The Mechanisms of Electrophilic Addition Reactions (HL) (DP IB Chemistry): Revision Note

The mechanisms of electrophilic addition reactions

Electrophilic addition

• Electrophilic addition is a reaction in which an electrophile (or Lewis acid) adds across the C=C double bond of an alkene

• The C=C bond is an area of high electron density

  • This makes it susceptible to attack by electrophiles

• The π bond in the double bond breaks, forming:

  • A single C–C bond

  • Two new σ-bonds from each carbon atom

• Electrophilic addition reactions include the addition of:

  • Hydrogen, H₂ (g) - catalytic hydrogenation

  • Steam, H₂O (g) - hydration of alkenes

  • Hydrogen halides, HX - formation of halogenoalkanes

  • Halogens, X₂ - formation of dihalogenoalkanes

Addition of steam (H₂O)

• Water is a weak electrophile

• Water does not readily react with alkenes without a strong acid catalyst

  • H₃O⁺ acts as the electrophile

• The electrophilic addition of water occurs in two steps:

  • Step 1

    • The π electrons in the C=C attack a proton from H₃O⁺

    • Heterolytic fission occurs and a carbocation is formed

  • Step 2

    • Water donates a lone pair of electrons to the carbocation 

    • This forms a protonated alcohol (oxonium ion)

    • An equilibrium is established between the protonated alcohol and the alcohol

    • This regenerates the H₃O⁺ catalyst

Electrophilic addition of H₂O mechanism

Addition of hydrogen halides (HX)

• Hydrogen halides such as HBr are permanently polar

  • This is due to the electronegativity difference between H and the halogen

• In a hydrogen bromide molecule, the bromine atom has a stronger pull on the electrons in the H-Br bond

• This causes:

  • The Br atom to have a partial negative charge

  • The H atom to have a partial positive charge

• The electrophilic addition of hydrogen halides occurs in two steps:

  • Step 1

    • The π electrons in the C=C attack the δ⁺ hydrogen atom in HBr

    • Heterolytic fission occurs forming a carbocation intermediate and a bromide ion

  • Step 2

    • The bromide ion donates a lone pair of electrons to the carbocation

    • This forms the halogenoalkane product

Addition of halogens (X₂)

• Halogen molecules like Br₂ are non-polar

• They become polarised when they approach the high electron density of a C=C bond

  • This induces a temporary dipole in the Br–Br bond

  • One Br atom becomes slightly δ⁺ and the other δ⁻

• The electrophilic addition of bromine occurs in two steps:

  • Step 1

    • The π electrons in the C=C attack the δ⁺ bromine atom in Br₂

    • Heterolytic fission occurs forming a brominated carbocation intermediate and a bromide ion

  • Step 2

    • The bromide ion donates a lone pair of electrons to the carbocation

    • This forms the 1,2-dibromoalkane product