Unsaturated Hydrocarbons

Hydrocarbons are organic compounds made entirely from carbon and hydrogen. One important way of classifying hydrocarbons is according to whether they are saturated or unsaturated.

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

An unsaturated hydrocarbon contains at least one carbon-carbon multiple bond. In the alkenes, this is a carbon-carbon double bond, C=C.

This structural difference is extremely important because the double bond changes the chemical behaviour of the molecule. In particular, alkenes are generally more reactive than alkanes and can undergo characteristic addition reactions.

What Does Saturated Mean?

A saturated hydrocarbon contains the maximum number of hydrogen atoms possible for a carbon chain containing only carbon-carbon single bonds.

Alkanes are saturated hydrocarbons.

For example, ethane has the molecular formula:

C₂H₆

Its condensed structural formula is:

CH₃–CH₃

The two carbon atoms are joined by a single bond:

C–C

Each carbon forms four covalent bonds, and there is no carbon-carbon double bond.

Other saturated hydrocarbons include:

• methane — CH₄
• ethane — C₂H₆
• propane — C₃H₈
• butane — C₄H₁₀
• pentane — C₅H₁₂

For simple alkanes, the general formula is:

CₙH₂ₙ₊₂

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What Does Unsaturated Mean?

An unsaturated hydrocarbon contains a carbon-carbon multiple bond.

For the alkenes studied here, this means a:

carbon-carbon double bond

or:

C=C

For example, ethene has the formula:

C₂H₄

Its condensed structural formula is:

CH₂=CH₂

Because it contains a C=C bond, ethene is unsaturated.

Other examples include:

propene: CH₂=CH–CH₃

but-1-ene: CH₂=CH–CH₂–CH₃

but-2-ene: CH₃–CH=CH–CH₃

Saturation Does Not Mean "Full of Liquid"

The words saturated and unsaturated have different meanings in different areas of chemistry.

For hydrocarbons, saturation refers specifically to bonding between carbon atoms.

A saturated hydrocarbon has only:

C–C

An alkene contains:

C=C

Therefore, saturation in organic chemistry is about molecular structure, not whether something has absorbed or dissolved as much material as possible.

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Comparing Ethane and Ethene

Ethane and ethene provide a useful comparison.

Ethene contains two fewer hydrogen atoms than ethane.

This difference results from the carbon-carbon double bond.

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Why Alkenes Have Fewer Hydrogen Atoms

Carbon normally forms four covalent bonds.

Consider ethane:

CH₃–CH₃

Each carbon forms:

• three C–H bonds
• one C–C bond

Now consider ethene:

CH₂=CH₂

Each carbon forms:

• two C–H bonds
• a double bond to the other carbon

The double bond means that fewer bonds are available for hydrogen atoms.

Therefore:

ethane = C₂H₆

but:

ethene = C₂H₄

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General Formulas

Simple alkanes follow:

CₙH₂ₙ₊₂

Simple acyclic alkenes containing one double bond follow:

CₙH₂ₙ

Compare:

For the same number of carbon atoms:

alkene = alkane minus H₂

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Identifying Unsaturated Compounds from Structures

The most reliable way to identify an alkene from a structural formula is to look for:

C=C

Consider:

CH₃–CH₂–CH₃

There are only single carbon-carbon bonds.

Therefore, this compound is saturated.

Now consider:

CH₂=CH–CH₃

It contains C=C.

Therefore, it is unsaturated.

Now consider:

CH₃–CH=CH–CH₃

Again, it contains C=C.

Therefore, it is unsaturated.

Identifying Unsaturation from Molecular Formulas

The general formulas can provide useful clues.

Suppose a compound has the formula:

C₅H₁₀

For a simple alkene:

CₙH₂ₙ

If n = 5:

H = 2 × 5 = 10

Therefore, C₅H₁₀ fits the general formula for a simple alkene.

Now consider:

C₅H₁₂

For a simple alkane:

CₙH₂ₙ₊₂

If n = 5:

H = 2(5) + 2 = 12

Therefore, C₅H₁₂ fits the alkane formula.

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An Important Limitation of Molecular Formulas

A molecular formula alone does not always prove whether a compound contains a C=C bond.

For example:

C₄H₈

could represent an alkene such as but-1-ene.

But C₄H₈ can also represent a cycloalkane, such as cyclobutane, which contains no carbon-carbon double bond.

Therefore:

CₙH₂ₙ suggests possible unsaturation, but structural information or chemical testing may be needed to confirm it.

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The Carbon-Carbon Double Bond

The C=C bond contains two shared pairs of electrons between the carbon atoms.

A double bond is not simply a "stronger version" of a single bond. Its bonding arrangement gives the molecule different geometry and different chemical behaviour.

The C=C bond consists of:

• one sigma (σ) bond
• one pi (π) bond

The π bond is more exposed and easier to disrupt during many chemical reactions.

This is one reason the double bond acts as a reactive region within an alkene molecule.

Why Are Alkenes More Reactive?

Alkenes are generally more reactive than corresponding alkanes because of their C=C double bond.

During many reactions, the π portion of the double bond can be broken and replaced by new bonds to other atoms.

This allows substances to be added across the double bond.

For example:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Before the reaction:

C=C

After the reaction:

C–C

and bromine atoms have become attached to the two carbon atoms.

This type of reaction is called an addition reaction.

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Addition Reactions

An addition reaction occurs when atoms are added to an unsaturated molecule.

A simple pattern is:

alkene + another substance → one larger product

The C=C bond provides the site where the reaction occurs.

Important addition reactions of alkenes include reactions with:

• bromine
• hydrogen
• steam
• other alkene molecules during polymerization

These reactions are characteristic of unsaturated hydrocarbons.

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Addition of Hydrogen

Ethene can react with hydrogen:

ethene + hydrogen → ethane

Symbol equation:

C₂H₄ + H₂ → C₂H₆

Structural representation:

CH₂=CH₂ + H₂ → CH₃–CH₃

Notice the change:

unsaturated → saturated

The process of adding hydrogen is called hydrogenation.

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Saturation Through Hydrogenation

Hydrogenation provides a useful way to understand the meaning of saturation.

Before hydrogenation:

CH₂=CH₂

The molecule is unsaturated.

After hydrogenation:

CH₃–CH₃

The molecule is saturated.

The alkene has gained hydrogen atoms and the carbon-carbon double bond has become a single bond.

Chemical Tests for Unsaturation

Because alkenes react readily at the C=C bond, their chemical behaviour can be used to distinguish them from saturated hydrocarbons.

A commonly taught test uses bromine water.

Bromine water has an:

orange-brown colour

An alkene reacts with bromine and causes the colour to disappear.

Observation:

orange-brown → colourless

This is called decolourisation.

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The Bromine Water Test

Suppose you have two unknown hydrocarbons.

Sample A is an alkane.

Sample B is an alkene.

When bromine water is added under appropriate test conditions:

Sample A

Bromine colour remains.

Sample B

Bromine water is decolourised.

Therefore, Sample B contains carbon-carbon unsaturation.

What Happens During the Bromine Test?

Ethene reacts with bromine.

Word equation:

ethene + bromine → dibromoethane

Structural representation:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

The C=C double bond becomes a C–C single bond.

One bromine atom becomes attached to each carbon.

Because bromine is consumed during the reaction, its characteristic colour disappears.

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Why Doesn't an Alkane Give the Same Result?

Alkanes contain only:

C–C

single bonds.

They do not contain the reactive C=C site characteristic of alkenes.

Therefore, under the usual bromine-water test conditions, an alkane does not rapidly undergo the same addition reaction.

The bromine colour therefore remains.

This gives us a practical method for distinguishing:

saturated hydrocarbon vs unsaturated hydrocarbon

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Interpreting Experimental Results

Suppose three unknown hydrocarbons are tested.

We can conclude:

A shows evidence of unsaturation

B shows no evidence of C=C under the test conditions

C shows evidence of unsaturation

The chemical test provides stronger evidence than simply looking at the substances.

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A Test Is Evidence, Not Just a Colour Change

Good scientific reasoning separates:

observation

from:

conclusion

For example:

Observation:

The orange-brown bromine colour disappeared.

Conclusion:

The sample contains a group that reacts with bromine, consistent with carbon-carbon unsaturation.

This is stronger scientific language than simply saying:

"The substance is an alkene because it changed colour."

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Double Bonds Affect Molecular Shape

The C=C double bond also affects the shape and movement of molecules.

Carbon-carbon single bonds generally allow relatively free rotation.

A carbon-carbon double bond restricts rotation.

This means groups attached to the double-bonded carbon atoms can be held in different spatial arrangements.

This becomes important when studying geometric isomerism, including cis/trans or E/Z isomers.

For example, but-2-ene can exist in different geometric arrangements because rotation around C=C is restricted.

Double Bonds Affect Chemical Properties

The double bond strongly affects how alkenes react.

Alkenes can undergo:

• bromination
• hydrogenation
• hydration
• polymerization

Alkanes generally do not undergo these same addition reactions because they lack C=C bonds.

Therefore, even compounds containing similar numbers of carbon atoms can have very different chemical properties.

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Double Bonds and Physical Properties

The C=C bond changes molecular shape and bonding, but many of the overall physical properties of alkenes remain similar to those of comparable alkanes.

Alkenes are generally:

• non-polar
• poorly soluble in water
• combustible
• soluble in many non-polar organic solvents

As molecular size increases, boiling points generally increase because intermolecular forces become stronger overall.

The largest difference between alkanes and alkenes is often seen in their chemical reactivity, rather than dramatic differences in basic physical appearance.

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Unsaturated Hydrocarbons and Combustion

Unsaturated hydrocarbons can burn in oxygen.

Complete combustion produces:

carbon dioxide + water

For ethene:

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

When oxygen is limited, incomplete combustion can produce:

• carbon monoxide
• carbon particles (soot)
• water

Like other hydrocarbons, alkenes are therefore combustible.

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Unsaturation and Polymerization

The double bond makes alkenes extremely important industrial chemicals.

Alkene molecules can join together through addition polymerization.

For example:

ethene → poly(ethene)

Many ethene molecules join as their C=C bonds participate in forming a long carbon chain.

Ethene is the:

monomer

The large molecule produced is the:

polymer

Why Unsaturation Is Industrially Important

Unsaturated hydrocarbons are valuable because the double bond can be used to build new molecules.

Alkenes are used to manufacture:

• plastics
• alcohols
• detergents
• solvents
• other organic chemicals

Ethene and propene are particularly important industrial feedstocks.

Their usefulness comes largely from the chemical possibilities created by the C=C bond.

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Saturated and Unsaturated Fats

The terms saturated and unsaturated are also commonly used when discussing fats and oils.

Fat molecules contain long hydrocarbon portions.

A saturated fat contains carbon chains without C=C bonds in those portions.

An unsaturated fat contains one or more C=C bonds.

A fat containing one such double bond is often described as monounsaturated.

A fat containing several is polyunsaturated.

The presence of C=C bonds affects molecular shape and therefore how molecules pack together.

This helps explain why many saturated fats are solids at room temperature while many unsaturated oils are liquids.

A More Detailed Look at Molecular Shape

Many naturally occurring unsaturated fatty acids contain cis C=C bonds.

These can introduce bends or "kinks" into hydrocarbon chains.

Straighter saturated chains can often pack together more closely.

Bent unsaturated chains generally pack less efficiently.

This changes intermolecular interactions and can affect melting point.

This is a useful real-world example of how a small difference in chemical bonding can influence the properties of a substance.

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Worked Example: Saturated or Unsaturated?

Consider:

CH₃–CH₂–CH₂–CH₃

Look for C=C.

There is none.

Conclusion:

saturated

This compound is butane.

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Worked Example: Propene

Consider:

CH₂=CH–CH₃

There is a:

C=C

Therefore:

unsaturated

This compound is propene.

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Worked Example: But-2-ene

Consider:

CH₃–CH=CH–CH₃

Again, there is a:

C=C

Therefore:

unsaturated

The double bond begins at carbon 2, so the compound is but-2-ene.

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Worked Example: Using a Chemical Test

An unknown hydrocarbon is shaken with bromine water under appropriate conditions.

Observation:

The orange-brown colour disappears.

What can we conclude?

The compound reacts with bromine in a way consistent with carbon-carbon unsaturation.

If the unknown is known to be either an alkane or an alkene, the evidence supports:

alkene

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Worked Example: Molecular Formula

Compound X has the formula:

C₇H₁₄

Does it fit the general formula of a simple alkene?

General formula:

CₙH₂ₙ

n = 7

2n = 14

Therefore:

C₇H₁₄

fits the alkene formula.

However, the molecular formula alone does not prove the presence of C=C.

Structural evidence or a chemical test would provide stronger confirmation.

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Common Mistakes

Thinking Unsaturated Means "Contains Less Carbon"

Unsaturation refers to carbon-carbon multiple bonds, not simply the number of carbon atoms.

Thinking Saturated Means "Contains Lots of Hydrogen"

More precisely, a saturated hydrocarbon contains only single carbon-carbon bonds and therefore has the maximum hydrogen content for that carbon framework.

Looking Only at the Molecular Formula

A formula such as C₄H₈ can represent different structures.

Look for C=C when structural information is available.

Confusing C=C with C–C

C–C → single bond

C=C → double bond

Calling Alkenes Saturated

Alkenes containing C=C are unsaturated.

Thinking Double Bonds Make Alkenes Less Reactive

The opposite is generally true.

The C=C bond provides a reactive site.

Saying Bromine Water "Turns Clear"

A better description is:

orange-brown bromine water is decolourised

Saying Bromine Water Proves Every Unknown Is an Alkene

The test provides evidence of unsaturation. Other functional groups can also react with bromine under some conditions.

Confusing Addition and Combustion

Addition reactions involve adding atoms across the double bond.

Combustion involves reaction with oxygen and releases energy.

Forgetting What Happens to C=C During Addition

The carbon-carbon double bond becomes a carbon-carbon single bond as new bonds form.

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Key Terms

Hydrocarbon — A compound containing only carbon and hydrogen.

Saturated hydrocarbon — A hydrocarbon containing only single carbon-carbon bonds.

Unsaturated hydrocarbon — A hydrocarbon containing at least one carbon-carbon multiple bond.

Alkane — A saturated hydrocarbon containing only C–C single bonds.

Alkene — An unsaturated hydrocarbon containing at least one C=C bond.

Carbon-carbon single bond — A covalent bond involving one shared electron pair between carbon atoms.

Carbon-carbon double bond — A bond involving two shared electron pairs between carbon atoms.

Covalent bond — A chemical bond formed through the sharing of electrons.

Sigma bond (σ bond) — The first bond formed directly between two bonded atoms.

Pi bond (π bond) — The additional bond present in a double bond; its bonding arrangement contributes to alkene reactivity.

Functional group — An atom or group of atoms responsible for characteristic reactions of an organic compound.

General formula — A formula describing the common composition pattern of a homologous series.

Homologous series — A family of compounds sharing a functional group and general formula.

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

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

Bromine water — A bromine-containing solution commonly used as a test for carbon-carbon unsaturation.

Decolourisation — The disappearance of a substance's colour during a reaction.

Chemical test — A reaction used to obtain evidence about the identity or properties of a substance.

Monomer — A small molecule capable of joining with others to form a polymer.

Polymer — A large molecule consisting of repeating structural units.

Addition polymerization — Formation of polymers by addition reactions involving unsaturated monomers.

Geometric isomerism — Isomerism caused by restricted rotation, often around a C=C bond.

Monounsaturated — Containing one carbon-carbon double bond in the relevant carbon chain.

Polyunsaturated — Containing more than one carbon-carbon double bond in the relevant carbon chain.

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Key Takeaways

• Hydrocarbons contain only carbon and hydrogen.
• Hydrocarbons can be classified as saturated or unsaturated.
• Saturated hydrocarbons contain only carbon-carbon single bonds.
• Alkanes are saturated hydrocarbons.
• Unsaturated hydrocarbons contain carbon-carbon multiple bonds.
• Alkenes contain at least one C=C double bond.
• The C=C bond is the characteristic functional group of alkenes.
• Simple alkanes follow CₙH₂ₙ₊₂.
• Simple acyclic alkenes containing one double bond follow CₙH₂ₙ.
• An alkene generally contains two fewer hydrogen atoms than the corresponding alkane with the same number of carbons.
• Structural formulas provide stronger evidence of unsaturation than molecular formulas alone.
• A formula such as C₄H₈ does not by itself prove that a compound is an alkene.
• The C=C double bond contains a sigma bond and a pi bond.
• The double bond creates an important reactive region in an alkene.
• Alkenes are generally more reactive than corresponding alkanes.
• Alkenes undergo characteristic addition reactions.
• During many addition reactions, C=C becomes C–C as new bonds form.
• Hydrogenation converts an unsaturated alkene into a saturated alkane.
• Bromine water can be used as a chemical test for carbon-carbon unsaturation.
• Unsaturated compounds such as alkenes decolourise bromine water under appropriate conditions.
• Saturated alkanes do not rapidly decolourise bromine water under the same simple test conditions.
• A chemical test provides evidence that should be interpreted alongside other information.
• Double bonds restrict rotation and can affect molecular shape.
• The presence of double bonds can therefore affect physical as well as chemical properties.
• Alkenes can undergo addition polymerization.
• Their reactivity makes alkenes valuable industrial starting materials.
• Saturated and unsaturated structures are also important when discussing fats and oils.

The most important structural comparison is:

C–C → saturated

C=C → unsaturated

And the most useful chemical test at this level is:

bromine water remains orange-brown → no evidence of C=C

bromine water decolourises → evidence of carbon-carbon unsaturation

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Check Your Understanding

1. Define a hydrocarbon.

2. What does saturated mean when describing a hydrocarbon?

3. What does unsaturated mean?

4. Which hydrocarbon family contains C=C bonds?

5. Are alkanes saturated or unsaturated?

6. Explain why ethane is saturated.

7. Explain why ethene is unsaturated.

8. Write the molecular formula of ethane.

9. Write the molecular formula of ethene.

10. Why does ethene contain fewer hydrogen atoms than ethane?

11. State the general formula for simple alkanes.

12. State the general formula for simple acyclic alkenes containing one double bond.

13. Is CH₃–CH₂–CH₃ saturated or unsaturated? Explain.

14. Is CH₂=CH–CH₃ saturated or unsaturated? Explain.

15. Is CH₃–CH=CH–CH₃ saturated or unsaturated? Explain.

16. Why does C₄H₈ not necessarily prove that a compound is an alkene?

17. Explain why alkenes are generally more reactive than alkanes.

18. What is an addition reaction?

19. What happens to C=C during a typical addition reaction?

20. Write the equation for hydrogen reacting with ethene.

21. What is hydrogenation?

22. Describe the appearance of bromine water before testing an alkene.

23. What happens to bromine water when it reacts with an alkene?

24. Why does the bromine colour disappear?

25. What would normally happen if an alkane were tested with bromine water under the same conditions?

26. Explain why bromine water can distinguish an alkane from an alkene.

27. How does a double bond affect rotation within a molecule?

28. Why can double bonds affect molecular shape?

29. Explain how unsaturation makes alkenes useful for polymer production.

30. Challenge: Two unknown hydrocarbons, X and Y, have the following properties:

X has the molecular formula C₆H₁₄.

Y has the molecular formula C₆H₁₂.

When bromine water is added:

• X leaves the orange-brown colour unchanged.
• Y decolourises the bromine water.

a. Which compound fits the alkane general formula?
b. Which compound fits the alkene general formula?
c. Which compound shows experimental evidence of unsaturation?
d. What observation provides this evidence?
e. What structural feature is likely present in Y?
f. Explain why Y is generally more reactive than X.
g. What type of reaction occurs between Y and bromine?
h. What happens to the C=C bond during this reaction?
i. Explain why the molecular formula C₆H₁₂ alone would not have been enough to prove that Y was an alkene.
j. Explain how the bromine test strengthens the conclusion.
k. Predict what would happen if Y reacted with hydrogen under suitable conditions.
l. Predict the molecular formula of the product.
m. Would the product be saturated or unsaturated? Explain.