Reactions of Alkenes (AQA AS Chemistry): Revision Note

Alkenes: Reactions

• Alkenes are very useful compounds as they can undergo many types of reactions

• They can therefore be used as starting molecules when making new compounds

Electrophilic addition

• Electrophilic addition is the addition of an electrophile to a double bond

• The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms

• Electrophilic addition reactions include the addition of:

  • Steam (H₂O (g))

  • Hydrogen halide (HX)

  • Halogen

The diagram shows an overview of the different electrophilic addition reactions alkenes can undergo

Electrophilic Addition

• Alkenes are more reactive than alkanes due to their carbon-carbon double bonds

• The double bond has high electron density, making it susceptible to electrophilic attack

• Electrophilic addition occurs when an electrophile adds to the double bond, breaking it and forming new single bonds with each carbon.

Reaction with HBr

• Hydrogen bromide (HBr) is polar due to differences in electronegativity between hydrogen and bromine

• Bromine pulls electrons more strongly, creating a partial negative charge on Br and a partial positive charge on H

 The polarity of a HBr molecule

The two atoms have different electronegativities resulting in the formation of a polar bond

• In electrophilic addition:

  • The partially positive (δ+) hydrogen atom acts as an electrophile

  • It is attracted to the high electron density of the C=C double bond in the alkene and accepts a pair of electrons

  • The H-Br bond breaks heterolytically, forming a Br^- ion

  • A highly reactive carbocation intermediate is formed which reacts with the bromide ion, Br^-

• The reaction of ethene with HBr forms bromoethane

Electrophilic addition of HBr mechanism

Reaction with Br₂

• Bromine (Br₂) is non-polar because both atoms share electrons equally

• When Br₂ approaches an alkene's double bond, the high electron density repels the electron pair in Br-Br

• This causes the Br atom closest to the double bond to become partially positive (δ+), while the other Br atom becomes partially negative (δ-)

The polarity of a Br₂ molecule

• In an addition reaction:

  • The closest Br atom acts as an electrophile and accepts a pair of electrons from the C=C bond in the alkene

  • The Br-Br bond breaks heterolytically, forming a Br^- ion

  • This results in the formation of a highly reactive carbocation intermediate which reacts with the Br^- (nucleophile)

• The reaction of ethene with Br₂ forms 1,2-dibromoethane

Electrophilic addition of Br₂ mechanism

Reaction with H₂SO₄

• Alkenes react with concentrated sulphuric acid in the cold to produce alkyl hydrogensulphates

  • Ethene reacts to give ethyl hydrogensulphate

• Concentrated sulfuric acid adds across the double bond

• The hydrogen atom in sulfuric acid has a partial positive charge so a sulfuric acid molecule acts as electrophile

The polarity of a H₂SO₄ molecule

• In electrophilic addition:

  • The partially positive (δ+) hydrogen atom acts as an electrophile

  • It is attracted to the high electron density of the C=C double bond in the alkene and accepts a pair of electrons

  • The H-O bond breaks heterolytically, forming a hydrogensulfate ion, HSO₄^-

  • A highly reactive carbocation intermediate is formed which reacts with the HSO₄^-

Electrophilic addition of H₂SO₄ mechanism

• The product formed reacts with water to form an alcohol and sulfuric acid

• Essentially, water adds across the double bond with sulfuric acid acting as a catalyst