Electrophilic Addition - Mechanism (Edexcel IAL Chemistry)

Spec point — 5.6

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

Br₂is 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 Br₂and ethene to form dibromoethane

Exam Hint The stability of the carbocation intermediate is as follows:

tertiary > secondary > primary

When more than one carbocations can be formed, the major product of the reaction will be the one that results from the nucleophilic attack of the most stable carbocation.