Edexcel A Level Chemistry:复习笔记3.3.4 Electrophilic Addition Mechanism

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

3.2-Hydrocarbons-Polarity-HBr

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)

3.2-Hydrocarbons-Electrophilic-Addition-HBr

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

3.2-Hydrocarbons-Polarity-Br2_1

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)

3.2-Hydrocarbons-Electrophilic-Addition-Br2_1

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

Exam Tip

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.

 

 

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