13.4 Electrophilic addition in alkenes

• The high electron density of the pi bond in the carbon-carbon double bond attracts electrophiles
• An electrophile is a partially positive or fully positively charged atom or group of atoms that is attracted to an electron-rich centre and accepts a pair of electrons
• Electrophilic addition is the mechanism of the reaction between an alkene and an electrophile
• Electrophiles "attacks" a region of electron density

  • curly arrows indicates the movement of electrons
  • arrow goes towards the electrophile
  • arrow starts in the middle of the double bond
  • arrow must touch the double bond
• A carbocation is formed where the double bond was
• If electrophilic occurs with a non-polar molecule, the high electron density of the pi bond induces a dipole in the non-polar molecule
• Addition reaction in unsymmetrical alkenes form two products

  • major product is formed from the secondary carbocation
  • minor product is formed from the primary carbocation
• Markownikoff's rule states that when an electrophile H-X reacts with an unsymmetrical alkene, the hydrogen atom attaches to the carbon atom with the greater number of hydrogen atoms
• More stable carbocations leads to a major product
• Carbocations are classified as primary, secondary or tertiary depending on the number of alkyl groups attached to the carbocation
• Alkyl groups "push" electrons towards the positive carbocation giving the structure stability

  • tertiary carbocations are the most stable