The Alkene Family

Alkenes are a family of hydrocarbons that contain at least one carbon-carbon double bond (C=C).

A hydrocarbon is a compound made only from:

• carbon
• hydrogen

Alkenes are described as unsaturated hydrocarbons because their molecules contain a carbon-carbon double bond. This double bond is the characteristic feature that distinguishes the simplest alkenes from the corresponding alkanes.

Examples of simple alkenes include:

ethene — C₂H₄

propene — C₃H₆

butene — C₄H₈

pentene — C₅H₁₀

What Is an Alkene?

An alkene is an unsaturated hydrocarbon containing at least one carbon-carbon double bond.

The simplest alkene is ethene:

C₂H₄

Its structural formula can be represented as:

CH₂=CH₂

The = represents the double bond between the two carbon atoms.

Compare this with ethane:

CH₃–CH₃

Ethane contains only a single carbon-carbon bond.

Ethene contains:

C=C

This small structural difference gives alkenes different chemical properties from alkanes.

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The Carbon-Carbon Double Bond

The most important feature of an alkene is the:

carbon-carbon double bond

written:

C=C

A single carbon-carbon bond can be represented:

C–C

A double carbon-carbon bond is:

C=C

The double bond consists of two shared pairs of electrons between the carbon atoms.

The presence of this double bond affects:

• molecular structure
• chemical reactivity
• reactions with other substances
• naming
• physical properties

Why Are Alkenes Unsaturated?

Alkenes are called unsaturated because they do not contain the maximum possible number of hydrogen atoms for their carbon skeleton.

Consider ethane:

C₂H₆

Structure:

CH₃–CH₃

Now compare ethene:

C₂H₄

Structure:

CH₂=CH₂

Ethene has two fewer hydrogen atoms because the carbon atoms are joined by a double bond.

The molecule can undergo reactions in which atoms are added across this double bond.

This is why we describe ethene as unsaturated.

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Saturated and Unsaturated Hydrocarbons

A saturated hydrocarbon contains only single carbon-carbon bonds.

An unsaturated hydrocarbon contains one or more carbon-carbon multiple bonds.

For the families studied here:

Alkanes

Contain only:

C–C

bonds.

They are saturated.

Alkenes

Contain:

C=C

They are unsaturated.

This distinction is one of the most important differences between the two families.

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Comparing Ethane and Ethene

Consider these two compounds.

Ethane

Molecular formula:

C₂H₆

Structural formula:

CH₃–CH₃

Family:

alkane

Bond between carbon atoms:

single

Type:

saturated

Ethene

Molecular formula:

C₂H₄

Structural formula:

CH₂=CH₂

Family:

alkene

Bond between carbon atoms:

double

Type:

unsaturated

The Alkene Homologous Series

Alkenes form a homologous series.

A homologous series is a family of organic compounds that:

• have the same functional group
• have the same general formula
• have similar chemical properties
• show gradual changes in physical properties
• differ from neighbouring members by CH₂

For simple alkenes containing one double bond, the general formula is:

CₙH₂ₙ

where n represents the number of carbon atoms.

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The General Formula

The general formula for the simple alkene homologous series is:

CₙH₂ₙ

This allows us to predict molecular formulas.

For example, suppose an alkene contains 5 carbon atoms.

n = 5

Number of hydrogen atoms:

2n = 2 × 5 = 10

Therefore:

C₅H₁₀

This is a molecular formula for a pentene.

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First Members of the Alkene Family

Notice the pattern:

number of H atoms = twice the number of C atoms

This follows:

CₙH₂ₙ

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Why Is There No "Methene"?

The first alkene is ethene, not methene.

Why?

A carbon-carbon double bond requires two carbon atoms.

A molecule containing only one carbon atom cannot contain:

C=C

Therefore, there is no simple alkene called methene.

The alkene homologous series begins with:

ethene — C₂H₄

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Recognizing Ethene

Ethene contains two carbon atoms.

Molecular formula:

C₂H₄

Structural formula:

CH₂=CH₂

Each carbon forms four bonds in total.

Each carbon:

• forms a double bond with the other carbon
• forms two single bonds with hydrogen atoms

Therefore, each carbon still has four bonds altogether.

Recognizing Propene

Propene contains three carbon atoms.

Molecular formula:

C₃H₆

One possible structural representation is:

CH₂=CH–CH₃

Count the atoms:

Carbon:

3

Hydrogen:

2 + 1 + 3 = 6

Therefore:

C₃H₆

This agrees with the general formula:

CₙH₂ₙ

For n = 3:

C₃H₆

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Recognizing Butene

Butene contains four carbon atoms.

Molecular formula:

C₄H₈

One possible structure is:

CH₂=CH–CH₂–CH₃

This is but-1-ene.

Another possible structure is:

CH₃–CH=CH–CH₃

This is but-2-ene.

Both molecules:

• contain four carbon atoms
• contain eight hydrogen atoms
• contain a C=C bond
• are alkenes

But the double bond occurs in a different position.

Naming Simple Alkenes

Alkene names usually end in:

-ene

The beginning of the name indicates the number of carbon atoms.

The ending:

-ene

indicates the presence of a carbon-carbon double bond.

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Locating the Double Bond

For larger alkenes, the position of the double bond may need to be identified.

Consider:

CH₂=CH–CH₂–CH₃

The double bond begins at carbon 1.

Therefore:

but-1-ene

Now consider:

CH₃–CH=CH–CH₃

The double bond begins at carbon 2.

Therefore:

but-2-ene

The carbon chain is numbered from the end that gives the double bond the lowest possible number.

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Alkane Names vs Alkene Names

The endings provide an important clue.

Alkane

Ends in:

-ane

Examples:

ethane
propane
butane
pentane

Alkene

Ends in:

-ene

Examples:

ethene
propene
butene
pentene

So:

-ane → alkane

-ene → alkene

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Comparing the General Formulas

Simple alkanes have the general formula:

CₙH₂ₙ₊₂

Simple alkenes with one C=C bond have:

CₙH₂ₙ

For example:

For the same number of carbon atoms, the simple alkene has two fewer hydrogen atoms than the corresponding alkane.

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Worked Example: Identifying the Family

Consider:

C₆H₁₂

Could this be a simple alkene?

Use:

CₙH₂ₙ

n = 6

2n = 12

So:

C₆H₁₂

fits the general formula.

Therefore, it could be an alkene.

However, an important caution is needed: molecular formula alone does not always prove that a compound is an alkene. Some other structures, such as cycloalkanes, can also have the formula CₙH₂ₙ.

To confirm that a structure is an alkene, look for:

C=C

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Worked Example: C₇H₁₄

Question:

Does C₇H₁₄ fit the simple alkene general formula?

For n = 7:

2n = 14

Therefore:

C₇H₁₄

fits:

CₙH₂ₙ

So it could represent an alkene.

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Worked Example: C₅H₁₂

Question:

Is C₅H₁₂ a simple alkene?

For an alkene with five carbons:

CₙH₂ₙ = C₅H₁₀

But the given formula is:

C₅H₁₂

Therefore, C₅H₁₂ does not fit the alkene general formula.

It fits the alkane formula:

CₙH₂ₙ₊₂

and is pentane.

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Molecular, Structural and Displayed Formulas

Organic molecules can be represented in different ways.

Molecular Formula

Shows the number of each type of atom.

Ethene:

C₂H₄

Propene:

C₃H₆

Condensed Structural Formula

Shows more information about how atoms are connected.

Ethene:

CH₂=CH₂

Propene:

CH₂=CH–CH₃

Displayed Formula

Shows individual bonds between atoms.

Displayed formulas are particularly useful because the C=C double bond is clearly visible.

Carbon Forms Four Bonds

Carbon normally forms four covalent bonds.

This rule helps us check whether an alkene structure is reasonable.

In ethene:

CH₂=CH₂

Each carbon has:

• two bonds to hydrogen
• two bonds represented by the C=C double bond

Total:

4 bonds

If a proposed structure gives carbon five bonds, or only three without another explanation, something is wrong with the structure.

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The Double Bond and Reactivity

The carbon-carbon double bond makes alkenes generally more chemically reactive than corresponding alkanes.

The double bond can participate in reactions in which new atoms are added to the molecule.

These are called addition reactions.

For example, ethene can react with bromine.

The double bond is replaced as atoms become attached to the carbon atoms.

This behaviour provides a useful chemical test for unsaturation.

Testing for an Alkene

Bromine water can be used to distinguish many alkenes from alkanes.

Bromine water has an orange-brown colour.

When an alkene reacts with bromine under appropriate test conditions, the bromine colour disappears.

Observation:

orange-brown → colourless

This is described as decolourisation.

The test works because bromine reacts with the C=C double bond.

An alkane does not normally decolourise bromine water under the same simple test conditions.

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Addition Reactions

The characteristic reactions of alkenes are often addition reactions.

An addition reaction occurs when atoms are added across the double bond.

For example:

ethene + bromine → dibromoethane

Simplified structural representation:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Notice what has happened:

Before:

C=C

After:

C–C

The double bond has been involved in forming new bonds to bromine atoms.

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Alkenes and Hydrogen

Alkenes can also react with hydrogen under suitable conditions.

For example:

ethene + hydrogen → ethane

Symbolically:

C₂H₄ + H₂ → C₂H₆

Structural representation:

CH₂=CH₂ + H₂ → CH₃–CH₃

The unsaturated alkene becomes a saturated alkane.

This process is called hydrogenation.

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Why "Unsaturated" Makes Sense

Think about ethene:

CH₂=CH₂

If hydrogen is added:

CH₂=CH₂ + H₂ → CH₃–CH₃

The product is ethane.

Ethene was able to accept additional hydrogen atoms because of its double bond.

That is the chemical meaning behind the term:

unsaturated

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Physical Properties of Alkenes

Alkenes share several general physical properties.

They are generally:

• non-polar
• insoluble or only very slightly soluble in water
• soluble in many non-polar organic solvents
• combustible

As molecular size increases, boiling points generally increase.

The smaller alkenes are gases at room temperature, while larger members eventually become liquids and then solids as molecular size increases.

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Alkenes and Combustion

Like other hydrocarbons, alkenes can burn in oxygen.

Complete combustion produces:

carbon dioxide + water

For ethene:

ethene + oxygen → carbon dioxide + water

Balanced:

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

If oxygen is limited, incomplete combustion can produce carbon monoxide and/or carbon particles.

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Where Do Alkenes Come From?

Alkenes are important in the petrochemical industry.

Long hydrocarbon molecules can be broken into smaller molecules by a process called cracking.

Cracking can produce:

• shorter alkanes
• alkenes

For example, a large hydrocarbon can be split to produce smaller molecules, including ethene or propene.

These alkenes are extremely useful chemical raw materials.

Alkenes and Plastics

One major use of alkenes is producing polymers.

For example, many ethene molecules can join together to form poly(ethene), also called polyethylene.

Ethene is the monomer.

Poly(ethene) is the polymer.

Similarly:

propene → poly(propene)

These materials are used in an enormous range of everyday products.

The C=C double bond is what allows alkene molecules to join together during addition polymerization.

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Homologous Series Patterns

Members of the alkene homologous series share:

The Same Functional Group

C=C

The Same General Formula

CₙH₂ₙ

for simple acyclic alkenes containing one double bond.

Similar Chemical Properties

They undergo reactions involving the double bond.

Gradual Changes in Physical Properties

For example, boiling points generally increase as molecular size increases.

A CH₂ Difference Between Neighbouring Members

Compare:

Ethene:

C₂H₄

Propene:

C₃H₆

Difference:

CH₂

Propene to butene:

C₃H₆ → C₄H₈

Again:

CH₂

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Identifying Alkenes from Structures

Look for the characteristic:

C=C

Consider:

CH₃–CH₂–CH₃

No C=C bond.

Therefore:

alkane

Now:

CH₂=CH–CH₃

Contains C=C.

Therefore:

alkene

Now:

CH₃–CH=CH–CH₃

Contains C=C.

Therefore:

alkene

The double bond is the clearest structural evidence.

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Worked Identification Problems

Compound A

CH₃–CH₃

Only single carbon-carbon bonds.

Alkane

Compound B

CH₂=CH₂

Contains C=C.

Alkene

Compound C

CH₂=CH–CH₂–CH₃

Contains C=C.

Alkene

Compound D

CH₃–CH₂–CH₂–CH₃

Only C–C single bonds.

Alkane

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Predicting Molecular Formulas

Suppose an alkene has eight carbon atoms.

General formula:

CₙH₂ₙ

n = 8

Hydrogen:

2 × 8 = 16

Formula:

C₈H₁₆

Now suppose an alkene has ten carbon atoms.

n = 10

Hydrogen:

2 × 10 = 20

Formula:

C₁₀H₂₀

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Finding the Number of Carbon Atoms

Suppose an alkene has the formula:

C₉H₁₈

The carbon subscript tells us directly:

9 carbon atoms

Check:

2 × 9 = 18

Therefore, the formula fits:

CₙH₂ₙ

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Comparing Alkanes and Alkenes

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Common Mistakes

Thinking Alkenes Contain Oxygen

Alkenes are hydrocarbons.

They contain only carbon and hydrogen.

Confusing -ane and -ene

-ane → alkane

-ene → alkene

Forgetting the Double Bond

An alkene must contain:

C=C

Calling Alkenes Saturated

Alkenes are unsaturated.

Using the Wrong General Formula

Simple alkanes:

CₙH₂ₙ₊₂

Simple alkenes:

CₙH₂ₙ

Thinking Ethene Is CH₃–CH₃

That structure is ethane.

Ethene is:

CH₂=CH₂

Inventing Methene

A carbon-carbon double bond requires at least two carbon atoms, so the simplest alkene is ethene.

Assuming CₙH₂ₙ Always Proves a Compound Is an Alkene

Some other structures, particularly cycloalkanes, can also have this molecular formula.

Look for the C=C bond when structural information is available.

Forgetting Carbon's Valency

Each carbon normally forms four covalent bonds.

Thinking the Double Bond Means Two Carbon Atoms

The double bond means two shared pairs of electrons, not two carbon atoms.

Thinking All Butenes Have the Same Structure

The double bond can occur in different positions, giving compounds such as but-1-ene and but-2-ene.

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Key Terms

Hydrocarbon — A compound containing only carbon and hydrogen.

Alkene — An unsaturated hydrocarbon containing at least one carbon-carbon double bond.

Unsaturated — Describes a molecule containing a carbon-carbon multiple bond and capable of undergoing addition reactions.

Saturated — Describes a hydrocarbon containing only single carbon-carbon bonds.

Carbon-carbon double bond — A C=C bond consisting of two shared pairs of electrons between carbon atoms.

Covalent bond — A chemical bond formed by sharing electron pairs.

Functional group — An atom or group of atoms responsible for characteristic chemical reactions; for alkenes, the key functional group is C=C.

Homologous series — A family of organic compounds with the same functional group, general formula and similar chemical properties.

General formula — A formula representing the pattern shared by members of a homologous series.

Molecular formula — A formula showing the number of each type of atom in a molecule.

Structural formula — A representation showing how atoms are connected within a molecule.

Displayed formula — A representation showing individual atoms and bonds.

Ethene — The simplest alkene, C₂H₄.

Propene — A three-carbon alkene, C₃H₆.

Butene — A four-carbon alkene, C₄H₈.

Isomer — A compound with the same molecular formula as another compound but a different arrangement of atoms.

Addition reaction — A reaction in which atoms are added across a multiple bond.

Hydrogenation — Addition of hydrogen across a carbon-carbon double bond.

Bromine water — A reagent commonly used to test for carbon-carbon unsaturation.

Decolourisation — Loss of colour during a chemical reaction.

Monomer — A small molecule that can join with many similar molecules to form a polymer.

Polymer — A large molecule made from many repeating monomer units.

Addition polymerization — Formation of a polymer by joining unsaturated monomers without eliminating a small molecule.

Cracking — Breaking larger hydrocarbon molecules into smaller hydrocarbons, often producing alkenes.

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Key Takeaways

• Alkenes are hydrocarbons.
• They contain only carbon and hydrogen.
• Alkenes are unsaturated hydrocarbons.
• The characteristic feature of an alkene is the C=C carbon-carbon double bond.
• The double bond consists of two shared pairs of electrons.
• Alkanes contain only carbon-carbon single bonds.
• Alkanes are saturated; alkenes are unsaturated.
• Simple acyclic alkenes containing one double bond follow:

CₙH₂ₙ

• Simple alkanes follow:

CₙH₂ₙ₊₂

• The first alkene is ethene because at least two carbon atoms are needed to form C=C.
• Ethene has the formula C₂H₄.
• Propene has the formula C₃H₆.
• Butene has the formula C₄H₈.
• Pentene has the formula C₅H₁₀.
• Alkene names normally end in -ene.
• The position of the double bond may need to be specified in larger molecules.
• But-1-ene and but-2-ene have the same molecular formula but different structures.
• Carbon normally forms four covalent bonds.
• Molecular formulas show numbers of atoms but do not always reveal their arrangement.
• Structural formulas make the C=C bond easier to identify.
• A molecular formula fitting CₙH₂ₙ could also belong to another type of compound, such as a cycloalkane.
• Alkenes are generally more reactive than corresponding alkanes because of the C=C bond.
• Alkenes undergo characteristic addition reactions.
• Bromine water can be used to test for carbon-carbon unsaturation.
• Alkenes decolourise bromine water under appropriate test conditions.
• Hydrogen can be added to an alkene to produce an alkane.
• Alkenes are important raw materials for producing polymers.
• Ethene can form poly(ethene).
• Propene can form poly(propene).
• Alkenes can be produced during the cracking of larger hydrocarbons.

The most important identification rule is:

Look for C=C → alkene

And the key general formula is:

CₙH₂ₙ

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Check Your Understanding

1. What is an alkene?

2. Why are alkenes described as hydrocarbons?

3. Why are alkenes described as unsaturated?

4. What is the characteristic bond found in an alkene?

5. Write the structural formula of ethene.

6. What is the molecular formula of ethene?

7. Why is there no simple alkene called methene?

8. State the general formula of the simple alkene homologous series.

9. Predict the formula of an alkene containing five carbon atoms.

10. Predict the formula of an alkene containing eight carbon atoms.

11. Does C₆H₁₂ fit the general formula for a simple alkene?

12. Does C₆H₁₄ fit the general formula for an alkene?

13. Explain the difference between a saturated and an unsaturated hydrocarbon.

14. State the general formula for simple alkanes.

15. Explain why an alkene with four carbon atoms has two fewer hydrogen atoms than the corresponding alkane.

16. Identify the family of:

CH₃–CH₂–CH₃

17. Identify the family of:

CH₂=CH–CH₃

18. Identify the family of:

CH₃–CH=CH–CH₃

19. What does the ending -ene tell you about a hydrocarbon?

20. What is the difference between but-1-ene and but-2-ene?

21. Why can molecular formula alone sometimes be insufficient to prove that a compound is an alkene?

22. How many covalent bonds does carbon normally form?

23. What happens when an alkene reacts with bromine water under appropriate conditions?

24. Why can bromine water be used to distinguish an alkene from an alkane?

25. What type of reaction commonly occurs at an alkene's double bond?

26. What product forms when ethene reacts with hydrogen?

27. What is hydrogenation?

28. Explain why alkenes are useful in polymer production.

29. What is cracking, and why is it important for producing alkenes?

30. Challenge: Consider the following compounds:

A: C₂H₆
B: C₃H₆
C: C₄H₈
D: C₅H₁₂
E: CH₂=CH–CH₂–CH₃
F: CH₃–CH₂–CH₂–CH₃

a. Which molecular formulas fit CₙH₂ₙ?
b. Which molecular formulas fit CₙH₂ₙ₊₂?
c. Which displayed structural example definitely contains an alkene functional group?
d. Identify E by name.
e. Identify F by name.
f. Which compounds are definitely saturated based on the structural information given?
g. Which are definitely unsaturated based on the structural information given?
h. Explain why B and C cannot be proven to be alkenes from their molecular formulas alone.
i. Predict what would happen if E were tested with bromine water.
j. Explain your prediction in terms of the carbon-carbon double bond.
k. Predict the molecular formula of an alkene containing nine carbon atoms.
l. Predict the corresponding alkane formula with nine carbon atoms.
m. Explain why these two formulas differ by two hydrogen atoms.