Free-radical reactions. Photochemistry

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Free-radical reactions. Photochemistry

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Outline 1. Stability of free radicals 2.
Detection of free radicals 3. Sources of free
radicals 4. Free-radical halogenation 5.
Free-radical oxidation 6. Free-radical
addition 7. Rearrangements of free radicals 8.
Radical fragmentations 9. SRN1 substitution 10.
General principles of photochemistry 11.
Photochemistry of carbonyl compounds 12.
Photochemistry of alkenes and dienes 13.
Chemiluminescence
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1. Stability of free radicals
Normally free radicals are very unstable species
with incomplete electron octets. Radical often
undergo reactions of the following types
Most radicals have a planar or slightly pyramidal
configuration
The Singly Occupied Molecular Orbital can
interact with both occupied and unoccupied
molecular orbitals
Barriers of rotation
Free radicals are stabilized by both electron
donating and electron withdrawing substituents
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For alkyl radicals 3ogt2ogt1o
A few radicals are stable
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2. Detection of free radicals
a. Electron spin resonance (ESR)
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b. Chemically Induced Dynamic Nuclear
Polarization (CIDNP)
Technically it is just a set of normal NMR
spectra, recorded during a free radical reaction.
Free radicals produced mess up Energy levels of
nuclei, causing some signals to increase, some
signals to decrease, and sometimes their
inversion. Example Radical decomposition of
benzoyl peroxide
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c. Chemical traps
3. Sources of free radicals
a. Decomposition of peroxides
b. Decomposition of azocompounds
c. Decomposition of N-nitrosoanilides
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4. Free-radical halogenation (ORA 441)
This reaction is an example of a chain reaction
Chlorine is more active and reacts the same
way, but much less selectively, because the
transition state is farther from the
radical intermediate Iodine is not active
enough to react Fluorine is too active and
breaks carbon-carbon bonds
N-bromosuccinimide (NBS) is a practically
convenient brominating reagent
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5. Free-radical oxidation (ORA 566)
Industrial production of phenol and acetone
Autoxidation of aldehydes
Autoxidation of ethers
Hydroperoxides are explosive!
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6. Free-radical addition (ORA 430)
Anti-Markovnikov hydrobromination
Anti-addition is favored over syn-addition, so
the intermediate radical is better described by
a bridged structure
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More examples of the radical addition
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7. Rearrangements of free radicals
Concerted rearrangements are not
characteristic for free radicals, because their
highest in energy MO is singly occupied.
Lowering in energy of a singly occupied orbital
gives a lesser gain of stability than lowering
of a doubly occupied orbital. Consequence
rearrangements of free radicals usually occur
through an intermediate. Phenyl, vinyl and in a
lesser extent ethynyl substituents tend to
migrate
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More examples of free-radical rearrangements
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8. Radical fragmentations
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Examples
a. Decarboxylation, alkoxy radical fragmentation,
cracking, polymerization
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Cracking (Heating alkanes at high temperatures
gt400 oC breaks up the molecule in many places).
This reaction accounts for one of the largest
scale industrial productions and can be done with
a catalyst or without a catalyst.
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b. Decarbonylation
c. 1,4-Diradical fragmentation
9. SRN1 substitution
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The reaction is often initiated by light and
carried out in Liquid ammonia (to keep the
anion-radicals alive)
Some more examples
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10. General principles of photochemistry
A photochemical reaction starts once the system
crosses the barrier of potential energy in any of
the excited states
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Energy transfers between excited states

• Photoelectron transfer
• (PET, redox-process)

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2. Sensitization (Collision is necessary)
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3. Forster
Mechanism Occurs at large distances when
the wavelength of the donors emission matches
the wavelength of the aceptors absorption.
However, no photon emission is involved
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Diabatic, adiabatic, and hot ground
state photoreactions
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Three methods for photogeneration of reactive
species
One of common singlet photosensitizers is
naphthalene One of common triplet sensitizers is
benzophenone
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Singlet oxygen is produced by sensitized
excitation of triplet (normal) oxygen
Poison of the St. Johns wort and insecticide of
marigolds
Sensitizers for the Photodynamic Therapy of cancer
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11. Photochemistry of carbonyl compounds
Behavior of the carbonyl group upon
photoexcitation
p-gtp excitation occurs faster due to the better
orbital overlap In the n-gtp excited state, the
oxygen is more electron deficient Formation of
the p-gtp excited state is more likely for highly
conjugated carbonyl compounds
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p-gtp states are more emissive than n-gtp states,
which makes possible design of luminescent
indicators of acids.
Chemical behavior of the excited carbonyl group
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Examples of photochemical reactions of carbonyl
compounds
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(No Transcript)
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12. Photochemistry of alkenes and dienes
Photostationary state - is a proportion of
isomers that does not vary upon further
irradiation
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Photochemistry of vision
The RGB cone cells use the same pigment, tuned
through-space with polar amino-acid residues due
to the different stabilization of ground and
excited states
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Concerted reactions are more characteristic for
singlet states, while triplet states tend to
undergo stepwise radical rearrangements.
Di-p-methane (Zimmerman) rearrangement
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Di-p-methane rearrangement may occur through both
singlet and triplet states
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13. Chemiluminescence
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Sensitized chemiluminescence. Chemistry of
colored light sticks.