Electrophilic Addition Mechanism (Edexcel A Level Chemistry) : Revision Note

Electrophilic Addition Mechanism

Electrophilic addition of hydrogen halides

• Hydrogen halides such as hydrogen bromide (HBr) are polar as the hydrogen and halogen atoms have different electronegativities

• The bromine atom has a stronger pull on the electrons in the H-Br bond

• As a result of this, the Br atom has a partial negative and the H atom a partial positive charge

Due to differences in electronegativities of the hydrogen and bromine atom, HBr is a polar molecule

• In an addition reaction, the H atom acts as an electrophile and accepts a pair of electrons from the C-C bond in the alkene

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

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

Example of an electrophilic addition reaction of HBr and propene to form 1-bromopropane and 2-bromopropane

Electrophilic addition of Halogens

• Halogens such as bromine (Br₂) are a non-polar molecules as both atoms have similar electronegativities and therefore equally share the electrons in the covalent bond

• However, when a bromine molecule gets closer to the double bond of an alkene, the high electron density in the double bond repels the electron pair in Br-Br away from the closest Br atom

• As a result of this, the closest Br atom to the double bond is slightly positive and the further Br atom is slightly negatively charged

Br₂ is a non-polar molecule however when placed close to an area of high electron density it can get polarised

• 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)

Example of an electrophilic addition reaction of Br₂ and ethene to form dibromoethane

Electrophilic addition of water

• Water is a weak electrophile, so does not undergo addition reactions with alkenes unless in the presence of a strong acid which can act as a catalyst

  • H₃O⁺ acts as the electrophile

• The reaction occurs in two steps:

  • Step 1

    • The π electrons in the C=C are attracted to H3O+

    • Heterolytic fission occurs and a carbocation is formed

  • Step 2

    • Water acts as a nucleophile and donates a pair of electrons to the positive carbon atom forming the C-O bond

    • An equilibrium is established between the positive product and the deprotonated product (the alcohol)  

    • The H₃O⁺ is regenerated as the catalyst

This mechanism is catalysed by concentrated acid