Detection of Radical Intermediates

A second type of structural information can be deduced from the hyperfine splitting in EPR spectra. The origin of this splitting is closely related to the factors that cause spin-spin splitting in proton NMR spectra. Certain nuclei have a magnetic moment Those which are of particular interest in organic chemistry include H, N, F, and P. 

Interaction of the electron with one or more of these nuclei sphts the signal arising from the unpaired electron. The number of lines is given by the following equation  

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and = 26.87 G and imply extensive delocalization of spin density through the a bonds. Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

EPR spectra have been widely used in the study of reactions to detect free-radical intermediates. An interesting example involves the cyclopropylmethyl radical. Much chemical experience has indicated that this radical is unstable, giving rise to 3-butenyl radical rapidly after being generated. 

The radical is generated by photolytic decomposition of di-/-butyl peroxide in methylcy-clopropane, a process that leads to selective abstraction of a methyl hydrogen from methylcy clopropane  

A second type of structural information can be deduced from the hyperfine splitting in EPR spectra. The origin of hyperfine splitting is closely related to the 

Analysis of the EPR spectrum of the nitroxide radical can usually provide information about the structure of the original radical. 

Below -140 C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above -140°C, however, the spectrum of a second species was seen, and above -100°C, this was the only spectrum observed. This spectrum could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to -140°C. The conclusion is that the ring opening of the 

For reviews of ttie preparation, reactions and uses of nitroxide radicals, see J. F. W. Keana, Chem. Rev., 78, 37 (1978) L. J. Berliner, ed., Spin-Labelling, Vol. 2, Academic Press, New York, 1979 S. Banerjee and G. K. Trivedi, J. Sci. Ind. Res., 54, 623 (1995) L. B. Volodarsky, V. A. Reznikov, and V. 1. Ovcharenko, Synthetic Chemistry of Stable Nitroxides, CRC Press, Boca Raton, FL, 1994. 

Current Org. Chem., 4, 55 (2000) F. Gerson and W. Huber, Electron Spin Resonance of Organic Radicals, Wiley-VCH, Weinheim, 2003. 

A spectroscopic method that is uniquely applicable to the study of free radicals exists, and is known as electron paramagnetic resonance (EPR) spectroscopy. Electron spin resonance (ESR) spectroscopy is synonymous. Very simply stated, EPR 

For a review of the preparation, reactions, and uses of nitroxide radicals, see J. F. W. Keana, Chem. Rev. 78, 37 (1978), and L. J. Berliner ed., Spin-Labelling, Vol. 2, Academic Press, New York, 1979. 

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Distinction between PL and ET mechanisms is not straightforward. Various experimental methods have been used so far to demonstrate the ET process, including spectroscopic detection of radical intermediates detection of products indicative of radical intermediates " and measurement of secondary deuterium " and carbonyl carbon kinetic isotope effects (KlEs) "" . The combination of several experimental methods, including KIE, substituent effect and probe experiments, was shown to be useful in distinguishing the ET process from the PL process for the addition reactions of the Grignard and other organometallic reagents . 

Jaeger CD, Bard AJ (1979) Spin trapping and electron-spin resonance detection of radical intermediates in the photo-decomposition of water at TO2 particulate systems. J Phys Chem 83 3146-3152... 

ESR spectroscopy has found wide-spread use for the detection of radical intermediates in electrode processes 40 For the same purpose, the newly developed technique of trapping short-lived radicals by nitrones or nitroso compounds 40d-> should be of considerable interest, as should also the chemically induced nuclear spin polarization (CINP) phenomenon 40e-1 be. 

A summary of various clashes of radicals has been given above. Obviously many radicals have not been directly specified but, hopefully, those of biological importance have largely been covered. In the following chapters many points raised herein are elaborated. In Chapter 2, we cover different ways in which radicals are generated, and ways in which biological systems protect themselves from radical induced damage. In Chapter 3, methods of direct and indirect detection of radical intermediates are outlined. 

C. Detection of Radical Intermediates in Heterogeneous Catalytic Processes... 

The only application of this technique in phosphole chemistry has been in the detection of radical intermediates in the cleavage of P-substituents with alkali metals at — 80°C. Radical anions at phosphorus were formed as the initial products from l-methyl-2,5-diphenylphosphole, 1,2,5-triphenylphosphole, and pentaphenylphosphole. The P nucleus exhibited strong coupling with the electron <71T5705>. Radical anions were also detected in the reaction of 1,2,5-triphenylphosphole oxide and pentaphenylphosphole oxide with alkali metals <71CC782>. 

We have noted that both 5 2 and SnAr reactions may occur through SET processes. There is good evidence that the SnAc reaction may involve such a pathway also. Figure 8.55 shows species identified by Bacaloglu and coworkers in a fast kinetic spectroscopy study of the reaction of hydroxide ion with l-chloro-2,4,6-trinitrobenzene (picryl chloride, 71). D ending on reaction conditions, these workers could see transients ascribed to the n complex (72), an intermediate produced by single electron transfer (73), and one or more cr complexes (74, 75). In addition, evidence was obtained for the reversible formation of a phenyl carbanion (76) and a dianion (77) that probably do not lead directly to the substitution product (78). Further support for the role of SET processes in SNAr reactions comes from the detection of radical intermediates by EPR spectrometry and by correlations of reactivity with the oxidation potentials of the nucleophiles in some studies. ... 

The HAS reaction proceeds via a sigma (a) complex (1) with substitution being completed by the loss of the leaving group Y, which is usually hydrogen (Scheme 9.1, Y = H). Examples where the cyclohexadienyl radicals become trapped by fast reductants to form cyclohexadiene [2] and the detection of radical intermediates by ESR or CIDNP provide evidence that the cyclohexadienyl radicals are intermediates in this reaction [3]. In some systems, the addition of a radical onto the arene is the rate-determining step, because of the loss of aromaticity. For example, the rate constant for the addition of the ferf-butyl radical to benzene at 79°C is 3.8 x 10 M s [4], which is clearly at the lower end of a useful radical reaction. The arene needs to be used at high concentration, or as the solvent, in order to compensate for poor rates. On the other hand, as the rate of addition of the phenyl radical to benzene is 4.5 x 10 s" [5], it is more useful in these kind of reactions. 

Direct ESR and Spin Trapping as Tools for the Detection of Radical Intermediates... 

DIRECT ESR AND SPIN TRAPPING AS TOOLS FOR THE DETECTION OF RADICAL INTERMEDIATES... 

Free radicals, like ions, play an important role in various chemical reactions. The understanding of chemical mechanisms, whether associated with combustion, the interstellar medium, or other areas, can be clarified by detection of radical intermediates. 

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