Edexcel A Level Chemistry:复习笔记3.3.3 Electrophilic Addition

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
  • 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:
    • Hydrogen (also known as hydrogenation reaction)
    • Steam (H2O (g))
    • Hydrogen halide (HX)
    • Halogen

3.2-Hydrocarbons-Electrophilic-Addition-Reactions

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

Manufacture of margarine

  • Hydrogenation is extensively used in industry to manufacture margarine
  • Naturally occurring vegetable oils are unsaturated and contain C=C double bonds
  • When these bonds react with hydrogen, some of the C=C double bonds become C-C single bonds
  • This process changes the properties of vegetable oil and converts it into a solid: margarine

Oxidation

  • Alkenes can also be oxidised by acidified potassium manganate(VII) (KMnO4) which is a very powerful oxidising agent
  • When shaken with cold dilute KMnO4 the pale purple solution turns colourless and the product is a diol
    • This colour change means this reaction can be used, like bromine, to distinguish alkanes from alkenes ( alkanes do not have double bonds and so are not oxidised in this way)

screenshot-2022-03-14-133409

  • Although you do not need to know the full details of the working of this reaction you can think of it as an oxidation followed by an addition
    • The potassium manganate provides an oxygen atom (oxidation)
    • Then water in the solution provides another oxygen atom and two hydrogen atoms, so there is addition of two OH groups across the double bond

Heterolytic Fission

  • Heterolytic fission is breaking a covalent bond in such a way that the more electronegative atom takes both the electrons from the bond to form a negative ion and leaving behind a positive ionscreenshot-2022-03-14-134345

The diagram shows heterolytic fission in which the most electronegative atom takes both electrons in the covalent bond

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