free radicals basics

free radicals basics

1. Detection

   • Free radical scavenger
     • Addition = inhibition
   • EPR spectroscopy
     • up/down orbital energy is different in a magnetic field
       • exposed to microwave radiation with energy = energy difference between orbitals
         • Unpaired electron (spin up or down) will resonate between two orbitals
           • microwave absorbance with a frequency corresponding to the energy gap created by the magnetic field
             • microwave frequency expressed as a fraction of magnetic field strength
           • Resonance frequency depends on the nature of the free radical when magnetic field strength is fixed
     • Electron Paramagnetic Resonance or Electron Spin Resonance
     • Easiest in condensed phase ( [FR] is higher)
   • Difficult because FRs are short lived, especially reactive ones - low concentration

2. Chlorine and Bromine

   • Cl has 7 electrons
     • Cl2 --> 2Cl*
       • Initiated by UV light
     • (Bromine behaves the same way)

3. Oxygen

   • Diatomic oxygen (O2)
     • 12 electrons, 2 unpaired in pi*x and pi*y orbitals
       • paramagnetic (but does not behave as a FR)
       • 3 peaks on EPR spectrum
         • 'triplet oxygen'
     • Excitation by UV
       • one pi* orbital is full, the other empty. no unpaired electrons.
         • 'singlet oxygen'
         • higher energy than triplet oxygen but transition back is slow. strongly forbidden.
         • reactive. t1/2 in gas about one hour
   • Ozone (O3)
     • not paramagnetic
       • Resonant structure
         • Split by UV. O3 --> O2 + O*
     • not a free radical
     • less stable than O2
   • Peroxides O2 (2-)
     • acid form: hydrogen peroxide. H2O2
       • can accept two electrons to form 2OH(-)
         • strongly exothermic
     • alkyl peroxides RO2H
     • Not FRs but strong oxidisers
   • Hydroxyl radicals OH*(-)
     • H2O --> H* + OH*
       • normally when a FR attacks H2O in the atmosphere
     • Highly reactive
     • Fenton reaction
   • Peroxy radicals ROO*
     • Formation
       • ROOH --> ROO* + H*
       • R* + O2 --> ROO*
         • O2 has two unpaired electrons. One joins with R and the other remains free
   • O (2-)
     • 6 electrons
       • requires 2 extra electrons for a stable octet
       • acquires 1 electron to become an oxygen radical anion *O-
       • 2 unpaired electrons (biradical)
         • highly reactive

4. Nitrogen compounds

   • nitrous oxide N2O
     • not a free radical
     • N=N=O
     • very stable
   • nitric oxide NO*
     • free radical
       • Similar to triplet oxygen but only one of the pi* orbitals is occupied
       • fairly reactive
         • half life of seconds (biological) to hours (atmospheric)
     • signalling molecule
       • important in tropospheric FR reactions
   • Nitrogen dioxide NO2
     • structure resemble ozone
       • O=N(+)-O(-)
       • one less electron so one electron is unpaired
         • paramagnetic
         • strong EPR signal
         • Not very reactive - not considered a FR although it can behave as one
           • acts as a free radical scavenger - its concentration is high because of its stability
           • as a FR: NO2 + *OH --> HNO3
     • low energy photon (around 400nm) capable of exciting an electron
       • reddish brown
       • NO2 -->NO* + O**
       • NO* + O3 --> NO2 + O2
         • main route by which NO2 is produced in the troposphere
           • brown smog

5. Kinetics

   • 3 stages
     • Initiation
       • reactive FR created
       • Often photochemical
         • usually follow 1st order kinetics
           • dependent on the amount of light in the spectral region absorbed by the initiator
             • usually UV
               • dependent on altitude
     • Propagation
       • Free radical + ordinary molecule --> free radical
       • majority are linear reactions. (one FR produced per FR)
         • branched chains lead to explosions
       • usually fast
     • Termination
       • FR -> unreactive species
       • often when two FRs react together
       • Usually second order kinetics
         • rate depends on [free radical]^2 or [product] if 2 different FRs react
           • [FR] is low so termination rates are low
             • stable FR like NO can reach higher concentrations = good terminator
   • Dependent on: number of conversion steps initiated per unit time & the length of each chain
     • Ratio of initiation to termination
     • number of propagation steps per initiation
       • typically 100s to 1000s in atmosphere