Here I share my knowledge about the topic "Free radical stability and detection"

1. Free Radical Stability and
Detection
SRINITHI G
23CHE44
I.M.Sc CHEMISTRY
SCHOOL OF CHEMISTRY
MADURAI KAMARAJ UNIVERSITY

2. CONTENTS
 INTRODUCTION
 STABILITY OF RADICALS
 Inductive effect
 Hyper conjucation effect
 Resonance effect
 DETECTION OF FREE RADICALS
 Magnetic susceptibility
 ESR technique
 Spin trapping technique

3. INTRODUCTION
• Free radicals are formed in the organic reactions
when homolytic cleavage of bond takes place.
• In general, the free radicals are formed in the
reactions which occur in the presence of light or at
high temperature or in the presence of organic
peroxides such as benzoylperoxide.
• Free radicals are the species with a single electron
• They are electron deficient in nature (a species with
single electron always tends to pair up with another
electron and thus, looks for an electron rich site).

4. Characteristic features of free radicals
• A carbon free radical is sp2 hybridized with a p
orbital carrying single unpaired electron.
• A free radical has a planar geometry.
• The carbon in a radical is trivalent and has seven
electrons (septet).

5. • A radical, once formed, reacts immediately to
extract another radical from a bond and thus,
results in generation of new radical.
• For this reason, the reactions, which involve
free radicals as intermediates, are chain
reactions.
• Such reactions terminate only when two free
radicals combine with each other.

6. Stability of free radicals
• A free radical may be 1o, or 2o, or 3o, depending upon
the number of carbons attached to carbon carrying single
electron.
• Further, the free radicals may be categorised as alkyl,
allyl or benzyl radicals.
• For example
• The stability aspects of different categories of free radicals
may be explained through inductive, hyperconjugation and
resonance effect.

7. Stability of alkyl free radicals:
Explanation through inductive effect
• Free radicals are electron deficient species where
carbon carries a single unpaired electron.
• The alkyl groups release electrons through inductive
effect (+I effect).
• More the number of alkyl group attached to a carbon
radical, more is the availability of electrons and more
is the stabilization of free radical.

8. • This electron deficiency in carbon radicals is
compensated to maximum in 3o radicals because of
the presence of three electron releasing groups.
• Thus, the order of stability of methyl substituted free
radicals is as
• follows:

9. Radicals are stabilized by electron-
withdrawing groups
• when a radical centre finds itself next to an electron-
withdrawing group.
• Groups like C=O and C≡N are electron withdrawing because
they have a low-lying empty π* orbital.
• By overlapping with the (usually p) orbital containing the
radical (the SOMO), two new molecular orbitals are generated
• One electron (the one in the old SOMO) is available to fill the
two new orbitals.
• It enters the new SOMO, which is of lower energy than the old
one, and the radical experiences stabilization because this
electron drops in energy.

10. Radicals are stabilized by electron-
withdrawing groups

11. • Electron-rich groups, such as RO groups, in a similar way.
• Ether oxygen atoms have relatively high-energy filled n
orbitals,
• Their lone pairs interacting this with the SOMO again gives
two new molecular
• Three electrons are available to fill them.
• The SOMO is now higher in energy than it was to start with,
but the lone pair is lower.
• Because two electrons have dropped in energy and only one
has risen, there is an overall stabilization of the system, even
though the new SOMO is of higher energy than the old one.
Radicals are stabilized by electron-Donating
groups

12. Radicals are stabilized by electron-Donating
groups

13. Stability of alkyl free radicals:
Explanation through hyperconjugation
• The stability of free radicals may be explained not
only through inductive effect but also through
hyperconjugation.
• The C–H σ bond which is in conjugation with the p
orbital carrying a single electron, participates in
delocalization.
• For example, in case of ethyl radical (CH3 H2), the
three C–H σ bonds (of CH3 group) are in conjugation
with the p orbital on CH2 (carrying a single electron),
as shown below:

14. Hyperconjugation

15. Hyperconjugation
• In (CH3)3C•, there are nine C–H σ bonds which participate in
delocalization with p orbital of the carbon carrying single
unpaired electron thus, nine contributing structures can be
obtained.
• In case of (CH3)2CH•,there are only six C–H σ bonds
available for participation in delocalization and only six
contributing structures are possible.
• There are three C–H σ bonds in CH3CH2•, available for
participation in delocalization and only three contributing
structures are possible.

16. • Thus, (CH3)3C• is more stable than (CH3)2CH• which in turn
is more stable than CH3CH2•. The CH3• is least stable as
there is no C–H σ bond available for participation in
delocalization with p orbital of the carbon carrying single
unpaired electron.
• Thus, the overall stability of free radicals can be given as
(CH3)3C• > (CH3)2C•H > CH3C•H2 > C•H3

17. Stability of allyl and benzyl radicals:
Explanation through Resonance
• The allyl and benzyl radicals are 1° free radicals but in
comparison to 1° alkyl radicals the allyl and benzyl radicals
are more stable.
• The stability of these radicals is attributed to resonance effect.
• In both the cases the p orbital, on carbon, carrying a single
electron is in conjugation with π bond and thus delocalization
occurs through p-π overlap.

18. • The benzyl radical is more stable compared to allyl
radical, because in radical, number of contributing
structures are more as compared to those in allylic
radical.
• Due to same reasons the diphenylmethyl radical (6
contributing structures) and triphenylmethyl radical
(9 contributing structures) are more stable compared
to benzyl radical (3 contributing structures).

19. • Due to resonance effect, the allyl and benzyl radicals are more
stable than 1o, 2o, or 3o alkyl free radicals.
• Thus, we can summarize the stability order of free radicals
discussed so far, as follows:
Benzylic > allylic > 3o > 2o > 1o > C•H3

20. Detection of free radicals
• The single electron have spinning motion and can be produced
magnetic field and magnetic momentum.
• While the single paired electrons can cancel the effects of each
other and cannot produced any magnetic field and magnetic
momentum.
• Therefore the free radicals can be detected by following
methods:
1. Magnetic susceptibility measurement
2. ESR Technique
3. Spin trapping technique

21. Magnetic susceptibility measurement
• The magnetic properties of free radical provide a
powerful tool for their detection and study.
• Detection of free radical associated with spin of
electron in magnetic field which can be expressed in
quantum number.
Quantum number of +1/2 and -1/2
• According to Pauli principle : any two electron
occupying the same orbital must have opposite spin
so that their magnetic momentum is zero for any
species in which all the electrons are paired.

22. Magnetic susceptibility
• Molecules with even numbers of paired electrons are
diamagnetic; i.e., they are slightly repelled by magnets
• Free radicals, however, are paramagnetic (attracted by
magnet) because of the spin of odd electron, the spins of
remaining paired electrons effectively cancelling each
other.
• Therefore in free radicals due to the presence of one or
more unpaired electrons there is net magnetic moment
and the species is paramagnetic.
• Therefore the free radicals can be detected by the
magnetic susceptibility.

23. Electron Spin Resonance spectroscopy
• The electron spin resonance spectra of free radicals
provide another technique for their detection and
study.
• A much more important technique is ESR this
method is similar to NMR technique except the
electron spin is involved rather than nuclear spin.
• The two electron spin states : +1/2 and -1/2
• ESR spectrum can detect both the presence of free
radical and their concentration.

24. • These two spin states are ordinarily of equal energy
but in magnetic field is applied the energies are
different and the electrons are excited from the lower
state to higher state by the absorption of appropriate
frequency signals.
• If two electrons are paired in orbital have opposite
spin cancel the effect of each other and hence ESR
spectrum arises only for a species that have one or
more unpaired electrons.
• Since the free radical only gives the ESR spectrum.
• ESR have been observed for the free radical with life
time considerably less than 1 second.

25. ESR spectrum of methyl radical

26. Spin Trapping Technique
• In case of failure of ESR technique another technique
is used called spin trapping technique for detection
and involvement of free radical.
• Spin trapping agent in this technique a compound is
added to reaction medium which combine to reactive
free radical to produced more persistent radical
which can be observed in ESR technique.

27. Spin trapping
• Free radicals are trapped by trapping agent and then
detected by ESR spectrum.
Trapping agent + free radical reaction ESR detector
• Example : azulenyl nitrone is used as spin trapping
reagent the most important spin trapping agents are
nitroso compounds, nitrosyl compounds which react
with to give stable nitroxide radicals which can be
determined by the ESR technique.

28. Examples