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

The Mechanisms of Electrophilic Addition Reactions

Electrophilic Addition

• Electrophilic addition is the addition of an electrophile (or Lewis acid) to an alkene double bond, C=C

• The alkene double bond, C=C, is an area of high electron density which makes it susceptible to attack by electrophiles

• The C=C bond breaks forming a single C-C bond and 2 new bonds from each of the two carbon atoms

• Electrophilic addition reactions include the addition of:

  • Hydrogen, H₂ (g)

  • Steam, H₂O (g)

  • Hydrogen halides, HX

  • Halogens, X₂

Different reactions of the alkenes including reagents and conditions

Alkene electrophilic addition reaction overview

Addition of hydrogen halides

• A hydrogen halide molecule is polar as the hydrogen and halogen atoms have different electronegativities

  • For example, in a molecule of hydrogen bromide, HBr, 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 charge and the H atom a partial positive charge

  Partial charges in the H–Br bond

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

• In electrophilic addition reactions with hydrogen halides, the H atom acts as an electrophile and Lewis acid by accepting a pair of electrons from the C=C bond in the alkene

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

• This results in the formation of a highly reactive carbocation intermediate which reacts with the bromide ion, Br^-

• For example, the mechanism for the electrophilic addition of hydrogen bromide and ethene is:

Electrophilic addition mechanism

Electrophilic addition reaction of HBr and ethene to form bromoethane 

Addition of halogens

• The mechanism for the electrophilic addition of halogens (and hydrogen) is the same as the electrophilic addition of hydrogen halides with one key exception:

• Hydrogen halide molecules have a permanent dipole (as shown above)

• Halogen molecules have a temporary (or induced) dipole caused by the repulsion of the halogens electrons by the high electron density C=C bond

The partial charges in a halogen molecule are caused by the high electron density in the C=C bond

The temporary (or induced) dipole in a halogen molecule

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

Electrophilic addition of water

This mechanism is catalysed by concentrated acid