3.3.3 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

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)

• 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 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 ion

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