AQA AS Chemistry

Revision Notes

3.4.3 Reactions of Alkenes

Alkenes: Reactions

  • Alkenes are very useful compounds as they can undergo many types of reactions
  • They can therefore be used as starting molecules when making new compounds

Hydrocarbons Reactions of Alkenes, downloadable AS & A Level Chemistry revision notes

Electrophilic addition

  • Electrophilic addition is the addition of an electrophile to a double bond
  • The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms
  • Electrophilic addition reactions include the addition of:
    • Steam (H2O (g))
    • Hydrogen halide (HX)
    • Halogen

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

The diagram shows an overview of the different electrophilic addition reactions alkenes can undergo

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 share equally  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.

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