CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.2.10 Electrophilic Addition of Alkenes

Alkenes: Electrophilic Addition

  • 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

 

Hydrocarbons Polarity HBr, downloadable AS & A Level Chemistry revision notes

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)

 

Hydrocarbons Electrophilic Addition HBr, downloadable AS & A Level Chemistry revision notes

Example of an electrophilic addition reaction of HBr and propene to form 1-bromopropane and 2-bromopropane

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

 

Hydrocarbons Polarity Br2, downloadable AS & A Level Chemistry revision notes

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)

 

Hydrocarbons Electrophilic Addition Br2, downloadable AS & A Level Chemistry revision notes

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.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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