708051
free radicals basics
- Detection
- Free radical scavenger
- Addition = inhibition
- 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
- 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
- microwave absorbance with a frequency corresponding to
the energy gap created by the magnetic field
- Unpaired electron (spin up or down)
will resonate between two orbitals
- exposed to microwave radiation with energy = energy difference between orbitals
- Electron Paramagnetic Resonance
or Electron Spin Resonance
- Easiest in condensed phase ( [FR] is higher)
- up/down orbital energy is different in a magnetic field
- Difficult because FRs are short lived,
especially reactive ones - low concentration
- Free radical scavenger
- Chlorine and Bromine
- Cl has 7 electrons
- Cl2 --> 2Cl*
- Initiated by UV light
- Initiated by UV light
- (Bromine behaves the same way)
- Cl2 --> 2Cl*
- Cl has 7 electrons
- 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'
- 'triplet oxygen'
- paramagnetic (but does not behave as a FR)
- 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
- 'singlet oxygen'
- one pi* orbital is full, the other empty.
no unpaired electrons.
- 12 electrons, 2 unpaired in pi*x and pi*y orbitals
- Ozone (O3)
- not paramagnetic
- Resonant structure
- Split by UV. O3 --> O2 + O*
- Split by UV. O3 --> O2 + O*
- Resonant structure
- not a free radical
- less stable than O2
- not paramagnetic
- Peroxides O2 (2-)
- acid form: hydrogen peroxide. H2O2
- can accept two electrons to form 2OH(-)
- strongly exothermic
- strongly exothermic
- can accept two electrons to form 2OH(-)
- alkyl peroxides RO2H
- Not FRs but strong oxidisers
- acid form: hydrogen peroxide. H2O2
- Hydroxyl radicals OH*(-)
- H2O --> H* + OH*
- normally when a FR attacks H2O in the atmosphere
- normally when a FR attacks H2O in the atmosphere
- Highly reactive
- Fenton reaction
- H2O --> H* + OH*
- Peroxy radicals ROO*
- Formation
- ROOH --> ROO* + H*
- R* + O2 --> ROO*
- O2 has two unpaired electrons. One
joins with R and the other remains free
- O2 has two unpaired electrons. One
joins with R and the other remains free
- ROOH --> ROO* + H*
- Formation
- 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
- highly reactive
- requires 2 extra electrons for a stable octet
- 6 electrons
- Diatomic oxygen (O2)
- Nitrogen compounds
- nitrous oxide N2O
- not a free radical
- N=N=O
- very stable
- not a free radical
- 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)
- half life of seconds (biological)
to hours (atmospheric)
- Similar to triplet oxygen but only
one of the pi* orbitals is occupied
- signalling molecule
- important in tropospheric FR reactions
- important in tropospheric FR reactions
- free radical
- 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
- acts as a free radical
scavenger - its concentration
is high because of its stability
- paramagnetic
- O=N(+)-O(-)
- 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
- brown smog
- main route by which NO2 is produced in the troposphere
- reddish brown
- structure resemble ozone
- nitrous oxide N2O
- 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
- dependent on altitude
- usually UV
- dependent on the amount of light
in the spectral region absorbed
by the initiator
- usually follow 1st order kinetics
- reactive FR created
- Propagation
- Free radical + ordinary molecule --> free radical
- majority are linear reactions.
(one FR produced per FR)
- branched chains lead to explosions
- branched chains lead to explosions
- usually fast
- Free radical + ordinary molecule --> free radical
- 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
- stable FR like NO can reach higher concentrations
= good terminator
- [FR] is low so termination rates are low
- rate depends on [free radical]^2 or [product] if 2 different FRs react
- FR -> unreactive species
- Initiation
- 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
- typically 100s to 1000s in atmosphere
- Ratio of initiation to termination
- 3 stages
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