Electrophilic Addition of Alkenes (CIE AS Chemistry)

• The double bond in alkenes is an area of high electron density (there are four electrons found in this double bond)
• This makes the double bond susceptible to attack by electrophiles (electron-loving species)
• An electrophilic addition is the addition of an electrophile to a double bond

Electrophilic addition of hydrogen bromide

• A molecule of hydrogen bromide (HBr) is polar as the hydrogen and bromine 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

 Explaining the polarity of a HBr molecule

Due to differences in electronegativities of the hydrogen and bromine atoms, 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 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 of HBr mechanism

Electrophilic addition reaction of HBr and ethene to form bromoethane 

Electrophilic addition of bromine

• Bromine (Br₂) is a non-polar molecule 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

The polarity of a Br₂ molecule

 

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)

Electrophilic addition of Br₂ mechanism

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