Properties, method of formation, stability, applications, types of reaction mechanisms, radical inhibitors, method of detection of free radicals

1. FREE
RADICALS
FROM,
VIKAS R. MATHAD
I M.PHARM.
DEPT. OF PHARMACHEMISTRY
SJMCP
TO,
H S BASAVARAJ SIR
DEPT. OF PHARMACHEMISTRY
SJMCP

2. CONTENTS
• DEFNITION
• PROPERTIES
• STRUCTURE OF FREE RADICALS
• METHOD OF FORMATION OF FREE RADICALS
• STABILITY OF FREE RADICALS
• APPLICATIONS OF FREE RADICALS
• TYPES OF REACTION MECHANISMS
• RADICAL INHIBITORS
• METHOD OF DETECTION OF FREE RADICALS

3. DEFNITION
• Free radicals are atoms, molecules or ions with unpaired electrons in
outer shell configurations.
• Free radicals may have positive, negative or zero charge.
• Unpaired electrons cause radicals to be highly reactive.
• Radicals are believed to be involved in degeneration diseases &
cancers.

4. PROPERTIES
• In gaseous state free radicals are easily formed.
• Most of them are formed in non polar solvents.
• Short lived radicals are difficult to isolate, because they are highly
reactive.
• Long lived radicals are stable, because they exist in equilibrium with
normal compound.
STRUCTURE OF FREE RADICALS

5. METHOD OF FORMATION OF FREE
RADICALS
The homolytic cleavage of covalent bonds produces radicals. Weaker
covalent bonds dissociate into radicals more than stronger covalent
bonds.
1) Thermal reaction
2) Photochemical reaction
3) Redox reaction

6. 1) Thermal reaction
At temperatures greater than 500°C & in the absence of oxygen,
mixtures of high molecular weight alkanes break down into smaller
alkane & alkene fragments. It is also called thermal cracking. It is
important in the refining of crude petroleum because of the demand for
lower boiling gasoline fractions.
a) Peroxides

7. b) Azo compounds

8. 2) Photo chemical reaction
Compounds having absorption bands in the visible or near violet
spectrum may be electronically excited to such a degree that weak
covalent bonds undergo homolyses. Examples include the halogens Cl₂,
Br₂, & I₂, alkyl hypo chlorides, nitrites, esters, & ketones.
3) Redox reactions
The action of inorganic oxidizing & reducing agents on organic
compounds may involve electron transfers that produce radical ionic
species.

9. a) Kolbe’s Reaction
The electrochemical oxidative decarboxylation of carboxylic acid salts that to radicals, which
demerize. It is best applied to the synthesis of symmetrical dimers.
The reaction mechanism involves a two-stage radical process, electrochemical decarboxylation gives
a radical intermediate, then two such intermediates combine to form a covalent bond.

10. b) Fenton’s reagent
Ferrous ion catalyses the decomposition of hydrogen peroxide &
organic peroxides.

11. STABILITY OF FREE RADICALS
Free radicals are highly reactive, they reacts themselves or with other compounds.
1) Bond dissociation energy
The bond dissociation energy is correlated to free radical stability. Low bond dissociation energies reflect the
formation of stable free radicals & high bond dissociation energies reflect the formation of unstable free
radicals.
Small energy is required for homolytic cleavage for more stable radical

12. This graph is plotted with bond dissociation energy on the Y-axis v/s various radicals on X-axis. This
graph represents the energy required for the bond dissociation for the formation of free radical in
simple methane radical is higher compared to energy required for formation of tertiary radical.
• More the bond dissociation energy, lesser will be the stablility of the radical.
• Order of stability
3° Radical > 2° Radical > 1° Radical > Simple Radical
(more (less
stable) stable)

13. 2) Hyper conjugation
The more alkyl substituents a radical carbon atom possesses, the more stabilized it becomes from
hyperconjugation.
The stability of radicals is increased by aromatic substituents at the radical carbon atom. The central radical
carbon atom of the triphenylmethyl radical, carries three phenyl groups. Therefore the radical is highly
stabilized.
Unstable charges on molecules are dispersed over structure or due to presence of hydrogen attached to radical
carbon atom stabilizes the molecule.
• Order of stability
3° Radical > 2° Radical > 1° Radical > Simple Radical
(more (less
stable) stable)

14. 3) Inductive effect
Radical with a electron releasing group is more stable than that of the
radical with electron withdrawing group.
Free radicals adjacent to an electron withdrawing group are less stable,
since electron density is being taken away from what is already an
electron deficient species (CF₃ or CN)

15. 4) Presence of double bond

16. More the phenyl groups or double bonds more the stability of the compound.
• Order of stability

17. 5) Presence of hetero atoms
• The more electronegative element has the least stable free radical.
• Order of stability
Methyl Radical > Amine Radical > Hydroxy Radical > Fluorine Radical
(more (less
stable) stable)

18. APPLICATIONS OF FREE RADICALS
Markovnikov’s Rule
The rule states that with the addition of a protic acid HX an asymmetric alkene, the acid hydrogen becomes
attached to the carbon with more hydrogen substituents & the halide (X) group becomes attached to the carbon
with more alkyl substituents.
OR
The rule can be stated that the hydrogen atom is added to the carbon with the greatest number of hydrogen
atoms while the X component is added to the carbon with the least number of hydrogen atoms.

19. Mechanism

20. Anti-Markovnikov’s Rule
Mechanism that do not involve a carbocation intermediate may react through other
mechanisms that have other regioselectivities not dictated by Markovnikov’s rule,
such as free radical addition. Such reactions are said to be Anti-Markovnikov’s,
since the halogen adds to the less substituted carbon, the opposite of a
Markovnikov’s reaction.
Here the halogen attacks the carbon with more number of hydrogen atoms across the
double bond where as hydrogen will attacks the carbon with less number of
hydrogen atoms across the double bond.

21. Mechanism

22. DIFFERENCES BETWEEN MARKOVNIKOV’S &
ANTI- MARKOVNIKOV’S REACTION
Markovnikov’s reaction
• Halide groups attacks the carbon with less
hydrogen across double bond where as hydrogen
will attack the carbon with more number of
hydrogens across the double bond.
• Formation of primary & secondary carbocation.
• Ionic reaction.
• Rearrangement of primary carbocation to
secondary carbocation.
• Product is 2-bromopropane
Anti- Markovnikov’s reaction
• Halogen attacks the carbon with more hydrogen
across double bond where as hydrogen will attacks
the carbon with less number of hydrogen across
double bond.
• Formation of primary & secondary radicals.
• Free radical reaction.
• Rearrangement of primary radical to secondary
radical.
• Product is n-bromopropane

23. TYPES OF REACTION MECHANISMS
1) Free radical halogenation
Step 1: Initiation
Separation of halogen into two radicals by the addition of UV light.
Step 2: Propagation
The first step is followed by propagation directly involved in the formation of the product. Isobutane will be
used in chlorination reaction. First step is abstraction of the hydrogen atom from the tertiary carbon & forms the
tertiary radical.
The tertiary radical then reacts with another one of the chlorine molecule to form the product. Notice that
another chlorine radical is regenerated, so this reaction can go on forever as long as there are reagents.

24. Step 3: Termination
Side reaction that can stop the chain reaction are called termination steps. These termination steps
involve the destruction of the radical intermediates, typically by two of them coming together.

25. 2) Free radical addition reaction
Anti-Markovnikov’s radical addition of haloalkene can only happen to HBr and there must be presence of
hydrogen peroxide. Hydrogen peroxide is essential for this process, as it start of the chain reaction in the
initiation step.
Step 1: Initiation
Hydrogen peroxide is an unstable molecule under heat or sun light forms two free radicals of OH. This OH
radical will go on and attack HBr which will take the hydrogen and create a bromine radical.
Step 2: Propagation
The bromine radical will go on & attack the less substituted carbon of the alkene. This is because after the
bromine radical attack the alkene & it is bonded to the less substituted carbon & a carbon radical is formed &
the radical will be formed at the more substituted carbon due to induction & hyperconjugation.
After formation of carbon radical , It will go on & attack the hydrogen of a HBr, by which a bromine radical
will be formed again.

26. 3) Free radical rearrangement reaction
Radical 1,2-rearrangement
The first radical 1,2-rearrangement reported by Heinrich Otto Wieland in 1911 was the conversion of
bis(triphenylmethyl)peroxide (1) to the tetraphenylethane(2).
The reaction proceeds through the triphenylmethoxyl radical (A), a rearrangement to diphenylphenoxymethyl
(C) & it’s dimerization. In this cyclohexadienyl radical intermediate (B) is a transition state or may be a reactive
intermediate.

27. 4) Hunsdiecker reaction
The Hunsdiecker reaction is the organic reaction of silver salts of carboxylic acids with halogens to give
organic halides. It is an example of halogenation reaction.
Mechanism
The reaction mechanism of the Hunsdiecker reaction is believed to involve organic radical intermediates. The
silver salt of the carboxylic acid 1 will quickly react with bromine to form the acyl hypohalite intermediate 2.
Formation of the diradical pair 3 allows for radical decarboxylation to form the diradical pair 4, which will
quickly recombine to form the desired organic halide 5. The yield of halide is primary>secondary>tertiary.

28. 5) Free radical polymerization
Free radical polymerization is catalysed by organic peroxides or other reagents which decompose to give free
radicals.
Step 1: Initiation
Organic peroxides undergo homolytic fission to form free radicals.
Step 2: Propagation
Free radical produced in the above step adds to an alkene molecule to form a new free radical.
This free radical can attack another alkene molecule and so on.

29. Step 3: Termination
The above chain reaction can come to an halt in two ways.
a) Chain combination
Two chains can combine at their propagating sites.
b) Disproportionation
Two chains undergo disproportionation, with one chain being oxidized to an alkene and the other being reduced
to an alkane as a result of hydrogen atom transfer.
Other polymers that can be produced by free radical chain polymerization and poly(vinyl chloride) and
polystyrene.

30. RADICAL INHIBITORS
• Radical reaction inhibitors or simply radical inhibitors are those compounds that
are capable of removing chain-carrying molecules and thereby terminating the
radical chain reaction.
• Radical reactions can be slowed or stopped by the presence of compounds called
Radical Inhibitors.
• An inhibitor combines with the free radical to form a stable molecule.
• Without an inhibitor, each initiation step will cause a chain reaction so that many
molecules will react.
• Vitamin E & Vitamin C are thought to protect living cells from free radicals.
• When an inhibitor reacts with the radical, it creates a stable intermediate, and any
further reactions will be endothermic and slow.
• Hydroquinone, oxygen & phenothiazine are some examples of radical inhibitors.
• Oxygen molecules can exist in the form of a diradical, which reacts readily with
other radicals.

31. • Hydroquinone is also often used as a radical inhibitor.
• Hydroquinone is a compound that contains a benzene ring attached to two
hydroxyl groups on opposing ends. The carbon-hydrogen bond one each hydroxyl
group interacts with a radical to from an intermediate, which then rearranges its
electrons to form a non-radical compound. In this reaction, the final product does
not contain any chain-carrying radicals and therefore terminates the radical
reaction.

32. METHOD OF DETECTION OF FREE
RADICALS
• Lead mirror is deposited on the inside wall of a glass tubes. These
mirrors are disappeared when attacked by free radicals. So by varying
distance of mirrors from the source of free radical generation &
velocity of carrier inert gas, free radicals can be detected.
• Several radical are coloured or produce colour reaction which can be
detected by colorimetry.
• Magnetic field is used to detect the free radicals.

33. THANK YOU