Addition Reactions

An addition reaction is a reaction in which atoms or groups of atoms are added across a carbon-carbon multiple bond, producing a larger product.

Addition reactions are characteristic reactions of alkenes because alkenes contain a carbon-carbon double bond, C=C.

A simplified pattern is:

alkene + reactant → addition product

During the reaction, the C=C double bond becomes a C–C single bond while new atoms or groups become attached to the two carbon atoms.

For example:

CH₂=CH₂ + H₂ → CH₃–CH₃

Ethene has become ethane.

Why Do Alkenes Undergo Addition Reactions?

Alkenes contain the functional group:

C=C

The double bond consists of:

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

The π bond has electron density above and below the line between the carbon nuclei and is more exposed than the σ bond.

During an addition reaction, the π portion of the double bond is disrupted and new covalent bonds form.

The carbon atoms remain joined by:

C–C

but each carbon can now form an additional bond to an incoming atom or group.

This is why the C=C bond acts as an important reaction site.

What Happens to the Double Bond?

Consider ethene:

CH₂=CH₂

During an addition reaction, the double bond changes:

C=C → C–C

At the same time, new atoms become bonded to the carbon atoms.

For example:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Before:

C=C

After:

C–C

One bromine atom has been added to each carbon.

Importantly, the two carbon atoms do not separate from one another.

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Addition vs Substitution

Addition reactions should not be confused with substitution reactions.

Addition

Atoms are added to the molecule.

Typical of:

alkenes

Example:

C₂H₄ + H₂ → C₂H₆

Substitution

One atom or group is replaced by another.

Substitution reactions are characteristic of alkanes reacting with halogens under suitable conditions.

The key distinction is:

addition → atoms are added

substitution → one atom/group replaces another

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Hydrogenation

The addition of hydrogen to an alkene is called hydrogenation.

General pattern:

alkene + hydrogen → alkane

For example:

ethene + hydrogen → ethane

Symbol equation:

C₂H₄ + H₂ → C₂H₆

Structural equation:

CH₂=CH₂ + H₂ → CH₃–CH₃

The unsaturated alkene becomes a saturated alkane.

Hydrogenation of Propene

Propene has the structure:

CH₂=CH–CH₃

When hydrogen is added:

CH₂=CH–CH₃ + H₂ → CH₃–CH₂–CH₃

The product is:

propane

Molecular equation:

C₃H₆ + H₂ → C₃H₈

Again:

C=C → C–C

and one hydrogen becomes attached to each carbon that was originally part of the double bond.

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Catalysts in Hydrogenation

Hydrogenation often requires a catalyst.

A catalyst increases reaction rate without being permanently consumed.

Common industrial hydrogenation catalysts include metals such as:

• nickel
• platinum
• palladium

For introductory chemistry, nickel is often used as the standard example.

The catalyst provides a surface on which the reacting particles can interact more effectively.

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Hydrogenation and Saturation

Hydrogenation demonstrates the meaning of unsaturated particularly clearly.

Before:

CH₂=CH₂

Ethene is unsaturated.

Add H₂:

CH₂=CH₂ + H₂ → CH₃–CH₃

After:

CH₃–CH₃

Ethane is saturated.

The molecule has gained additional hydrogen atoms.

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Hydrogenation of Oils

Hydrogenation also has industrial importance in food chemistry.

Many vegetable oils contain molecules with carbon-carbon double bonds in their fatty acid portions.

Hydrogenation can reduce the number of these double bonds.

This can change properties such as:

• melting point
• hardness
• texture

Historically, partial hydrogenation was used extensively to modify oils. However, partial hydrogenation can produce trans fats, which is why their use in foods has been heavily restricted in many places.

The chemistry remains an important example of addition reactions in industry.

Addition of Halogens

Alkenes also undergo addition reactions with halogens.

Halogens include:

• fluorine
• chlorine
• bromine
• iodine

Bromine is particularly useful in introductory chemistry because its reaction with alkenes forms the basis of a test for unsaturation.

General pattern:

alkene + halogen → dihaloalkane

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Ethene and Bromine

Ethene reacts with bromine:

ethene + bromine → 1,2-dibromoethane

Structural equation:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Molecular equation:

C₂H₄ + Br₂ → C₂H₄Br₂

One bromine atom becomes attached to each of the carbon atoms that formed the original double bond.

Propene and Bromine

Propene:

CH₂=CH–CH₃

Add Br₂:

CH₂=CH–CH₃ + Br₂ → CH₂Br–CHBr–CH₃

The product is:

1,2-dibromopropane

Again, the bromine atoms become attached across the original C=C bond.

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The Bromine Water Test

The bromine addition reaction provides a useful test for carbon-carbon unsaturation.

Bromine water has an orange-brown colour.

When an alkene reacts with bromine under the usual test conditions:

orange-brown → colourless

The bromine is consumed in an addition reaction.

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

Therefore:

decolourisation of bromine water → evidence of C=C unsaturation

Why Does Bromine Water Lose Its Colour?

Bromine molecules are responsible for the characteristic colour.

During the reaction:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Br₂ is consumed.

The product does not have the same bromine colour.

Therefore, the solution becomes decolourised.

This is chemical evidence that the original substance contained a reactive carbon-carbon double bond.

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Addition of Chlorine

Chlorine can also add across a carbon-carbon double bond.

For ethene:

CH₂=CH₂ + Cl₂ → CH₂Cl–CH₂Cl

The product is:

1,2-dichloroethane

Again:

C=C → C–C

and one chlorine atom becomes attached to each carbon.

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Addition of Water

Water can also be added across an alkene double bond.

This reaction is called hydration.

General pattern:

alkene + water → alcohol

Industrially, the reaction commonly uses steam rather than liquid water.

For ethene:

ethene + steam → ethanol

Molecular equation:

C₂H₄ + H₂O → C₂H₅OH

Structural representation:

CH₂=CH₂ + H–OH → CH₃–CH₂OH

The product is ethanol.

What Is Added During Hydration?

Water can be thought of as:

H–OH

During the addition:

• H becomes attached to one carbon
• OH becomes attached to the other carbon

The original:

C=C

becomes:

C–C

For ethene:

CH₂=CH₂

becomes:

CH₃–CH₂OH

The product belongs to the alcohol family.

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Hydration of Propene

Propene can also react with water under suitable conditions.

Propene:

CH₂=CH–CH₃

The major product under typical acid-catalyzed conditions is:

CH₃–CH(OH)–CH₃

This is:

propan-2-ol

At a more advanced level, the position where H and OH are added can matter when the alkene is not symmetrical.

For ethene, this issue does not arise because the two carbon atoms of the double bond are equivalent.

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Industrial Production of Ethanol

Hydration of ethene is an important industrial method of manufacturing ethanol.

Overall reaction:

C₂H₄ + H₂O ⇌ C₂H₅OH

The process uses:

• ethene
• steam
• a catalyst
• controlled temperature and pressure

The reaction is reversible, which means ethanol can also form ethene and water under appropriate conditions.

This process allows ethanol to be produced from ethene obtained from petrochemical feedstocks.

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Addition of Hydrogen Halides

Alkenes can also undergo addition with hydrogen halides such as:

• HCl
• HBr
• HI

For example:

ethene + hydrogen bromide → bromoethane

Structural equation:

CH₂=CH₂ + HBr → CH₃–CH₂Br

One carbon gains H.

The other gains Br.

Again, the reaction occurs across the C=C bond.

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Recognizing an Addition Reaction

Look for three clues.

A Carbon-Carbon Multiple Bond Is Present Initially

Usually:

C=C

Atoms Are Added

Atoms or groups become attached to the carbon atoms.

One Main Product Is Formed

The reactants combine into a larger molecule.

For example:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Two reactants become one product.

This is characteristic of addition.

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Predicting Products

A useful strategy is:

Step 1: Find the C=C bond.

Step 2: Change C=C to C–C.

Step 3: Identify what is being added.

Step 4: Attach one part to each carbon of the original double bond.

Step 5: Check that each carbon has four bonds.

This method works well for simple addition reactions.

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Worked Example: Adding Hydrogen

Question:

Predict the product:

CH₂=CH–CH₃ + H₂ → ?

Start:

CH₂=CH–CH₃

Change:

C=C → C–C

Add one H to each carbon.

Product:

CH₃–CH₂–CH₃

Therefore:

propene + hydrogen → propane

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Worked Example: Adding Bromine

Question:

Predict:

CH₂=CH–CH₃ + Br₂ → ?

Change C=C to C–C.

Add Br to each carbon.

Product:

CH₂Br–CHBr–CH₃

Therefore:

propene + bromine → 1,2-dibromopropane

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Worked Example: Adding Water

Question:

What forms when ethene reacts with steam?

Start:

CH₂=CH₂

Add:

H–OH

Product:

CH₃–CH₂OH

Therefore:

ethene + steam → ethanol

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Worked Example: Adding Chlorine

Question:

Predict:

CH₂=CH₂ + Cl₂ → ?

Change:

C=C → C–C

Add one chlorine to each carbon.

Product:

CH₂Cl–CH₂Cl

Name:

1,2-dichloroethane

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Checking Carbon's Four Bonds

When predicting an addition product, always remember:

carbon forms four covalent bonds

For example:

CH₂Br–CH₂Br

Each carbon has:

• one C–C bond
• two C–H bonds
• one C–Br bond

Total:

4 bonds

This provides a quick way to check whether your proposed product is reasonable.

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Addition Polymerization

One of the most economically important uses of alkene addition chemistry is addition polymerization.

Many alkene molecules can join together to form a long-chain polymer.

For example:

ethene → poly(ethene)

Ethene is the:

monomer

Poly(ethene) is the:

polymer

During polymerization, the C=C bonds participate in forming new C–C bonds linking many molecules together.

Poly(ethene)

Ethene:

CH₂=CH₂

can form long chains containing repeating units based on:

–CH₂–CH₂–

Poly(ethene), also called polyethylene, is used in products such as:

• packaging
• plastic bags
• containers
• bottles
• electrical insulation
• pipes

Different manufacturing conditions can produce poly(ethene) with different properties.

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Poly(propene)

Propene can also undergo addition polymerization.

Propene:

CH₂=CH–CH₃

forms poly(propene).

Poly(propene), also called polypropylene, is used in:

• food containers
• fibres
• ropes
• automotive components
• reusable plastic products
• laboratory equipment

The starting alkene determines the structure and properties of the polymer.

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Why Addition Reactions Matter Industrially

Alkenes are among the most important starting materials in the chemical industry because the C=C bond allows them to be converted into many other compounds.

Addition reactions can be used to produce:

• alkanes
• alcohols
• halogen-containing compounds
• polymers
• chemical intermediates

Ethene and propene are particularly important industrial feedstocks.

Their value comes largely from the chemical reactivity of their carbon-carbon double bonds.

From Crude Oil to Useful Products

Large hydrocarbons obtained from petroleum can undergo cracking.

Cracking produces:

• smaller alkanes
• alkenes

Alkenes such as ethene and propene can then undergo addition reactions to produce many useful materials.

A simplified industrial pathway is:

crude oil hydrocarbons → cracking → alkenes → addition reactions → useful products

This is one reason alkene chemistry is so economically important.

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Addition Reactions and Reaction Conditions

Different addition reactions require different conditions.

For example:

Hydrogenation

Often requires:

• H₂
• metal catalyst
• suitable temperature

Hydration

Industrial hydration of ethene uses:

• steam
• catalyst
• controlled temperature
• controlled pressure

Halogen Addition

Bromine reacts readily with many alkenes and provides a convenient chemical test for unsaturation.

Reaction conditions are therefore an important part of industrial chemistry.

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Addition Reactions and Atom Economy

Addition reactions often have good atom economy because atoms from the reactants become incorporated into the product.

For example:

C₂H₄ + H₂ → C₂H₆

All atoms from both reactants appear in the desired product.

Similarly:

C₂H₄ + Br₂ → C₂H₄Br₂

Again, all reactant atoms are incorporated into the product.

High atom economy is desirable because it can reduce waste.

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Environmental Considerations

Addition chemistry creates many useful materials, but its industrial use also raises environmental considerations.

For example:

• many alkene feedstocks originate from fossil fuels
• polymer products can persist in the environment
• industrial processes require energy
• catalysts and reaction conditions must be managed efficiently
• plastic waste requires appropriate reuse, recycling or disposal strategies

Chemists therefore investigate ways to make industrial reactions:

• more energy efficient
• less wasteful
• safer
• less dependent on fossil resources

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Comparing Major Addition Reactions

The common feature is:

reaction at C=C

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Common Mistakes

Thinking the Carbon-Carbon Bond Completely Breaks

The carbon atoms remain bonded.

C=C becomes C–C.

Forgetting to Add Atoms to Both Carbon Atoms

In many simple addition reactions, components are added across the two carbons of the original double bond.

Confusing Addition with Substitution

Addition adds atoms across a multiple bond.

Substitution replaces an atom or group.

Thinking Alkanes Commonly Undergo Addition

Addition is characteristic of unsaturated compounds such as alkenes.

Forgetting Hydrogenation Produces an Alkane

alkene + H₂ → alkane

Confusing Hydrogenation and Hydration

hydrogenation → add H₂

hydration → add H₂O

Saying Bromine Water "Turns Clear"

Better scientific language is:

bromine water is decolourised

Forgetting the Alcohol Product

Adding water to an alkene produces an alcohol.

Breaking Carbon's Valency

Each carbon should normally have four covalent bonds.

Forgetting the Catalyst

Some addition reactions, particularly industrial hydrogenation and hydration, require catalysts.

Confusing Polymerization with Simple Addition

Addition polymerization involves many alkene molecules joining repeatedly rather than one small molecule simply adding to one alkene.

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Key Terms

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

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

Unsaturated — Containing a carbon-carbon multiple bond.

Saturated — Containing only carbon-carbon single bonds in the hydrocarbon structure.

Carbon-carbon double bond — A C=C bond consisting of a sigma bond and a pi bond.

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

Pi bond (π) — The additional bond present in a double bond that contributes strongly to alkene reactivity.

Functional group — The part of a molecule responsible for its characteristic reactions.

Hydrogenation — Addition of hydrogen across a multiple bond.

Hydration — Addition of water across a multiple bond.

Halogen — An element in Group 17, such as chlorine or bromine.

Halogenation — Addition of a halogen to an unsaturated molecule.

Hydrogen halide — A compound such as HCl or HBr containing hydrogen and a halogen.

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

Decolourisation — Loss of colour as a coloured reactant is consumed.

Catalyst — A substance that increases reaction rate without being permanently consumed.

Alcohol — An organic compound containing a hydroxyl, –OH, functional group.

Haloalkane — An organic compound containing a halogen bonded to an alkane-type carbon framework.

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

Polymer — A large molecule made from many repeating units.

Addition polymerization — Polymer formation involving repeated addition of unsaturated monomers.

Cracking — Breaking larger hydrocarbons into smaller hydrocarbons, including alkenes.

Atom economy — A measure of how much of the reactants becomes part of the desired product.

Industrial feedstock — A starting material used to manufacture other chemicals.

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Key Takeaways

• Addition reactions are characteristic reactions of alkenes.
• Alkenes undergo addition because they contain a reactive C=C double bond.
• During addition:

C=C → C–C

• New atoms or groups become attached to the carbon atoms of the original double bond.
• Addition reactions usually produce one larger product from two reactants.
• Hydrogenation is the addition of H₂.
• Hydrogenation converts an alkene into an alkane.
• Ethene + hydrogen produces ethane.
• Propene + hydrogen produces propane.
• Hydrogenation commonly uses a metal catalyst.
• Halogens such as bromine can add across C=C.
• Ethene + bromine produces 1,2-dibromoethane.
• Bromine water is decolourised by alkenes under appropriate conditions.
• This provides a useful test for carbon-carbon unsaturation.
• Chlorine can also undergo addition with alkenes.
• Hydration is the addition of water.
• Alkene + water produces an alcohol.
• Ethene + steam produces ethanol.
• Hydrogen halides such as HBr can also add across C=C.
• Addition reactions can be predicted by locating the C=C bond and adding atoms across it.
• Carbon should have four bonds in the completed product.
• Addition polymerization uses the reactivity of C=C to join many alkene molecules.
• Ethene forms poly(ethene).
• Propene forms poly(propene).
• Addition reactions are extremely important in the petrochemical and polymer industries.
• Hydrogenation has applications in food and chemical manufacturing.
• Hydration of ethene provides an industrial route to ethanol.
• Addition reactions often have high atom economy because most or all reactant atoms enter the desired product.

The central pattern to remember is:

Find C=C → change it to C–C → add the incoming atoms across the two carbon atoms.

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Check Your Understanding

1. Define an addition reaction.

2. Why do alkenes undergo addition reactions?

3. What happens to C=C during an addition reaction?

4. Why are alkanes less likely to undergo addition reactions?

5. Define hydrogenation.

6. Complete:

ethene + hydrogen → ______

7. Write the symbol equation for hydrogenation of ethene.

8. Predict the product:

CH₂=CH–CH₃ + H₂ → ?

9. What type of hydrocarbon forms after complete hydrogenation of an alkene?

10. What is the role of a catalyst during hydrogenation?

11. Define halogenation.

12. Complete:

ethene + bromine → ______

13. Write the structural equation for ethene reacting with Br₂.

14. Predict:

CH₂=CH–CH₃ + Br₂ → ?

15. What happens to bromine water when an alkene is present?

16. Why does this colour change occur?

17. Predict the product:

CH₂=CH₂ + Cl₂ → ?

18. Define hydration.

19. Complete:

ethene + steam → ______

20. Write the molecular equation for hydration of ethene.

21. What functional group is found in the product of alkene hydration?

22. What forms when ethene reacts with HBr?

23. Explain the difference between hydrogenation and hydration.

24. Explain the difference between addition and substitution.

25. Describe a systematic method for predicting a simple addition product.

26. Why should you check that every carbon has four bonds?

27. What is addition polymerization?

28. Explain why alkenes are useful monomers.

29. Give three industrially important products or product types that can be made using alkene addition reactions.

30. Challenge: Consider propene:

CH₂=CH–CH₃

a. Identify the C=C bond as the reaction site.
b. Draw or write the product when H₂ is added.
c. Name the product.
d. Write its molecular formula.
e. Predict the product when Br₂ is added.
f. Name this product.
g. Predict what happens to bromine water during this reaction.
h. Explain the observation.
i. Predict the product formed by adding HBr.
j. Explain what happens to the C=C bond in each reaction.
k. State which reactions convert the molecule from unsaturated to saturated.
l. Explain why these reactions are classified as addition rather than substitution.
m. Explain how propene can also be used in addition polymerization.
n. Name the polymer produced.
o. Explain why addition reactions make alkenes valuable industrial feedstocks.