Reactions of Alkenes (Edexcel International AS Chemistry): Revision Note

Alkenes - Reactions

• 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 π bond of the C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms

• Addition reactions of alkenes include the addition of:

  • Hydrogen

    • This is also known as hydrogenation, which is a catalytic addition using a nickel catalyst, not an electrophilic addition

  • Steam (H₂O (g))

  • Hydrogen halide (HX)

  • Halogen

• Conditions and products:

  • Hydrogen with a nickel catalyst and heat gives an alkane

  • Steam with an acid catalyst (H₃PO₄) and heat gives an alcohol

  • Hydrogen halides such as HBr react at room temperature to give a halogenoalkane (propene gives 2-bromopropane as the major product and 1-bromopropane as the minor product)

  • Halogens react at room temperature to give a dihalogenoalkane (ethene and bromine give 1,2-dibromoethane)

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

Oxidation

• Alkenes can also be oxidised by acidified potassium manganate(VII) (KMnO₄) which is a very powerful oxidising agent

• When shaken with cold dilute acidified KMnO₄ the 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(VII) 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