C10.1 - Reaction of the alkenes

• Alkenes have a C=C double bond between two of the carbon atoms in their chains. The C=C double bond means that alkenes have two fewer hydrogens compared with alkanes containing the same number of carbon atoms. This makes them unsaturated.
• The general formula for an alkene is CnH2n.
• The C=C double bond can open up to make a single bond, allowing the two carbon atoms to bond with other atoms. This makes alkenes reactive - far more reactive than alkanes.
• Alkenes:

  
• Alkenes burn with a smoky flame. In a large amount of oxygen, alkenes combust completely to produce only water and carbon dioxide. However, there isn't enough oxygen in the air for this, so when you burn them they tend to undergo incomplete combustion.
• Incomplete combustion of alkenes: alkene + oxygen → carbon + carbon dioxide + water
• Complete combustion of alkenes: alkene + oxygen → water + carbon dioxide
• Alkenes react via addition reactions. All alkenes have the functional group C=C so they react in similar ways. Most of the time, alkenes react via addition reactions. The carbon - carbon bond will open up to leave a single bond and a new atom is added to each carbon.

  
• Addition of hydrogen is known as hydrogenation. Hydrogen reacts with the double bond carbons to open up the double bod and form the equivalent, saturated, alkane. The alkene is reacted with hydrogen in the presence of a catalyst.

  
• Steam can react with alkenes to form alcohols. When alkenes react with steam, water is added across the double bond and an alcohol is formed. For example, ethanol can be made by mixing ethene with steam and then passing it over a catalyst.

  
• The conversion of ethene to ethanol is one way of making ethanol industrially. After the reaction has taken place, the reaction mixture is passed from the reactor into a condenser. Ethanol and water have a higher boiling point point than ethene, so both condense whilst any unreacted ethene gas is recycled back into the reactor. The alcohol can then be purified from the mixture by fractional distillation.
• Halogens can react with alkenes. Alkenes will also react in addition reactions with halogens such as bromine, chlorine and iodine. The molecules formed are saturated, with the C=C carbons each becoming bonded to a halogen atom.

  
• The addition of bromine to a double bond can be used to test for alkenes:

  1. When orange bromine water is added to a saturated compound, like an alkane, no reaction will happen and it will stay bright orange.
  2. If it's added to an alkene the bromine will add across the double bond, making a colorless dibromo-compound - so the bromine water is decolorized