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CHEMISTRY PAPER No. 5:Organic Chemistry-2 (Reaction Mechanism-1) MODULE No ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ 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 ____________________________________________________________________________________________________ • 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
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