Alkenes are unsaturated: they contain a C=C double bond, so more atoms can be added to the molecule. Their typical reaction is addition, in which two reactant molecules join together to form a single product.During addition the weaker π bond of the C=C breaks, and two new σ bonds form, one to each of the carbon atoms that were double bonded. The product is saturated. Because every atom of the reactants ends up in the one product, an addition reaction has an atom economy of 100%.The general pattern for an alkene reacting with a molecule X-Y is:C=C + X-Y → X-C-C-Y The Edexcel specification covers these addition reactions of alkenes:• hydrogen (nickel catalyst) → alkane
• halogens → dihalogenoalkane
• hydrogen halides → halogenoalkane
• steam (acid catalyst) → alcohol
• potassium manganate(VII) in acid solution → diol
The last is an oxidation: two OH groups are added across the double bond. For example, ethene gives ethane-1,2-diol, HOCH2CH2OH, and the purple manganate(VII) solution turns colourless. This colour change can also be used to show that a C=C bond is present.Each of the other reactions is covered in its own lesson.
• halogens → dihalogenoalkane
• hydrogen halides → halogenoalkane
• steam (acid catalyst) → alcohol
• potassium manganate(VII) in acid solution → diol
The last is an oxidation: two OH groups are added across the double bond. For example, ethene gives ethane-1,2-diol, HOCH2CH2OH, and the purple manganate(VII) solution turns colourless. This colour change can also be used to show that a C=C bond is present.Each of the other reactions is covered in its own lesson.