R3.4.5 Addition reactions of alkenes

Why alkenes are reactive

Due to the carbon-carbon double bond (alkene group).

Double bond opens to form new bonds in addition reactions.

Type of reaction in alkenes

Electrophilic addition.

Two reactants combine to form one product.

Double bond breaks during reaction.

Hydrogenation of alkenes

Alkene + H₂ → alkane.

Uses nickel catalyst.

Adds hydrogen atoms to saturate the molecule.

Halogenation of alkenes

Alkene + X₂ → dihalogenoalkane.

Example: Ethene + Br₂ → 1,2-dibromoethane.

Reaction with hydrogen halides

Alkene + HX → halogenoalkane.

Example: Ethene + HBr → bromoethane.

Hydration of alkenes

Alkene + steam → alcohol.

Uses H₂SO₄ catalyst.

Example: Ethene + H₂O → ethanol.

Symmetrical vs. unsymmetrical alkenes

Symmetrical alkenes form one product.

Unsymmetrical alkenes may form two products.

Saturated vs unsaturated molecules

Saturated: only C–C single bonds (alkanes).

Unsaturated: contains C=C or C≡C bonds (alkenes, alkynes).

Example of saturated molecule

Ethane (C₂H₆). Has only single bonds between carbon atoms.

Classified as saturated.

Examples of unsaturated molecules

Ethene (C₂H₄) has a double bond.

Ethyne (C₂H₂) has a triple bond.

Both are unsaturated.

Test for unsaturation

Use bromine water.

Brown solution turns colorless if C=C is present.

No change if saturated.

Bromine water with alkane

No color change.

Indicates saturated molecule with only C–C single bonds.

Bromine water with alkene

Turns from brown to colorless.

Indicates presence of C=C bonds, confirming unsaturation.