Free Radicals stability and detection.PPT

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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 b ecause 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 i nteracting 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 lowe r. 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: Magnetic susceptibility measurement ESR Technique 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