alkenes

• alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond
• the chemical reactivity of alkenes is due to the pi electrons being open to attack from electrophiles
• the ways to prepare an alkene

  • dehydration of an alcohol
  • base-induced elimination of hydrogen halides from monohydroalkanes
• during the dehydration of an alcohol to prepare an alkene, the vapour of the alcohol can be passed over hot aluminium oxide, or, the alcohol can be treated with concentrated sulphuric acid or concentrated phosphoric acid
• during the dehydration of an alcohol, the OH- group is removed, along with the H atom on an adjacent carbon atom

  this can be described as an elimination process
• with some alcohols, two or more alkenes can be formed, but with other alcohols such as: butan-1-ol or propan-2-ol, only one alkene can be formed
• base-induced elimination of hydrogen halides from monohaloalkanes occurs when the monohaloalkane is heated under the reflux with ethanoic potassium hydroxide dissolved in ethanol
• during base-induced elimination of hydrogen halides from monohaloalkanes the basic OH- ion initially attacks a H atom on the carbon adjacent to the halogen bearing carbon atom in the haloalkane
• in base-induced elimination of hydrogen halides from monohaloalkanes, a bond is formed between the OH- ion and the H atom and a HO-H molecule is generated
• in base-induced elimination of hydrogen halides from monohaloalkanes, the pair of electrons in the C-H bond moves between the carbon atoms on the left hand side of the haloalkane to form a double bond
• alkenes take part in electrophilic addition reactions with:

  • hydrogen to form alkanes
  • halogens to form dihaloalkanes
  • hydrogen halides to form monohaloalkanes
  • water using an acid catalyst to form alcohol
• the addition of an alkene with:

  • hydrogen is known as hydrogenation and is catalysed by nickel or platinum
  • a halogen is also known as halogenation
  • a hydrogen halide is hydrohalogenation
  • water is known as hydration and is catalysed by acids
• markovnikov's rule

  this states that when a hydrogen halide or water is added to an unsymmetrical alkene, the hydrogen atom becomes attached to the carbon with the most hydrogen atoms already attached
• markovnikov's rule can be used to predict major or minor products formed during the reaction of a hydrogen halide or water with alkenes
• the reaction mechanisms for the addition of a halide and the acid-catalysed addition of water can be represented by curly arrows and showing the intermediate carbocation
• the inductive stabilisation of intermediate carbocations formed during these reactions is used to explain the products formed
• the reaction mechanism for the addition of a halogen can be represented using curly arrows