free radicals

• Free radicals definition:

  A chemical species with an unpaired electron
• Formation of Free Radicals

  Through homolytic cleavage, radical chain reactions, UV and visible light, oxidation, and radiation
• Homolytic Cleavage of Hydrogen Peroxide

  H2O2 → 2OH• + H•, breaking the O-O bond unevenly
• Characteristics of Radicals

  Unpaired electron, highly reactive, short lifespan, non-pairing, high-energy state
• Free radicals & halogens

  free radicals can be formed when halogen molecules are exposed to ultraviolet light
• a quantum of uv radiation has enough energy to break the cl-cl bond, this is called initiation
• Propagation:

  • a chlorine radical reacts with methane
  • the methyl free radical then reacts with more molecular chlorine
• What makes free radicals usually highly reactive?


  The unpaired electron is available to form a new covalent bond.
• What is the result of the combination of two radicals?


  It forms a molecule.
• What is the reaction of a radical with a molecule?


  It forms another molecule and another free radical.
• How are free radicals formed?


  By homolysis of a covalent bond.
• What happens during homolysis of a covalent bond?


  The two bonded atoms take the same share in the bond electrons.
• What is the result of the reaction between chlorine and methane in the presence of UV light?


  One or more hydrogen atoms in methane are substituted by a chlorine atom.
• What are the steps involved in the substitution of alkanes by halogens?


  1. Initiation: UV light breaks halogen bonds.
  2. Propagation: Halogen radicals react with alkanes.
  3. Termination: Radicals combine to form stable molecules.
• What is the product of the reaction between CH4 and Cl2?


  CH3Cl and HCl.
• Why is iodine not reactive enough for substitution reactions with alkanes?


  It does not react under the same conditions as chlorine and bromine.
• What is the effect of fluorine in substitution reactions?


  Fluorine is so reactive that reactions can be explosive and difficult to control.
• How does the molar ratio of alkane to halogen affect the products formed?


  The proportions of different halogenoalkanes depend on this ratio.
• What is a disadvantage of producing a mixture of halogenoalkanes in organic synthesis?


  Separation of the components is necessary before the next stage of synthesis.
• What is the nature of homolytic bond fission?


  It is an endothermic process.
• What are mean bond enthalpies used for?


  To compare the energies required to break different covalent bonds.
• What is the formula for the energy associated with one photon of electromagnetic radiation?


  E = hf
• What is Planck's constant?


  1. 626 x 10^-34 Js.
• What is the mechanism of the substitution of alkanes by halogens?


  • Initiation: UV radiation breaks halogen bonds.
  • Propagation: Halogen radicals react with alkanes.
  • Termination: Radicals combine to form stable molecules.
• What is the initiation reaction for chlorine or bromine in free radical substitution?


  X2 → 2X where X = Cl or Br.
• What happens during the propagation stage of free radical substitution?


  The halogen free radical reacts with an alkane, abstracting a hydrogen atom.
• Why is the process of free radical substitution considered a chain reaction?


  Because the free radical product of one reaction enables the next reaction to occur.
• What is the expected ratio of positional isomers formed during mono-substitution of propane?


  A ratio of 3:1 for 1-chloropropane to 2-chloropropane.
• Why is the experimental ratio of isomers different from the expected ratio?


  A secondary free radical is more stable than a primary free radical.
• What is the stability order of free radicals?


  • Tertiary FR > Secondary FR > Primary FR
• What is the termination stage in free radical reactions?


  It is when two radicals combine to form a stable molecule.
• What are the possible termination reactions in free radical reactions?


  X • + X • → X2, C • + X • → C-X, C • + C • → C-C.
• What is the concentration of ozone in the ozone layer?


  1. 10 ppm of ozone (O3).
• What is the role of the ozone layer?


  To absorb most of the Sun's harmful ultraviolet radiation.
• What is the Ozone-Oxygen Cycle?


  It maintains the concentration of ozone in the ozone layer.
• How is ozone formed in the ozone layer?


  By the combination of an oxygen radical with an oxygen molecule.
• What happens during ozone removal?


  Ozone can react with an oxygen radical or absorb lower energy UV radiation.
• What is the significance of the reactions that absorb harmful UV in the ozone cycle?


  They help maintain a constant concentration of ozone in the ozone layer.
• What is the role of nitrogen monoxide in the ozone layer?


  It can cause the loss of ozone by removing oxygen atoms.
• Why are chlorine and bromine free radicals significant in ozone depletion?


  They can catalyse the decomposition of ozone.