliance be dangerous?

Limited oxygen can cause:

incomplete combustion

which can produce:

carbon monoxide

Carbon monoxide interferes with oxygen transport in the blood.

Worked Example 7: Environmental Impact

A natural-gas heater burns methane completely.

Does this mean it has no environmental impact?

No.

Complete combustion produces:

CO₂

Carbon dioxide is a greenhouse gas.

Worked Example 8: Conservation of Mass

Consider:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Count the atoms.

Reactants:

C = 3

H = 8

O = 10

Products:

C = 3

H = 8

O = 10

The equation obeys the:

law of conservation of mass

Worked Example 9: Incomplete Combustion

Balance:

CH₄ + O₂ → CO + H₂O

Start by balancing carbon and hydrogen:

CH₄ + O₂ → CO + 2H₂O

There are 3 oxygen atoms on the right.

Use:

3/2O₂

Then multiply everything by 2:

2CH₄ + 3O₂ → 2CO + 4H₂O

Worked Example 10: Pentane Combustion

Write the balanced complete-combustion equation for pentane.

Pentane:

C₅H₁₂

Start:

C₅H₁₂ + O₂ → CO₂ + H₂O

Balance C:

C₅H₁₂ + O₂ → 5CO₂ + H₂O

Balance H:

C₅H₁₂ + O₂ → 5CO₂ + 6H₂O

Balance O:

Right side contains:

10 + 6 = 16 oxygen atoms

Therefore:

8O₂

Final:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Predicting Complete Combustion Products

For any alkane:

Step 1: Recognize that the fuel contains carbon and hydrogen.

Step 2: Check that sufficient oxygen is available.

Step 3: Carbon becomes:

CO₂

Step 4: Hydrogen becomes:

H₂O

Therefore:

alkane + oxygen → carbon dioxide + water

Then balance the equation.

A Reliable Combustion Equation Strategy

Step 1: Write the correct formula for the fuel.

Step 2: Add O₂ as a reactant.

Step 3: For complete combustion, write CO₂ and H₂O as products.

Step 4: Balance carbon atoms first.

Step 5: Balance hydrogen atoms second.

Step 6: Balance oxygen atoms last.

Step 7: If a fractional O₂ coefficient appears, multiply the entire equation to obtain whole-number coefficients when required.

Step 8: Count every atom on both sides to check the equation.

 

 

 

5

Complete vs Incomplete Combustion: Cause and Effect

Sufficient oxygen

↓

Complete combustion

↓

CO₂ + H₂O

↓

More complete release of chemical energy

Limited oxygen

↓

Incomplete combustion

↓

CO and/or C + H₂O, often with some CO₂

↓

Less complete energy release + additional harmful pollutants

This cause-and-effect relationship is important for understanding both chemistry and environmental impacts.

Environmental Impacts Summary

Combustion can affect the environment in several ways.

Carbon dioxide

• greenhouse gas
• contributes to climate change when atmospheric concentrations increase

Carbon monoxide

• toxic to humans and other animals
• associated with incomplete combustion

Soot and particulate matter

• reduce air quality
• can harm respiratory and cardiovascular health
• black carbon can contribute to warming

Nitrogen oxides

• contribute to smog
• contribute to acid deposition
• affect air quality

Sulfur dioxide

• may form from sulfur-containing fuels
• contributes to acid deposition
• affects air quality

 

 

 

6

Reducing the Environmental Impact of Combustion

Several approaches can reduce combustion-related environmental impacts:

• improving energy efficiency
• reducing unnecessary fuel use
• ensuring sufficient oxygen for efficient combustion
• maintaining engines and combustion equipment
• using catalytic converters and pollution-control systems
• reducing sulfur in fuels
• using lower-carbon energy sources where appropriate
• replacing some combustion-based processes with renewable electricity
• using public transportation or other efficient transport systems
• developing cleaner industrial technologies

Different approaches target different pollutants, so there is no single solution to every environmental impact of combustion.

Common Mistakes

Mistake 1: Saying combustion means burning without mentioning oxygen

Combustion involves reaction with:

oxygen

Mistake 2: Saying complete combustion produces carbon monoxide

Complete hydrocarbon combustion produces:

CO₂ + H₂O

Carbon monoxide is associated with:

incomplete combustion

Mistake 3: Forgetting water

Hydrocarbons contain hydrogen.

During combustion, hydrogen forms:

H₂O

Mistake 4: Changing subscripts when balancing equations

Never change:

CH₄

into:

CH₂

to balance an equation.

Changing a subscript changes the substance.

Change coefficients, not chemical formulas.

Mistake 5: Balancing oxygen first

For hydrocarbon combustion, it is usually easier to balance:

carbon → hydrogen → oxygen

Mistake 6: Thinking a yellow flame always proves exactly which products are present

A luminous or sooty flame can indicate incomplete combustion, but actual combustion products depend on the fuel and conditions.

Mistake 7: Saying complete combustion has no environmental effects

Complete hydrocarbon combustion still produces:

carbon dioxide

Mistake 8: Saying pure alkane combustion directly produces sulfur dioxide

Pure alkanes contain only carbon and hydrogen.

Sulfur dioxide comes from burning sulfur-containing material.

Mistake 9: Thinking carbon monoxide can be detected by smell

Carbon monoxide is:

colourless and odourless

Mistake 10: Forgetting conservation of mass

Every balanced chemical equation must contain the same number of each type of atom on both sides.

Did You Know?

Combustion has powered human technology for thousands of years, from early fires to modern engines and power stations.

 

 

 

However, the large-scale combustion of fossil fuels has also created major environmental challenges.

Understanding combustion chemistry helps scientists and engineers design systems that:

• use fuel more efficiently
• produce fewer harmful pollutants
• reduce greenhouse-gas emissions
• recover more useful energy
• transition toward alternative energy technologies

Combustion chemistry therefore connects molecular reactions with some of the largest energy and environmental issues faced by society.

Key Terms

• Combustion: Reaction of a substance with oxygen that releases energy.
• Fuel: Substance used to release useful energy.
• Hydrocarbon: Compound containing only carbon and hydrogen.
• Complete combustion: Combustion with sufficient oxygen.
• Incomplete combustion: Combustion occurring when oxygen is insufficient for complete oxidation.
• Exothermic: Describes a reaction that transfers energy to the surroundings.
• Carbon dioxide (CO₂): Product of complete hydrocarbon combustion and a greenhouse gas.
• Carbon monoxide (CO): Toxic gas that may form during incomplete combustion.
• Soot: Carbon-rich particulate material that can form during incomplete combustion.
• Particulate matter: Tiny solid or liquid particles suspended in air.
• Greenhouse gas: Atmospheric gas that absorbs and re-emits infrared radiation.
• Nitrogen oxides (NOₓ): Pollutants that can form during high-temperature combustion.
• Sulfur dioxide (SO₂): Pollutant produced when sulfur-containing material burns.
• Catalytic converter: Device that reduces several harmful vehicle exhaust pollutants.
• Fossil fuel: Carbon-rich fuel formed from ancient biological material over geological time.
• Conservation of mass: Principle that atoms are rearranged but not created or destroyed in chemical reactions.

Key Equations

General complete hydrocarbon combustion:

hydrocarbon + oxygen → carbon dioxide + water

Methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

Ethane:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Butane:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

Pentane:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

Example incomplete methane combustion producing CO:

2CH₄ + 3O₂ → 2CO + 4H₂O

Key Takeaways

• Combustion is a reaction between a fuel and oxygen.
• Combustion reactions are generally exothermic and release energy.
• Alkanes are important hydrocarbon fuels.
• Complete combustion occurs when sufficient oxygen is available.
• Complete combustion of hydrocarbons produces carbon dioxide and water.
• Incomplete combustion occurs when oxygen is limited.
• Incomplete combustion may produce carbon monoxide, carbon/soot, water, and some carbon dioxide.
• Carbon monoxide is toxic because it interferes with oxygen transport in the blood.
• Soot and other particulate matter can reduce air quality and affect health.
• Combustion equations must obey the law of conservation of mass.
• A reliable balancing order is carbon → hydrogen → oxygen.
• Coefficients may be changed when balancing equations; chemical subscripts should not be changed.
• Carbon dioxide from fossil-fuel combustion contributes to increased atmospheric greenhouse-gas concentrations.
• High-temperature combustion can produce nitrogen oxides.
• Sulfur-containing fuels can produce sulfur dioxide.
• Complete combustion reduces CO and soot formation but does not eliminate the environmental effects of burning fossil fuels.
• Understanding combustion connects chemical equations with energy production, air quality, climate, transportation, and everyday fuel use.