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708051
free radicals basics
Description
lecture 1
Mind Map by
ilovesoil
, updated more than 1 year ago
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Created by
ilovesoil
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Resource summary
free radicals basics
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
Chlorine and Bromine
Cl has 7 electrons
Cl2 --> 2Cl*
Initiated by UV light
(Bromine behaves the same way)
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
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
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
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