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Subject Chemistry
Paper No and Title 5: Organic Chemistry-II
Module No and Title Generation, structure, stability and reactivity of free radicals.
Module Tag CHE_P5_M8
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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TABLE OF CONTENTS
1. Learning outcomes 2. Introduction 3.Generation of free radicals 3.1Thermal cleavage 3.2 Photochemical cleavage 3.3 Decomposition reaction 4. Features of free radical 5. Stability of free radical 5.1 Inductive effect 5.2 Hyperconjugative effect 5.3 Resonance effect 6. Reactions of free radical 7. Summary
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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1. Learning Outcomes
After studying this module, you shall be able to
• Know what are free radicals • Learn about their structure • Known how radicals are generated • Learn the reactivity of radicals
2. Introduction
The species formed as an intermediate in a chemical reaction which have one or more unpaired electron are known as free radical (often called a radical). The unpaired electron is represented by a dot. Life of a radical depends on its stability and conditions of its generation. These species aregenerally short lived, extremely short lived in solution, but can have longerlife time in crystal • • • • •- • lattices of other molecules. Examples of some free radicals are: H , Cl , HO , O2 , O2 , H3C
The free radicals differ from carbocations and carbanions as shown below.
Carbocation carbanion
6 valence electron 7 valence electrons 8 valence electrons
Persistent free radicalshave a long lifetime and they are resistant to dimerization, disproportionation and other routes to self-annihilation, though they may not be very stable.
The triphenylmethyl radical was the first radical to be observed by Gomberg in 1900, although it took 30 more years to know what he had made.It can be generated by the treatment of triphenylmethyl chloride (trityl chloride) with silver. The presence of the trityl radical in solution can be detected by electron spin resonance.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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Other examples are 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) radical, phenalenyl radical, 1,1- diphenyl-2-picrylhydrazyl (DPPH) radical.
TEMPO radical
Phenalenyl radical
DPPH radical
Some examples of stable free radicals are shown below where the shape and the steric hindrance prevents dimerization. The stability may also be increased due to resonance stabilisation or delocalisation.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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Electron Spin Resonance (ESR) to detect radicals
A radical can be detected spectroscopically using electron spin resonance (ESR) technique as an ESR spectrum arises only from species that have one or more unpaired electrons i.e., free radicals. This method can be used to detect the presence of radicals and to determine their concentration. Furthermore, information concerning the electron distribution and hence the structure of free radicals can be obtained from the splitting pattern of the ESR spectrum.
Electrons have a magnetic moment. When electrons are paired they have opposite spins which leads to cancelling of their magnetic moments. Such species with paired electron cannot be detected by ESR. But, species with unpaired electrons have a net magnetic moment and can be detected by ESR.
Certain nuclei have a magnetic moment, such as 1H, 13C, 14N, 19F, and 31P. Interaction of the electron with one or more of these nuclei splits the signal arising from the unpaired electron. The number of lines is given by (2nI+1), where I is the nuclear spin quantum number and n is the number of equivalent interacting nuclei.
Hyperfine splitting of peaks is observed if the carbon bearing radical is attached to proton, due to the interaction of the equivalent hydrogen atoms present with the unpaired electron. For example, . . the signal for CH radical splits into a doublet. The CH3 radical has four signals in its spectrum. . Benzene radical ( C6H6) has seven peaks in ESR spectrum.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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Fig.8.1 ESR of benzene radical
Failure to observe ESR spectrum does not prove that radicals are not involved, since the concentration may be too low for direct observation. In such cases, the spin trapping technique can be used. In this technique, a compound is added that is able to combine with the very reactive radicals to produce more persistent radicals that can be observed by ESR. The most important spin trapping compounds are nitroso compounds, which react with radicals to give fairly stable nitroxide radicals.
3. Generation of Free Radicals
A free radical is formed during a homolytic bond cleavage such that each generated fragment has one electron with it.
The various ways by which free radicals may be generated are:
• Thermal cleavage • Photochemical cleavage • Decomposition reaction
3.1. Thermal cleavage
Free radicals may be generated by thermal cleavage. In the gaseous phase some molecules break down at high temperature. When the molecule contains bonds with 20 to 40 kcal/mol of energy, cleavage can be caused in the liquid phase. For example, the thermal cleavage of azo compounds yields free radical.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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3.2. Photochemical cleavage
The energy of light of 600 to 300 nm is 48 to 96 kcal/mol, which is of the order of magnitude of covalent bond energies. Certain molecules undergo homolytic fission in the presence of light of that particular wavelength. For example, the photochemical cleavage of chlorine molecule results into the formation of chlorine free radicals.
3.3. Decomposition reaction
At times a radical molecule may undergo decomposition to form another free radical. For example, decomposition of benzoxy radical gives phenyl radical and carbondioxide.
4. Features of Free Radical
A free radical has the following characteristic feature: • Free radicals are electron deficient species. • They are usually uncharged. • They contain odd number of electrons.
• The alkyl radical (·CR3) has seven electron around the carbon bearing radical character. • In methyl radical or other alkyl radicals, the radical centre is trivalent and trigonally hybridized.
• The carbon is sp2 hybridized. • It has planar structure.
• The unpaired electron occupies a 2p atomic orbital of carbon. This singly occupied orbital is often referred as singly occupied molecular orbital (SOMO).
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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• As stated by Pauli, that the two electrons occupying same orbital must have opposite spins and thus magnetic moment of such species becomes zero. However, free radicals have a net magnetic moment (paramagnetic) due to the presence of one or more unpaired electrons and thus can be detected by ESR. • These species are highly reactive due to the presence of unpaired electron which gets paired easily with another electron to fill their outer shell. • The alkyl radical have shallow pyramid geometry i.e., between sp2 and sp3 hybridization. But the energy required to invert the pyramid is very small. Practically speaking alkyl radicals are considered sp2 hybridized.
• The order of stability of alkyl radicals is 3° > 2° > 1°.
5. Stability of Free Radical .
Since bond dissociation energies give us an idea of the ease with which radicals can form, they can also give us an idea of the stability of those radicals once they have formed. The lower the bond dissociation energy, the higher will be the stability. Alkyl radicals are stabilized by adjacent lone-pair-bearing heteroatoms and by the π bonds. The various factors responsible for the stability of free radicals are:
• Inductive effect • Hyperconjugative effect • Resonance effect
5.1 Inductive effect
Greater the number of alkyl groups attached to the free radical carbon centre more will be the stability of the radical. This is due to the electron donating inductive effect of the alkyl groups which decrease the electron deficiency of the radical.
The bond dissociation energies, of the C-H bonds for the formation of a free radical of methane, ethane, and other alkanes, clearly shows that radical centres are stabilized by the replacement of one, two, or three of the hydrogens of the methyl radical by alkyl groups.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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For example the bond dissociation energy of ethane (98 kcal/mol) is less than methane (105kcal/mol).
Thus, the order of stability is 3o>2o>1o
5.2 Hyperconjugation
Hyperconjugative effect also give stability to free radicals as in the case of carbocations. The stability order of alkyl free radicals is tertiary >secondary > primary > CH3. This stability order can be explained by hyperconjugation. The odd electron in the alkyl radical is delocalized onto the β-hydrogens, through hyperconjugation, which confers stability to the radical. Thus, tert-butyl radical is more stable than sec-butyl radical which in turn more stable than n-butyl radical. As tert-butyl radical has three methyl groups or nine beta hydrogens which give it nine hyperconjugative structures. sec-butylradical has six and n-butyl radical has three hyperconjugative structures. Greater the number of hyperconjugative structures more will be the stability of the radical.
Therefore the order of stability is:
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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5.3 Resonance Effect
In the free radicals where the carbon centre is in conjugation to a double bond, the resonance effect leads to stabilisation of these molecules. The stabilising effects of vinyl groups (in allyl radicals) and phenyl groups (in benzyl radicals) are very significant and can be satisfactorily explained by resonance. Allyl and benzyl free radicals are more stable than alkyl free radicals but still have only a transient existence under ordinary conditions.
Resonance in allyl free radical
Resonance in benzyl free radical
6. Reactions of Free Radicals . .
A free radical may undergo the following types of reactions:
6.1. Abstraction of another atom or group
For example, halogenation of alkanes proceeds via free radical mechanism. The chlorination of methane is shown below:
It involves three main steps (i) Initiation (ii) Propagation and (ii) Termination. (i) Initiation: In this step, free radicals required for the reaction are generated in situ by irradiation or heating of the reagent or by carrying out the reaction in the presence of an initiator like peroxides. The process is always endothermic. The first step is initiation which involves bond CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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dissociation in chlorine molecule. The bond dissociation energy of the chlorine molecule is 58 kcal/mol, so chlorine readily undergoes a homolytic bond dissociation.
(ii) Chain propagation: Second step is chain propagation. In this step the highly reactive chlorine radicals with unpaired electron reacts further. They are electrophilic, thus each seeks an electron to complete its unfilled shell of electrons. In a reaction with methane, a chlorine atom readily removes a hydrogen from the methane. Free radical chain reactions work best when all propagation steps are exothermic.
The resulting methyl radical, which also is very electrophilic, then removes a chlorine atom from a chlorine molecule.
(iii) Chain termination: The final step is chain termination in which two reactive radicals combine together.
6.2. Addition to multiple bonds
Example: The addition of HBr to alkenes
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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The anti-Markovnikov addition of HBr to alkenes was probably the first free radical addition reaction to be discovered. For example, addition of HBr to 2-methylpropene in the presence of a peroxide, RO-OR (eg. t-BuO-O-t-BuO or PhCOOOCOPh) regiospecifically yields1-bromo-2- methylpropane, the anti-Markvonikov addition product.
Mechanism:
Initiation steps:
Propagation steps:
Termination steps include combination of free radicals in all possible ways.
Another, example is the addition of CCl4 across the double bond of norbornenei,ebicyclo[2.2.1]hept-2-ene.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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6.3. Rearrangement reaction
Free radicals may also undergo rearrangement to form a more stable radical and then the final product.
6.4. Radical Reduction
In a radical reduction reaction, hydrogen is added to a π bond.
For example, the radical reduction of alkyne to alkene with sodium-ammonia is a trans addition of hydrogen to an alkyne.
Another example of radical reduction is the Birch reduction, as shown here benzene is reduced to 1,4-Cyclohexadiene in presence of sodium-EtOH in ammonia.
Mechanism of Birch reduction: From the mechanism it is clear that sodium radical is involved in the reduction, which also forms some other carbon radicals as intermediate in the whole process.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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6.5. Free-Radical Polymerization of Alkenes
Polymerisation may also occur via generation of free radicals. The most important free radical chain reaction is the free radical polymerization of ethylene to polyethylene. Industrially (t-BuO)2 is used as a initiator. The t-BuO·radical adds to ethylene to give the beginning of a polymer chain. Thepropagation part has only one step: the addition of an alkyl radical atthe end of a growing polymer to ethylene to give a new alkyl radical atthe end of a longer polymer. The termination steps are the usual radical–radical combination and disproportionation reactions.
Overall:
Initiation:
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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Propagation:
Another example is polymerization of styrene to polystyrene.
6.6. Allylic Bromination
Alkenes can be brominated in the allylic position using NBS, the reaction is known as Wohl- Ziegler reaction. A mechanism for allylic bromination by NBS is shown below. The Br2 in this process is formed in a side reaction between HBr and NBS. The function of NBS is to provide a low, constant concentration of bromine and to use up the HBr formed during the propagation step.
6.7. Ozone formation and ozone depletion
Ultraviolet radiations if reach earth surface damages the plant and animal life. The ozone layer prevents these harmful UV rays from reaching earth’s surface.
Ozone formation: Ozone is formed when the ultraviolet radiation act upon molecular oxygen. Ozone formation involves homolytic bond dissociation in molecular oxygen in presence of UV radiations.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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Further in the presence of ultraviolet radiation the oxygen radicals react with another oxygen molecule to produce ozone.
Ultraviolet radiation also dissociates molecules of ozone to produce an electronically excited oxygen atom and an oxygen molecule.
These reactions make up a chain reaction that continue as long as oxygen and ultraviolet radiation are available. The net result of these three reactions is the absorption of most of the incoming ultraviolet radiation and formation of ozone.
Ozone depletion: Chlorofluoro carbons (CFCs), CFCl3 and CF2Cl2, are widely used as refrigerant which absorb radiation at the same wavelengths as molecular oxygen and ozone. When these CFCs absorb ultraviolet radiation, a C—Cl bond homolytically cleaves to form a chlorine radical.
Once formed, the chlorine radical can react with ozone to produce ClO and molecular oxygen. The ClO, in turn, reacts with oxygen radical to form a chlorine radical and a molecule of oxygen.
The net result is a catalytic cycle that destroys a molecule of ozone while regenerating the chlorine radical. This is how ozone layer gets depleted due to CFCs.
6.8. Autooxidation
The spontaneous oxidation of organic molecules in the presence of oxygen is called autooxidation.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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Ether solvents undergo autooxidation so readily that if stored for a long time oxidize to form some amount of hydroperoxide products. Hydroperoxide products are unstable and decompose violently when heated.
The mechanism for the autooxidation involves abstraction a hydrogen from the carbon bearing the ether oxygen by an oxygen molecule.It leads to formation of a radical and a hydroperoxide radical. These two radicals then react with each other to form the ether hydroperoxide.
Similarly furan also undergoes autooxidation to form explosive peroxide.
Synthetic antioxidants such as BHT are added to packaged and prepared food to prevent oxidation and spoilage. Vitamin E and BHT are radical inhibitors so they terminate radical mechanism by reacting with radical.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals
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The main causes of food spoilage are microorganisms and autooxidation. Antioxidants such as Butylated Hydroxy Anisole (BHA) and Butylated Hydroxy Toluene (BHT) are added to the food or packaging materials, to prevent their autooxidation.
5. Summary
• Free radical have unpaired electron, have a planar trigonal sp2geometry. • N-Bromosuccinimide is an excellent source of bromine in low concentrations for allylic • bromination. • Benzylic and allylic radicals readily form because both are resonance-stabilized. • The halogenation of an alkane is a radical chain reaction. The anti-Markovnikov addition of HBr proceeds via a radical intermediate.
CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No. 8: Generation, structure, stability and reactivity of free radicals