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)

 

 

 

5

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

 

 

 

5

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

 

 

 

5

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

 

 

 

5

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.

 

 

 

5

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

 

 

 

5

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.

 

 

 

6

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

 

 

 

4

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.

 

 

 

4

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

 

 

 

6

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.

 

 

 

5

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.

 

 

 

4

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

 

 

 

5

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.

 

 

 

4

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.

 

 

 

6

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

 

 

 

5

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.

 

 

 

5

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.