Detection and Characterization of Radicals

Radicals are neutral and so are far less solvated than carbocations or carbanions. Therefore, they are smaller and can operate in polar, hindered environments, where ionic chemistry fails. 

Unhke carbocations, radicals on the whole are less prone to rearrangements. Unhke carbanions, alkoxy and sulfonyloxy groups p-to a radical center do not undergo elimination. However, halogen, sulfur, and selenium groups P-to radicals are ehminated. Thus, radical elimination chemistry can be complementary to that of anions. 

Protection of -OH and -NH2 groups is unnecessary in radical reactions (unlike carbanions, radicals are not basic). In many situations, cumbersome and yield-lowering protection-deprotection steps become unnecessary. 

The utility of these features for organic transformations will emerge in the sections below, where actual examples are presented and discussed. One of the major disadvantages of radical chemistry is that they readily react with oxygen, which means that their reactions must be carried out under an inert atmosphere 

The distinguishing characteristic of a free radical is the presence of an unpaired electron. Species with an unpaired electron are said to be paramagnetic. The relative stability of the triphenylmethyl radical allows it to be studied by magnetic susceptibility measurement, which involves weighing it both inside and outside a magnetic field. The unpaired electron makes the radical paramagnetic, so the sample is drawn into the magnetic field. By this technique the dissociation of hexaphenylethane to the triphenylmethyl radical was determined to occur to the extent of 2% in a 0.1 M sample. 

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R.H. Magnuson, W.P. Giering et al. - Detection and Characterization of Radical Cations Resulting from the Oxidation of Methyl and Acetyl Iron Complexes,... 

One of the most powerful spectroscopic techniques for the detection and characterization of persistent and transient phenoxyls is time-resolved resonance Raman (RR) spectroscopy. Vibrational frequencies and the relative intensities of the resonance-enhanced bands have proven to be sensitive markers for tyrosyl radicals in proteins. For example, Sanders-Loehr and co-workers (31) detected the tyrosyl radical in native ribonucleotide reductase from Escherichia coli by a resonance-enhanced Raman mode at 1498 cm 1 that they assigned to the Ula Wilson mode of the tyrosyl, which is predominantly the u(C=0) stretching mode. 

Khindaria A, Nie G, Aust SD (1997) Detection and characterization of the lignin peroxidase compound II- veratryl alcohol cation radical complex. Biochemistry 36 14181-14185... 

In this chapter, we will critically examine the potential use of ESR methodology to detect and characterize oxy radicals formed in myocardial ischemia and reperfusion injuries. 

The use of resonance Raman spectroscopy for the study of transient species, including free radicals, has been reviewed be Hester (1978). Provided only that excitation may be achieved within an intense absorption band specific to the intermediate species, selective resonance enhancement permits the detection and characterization of such species at concentrations in the lO" - 10 molar range without serious interference from spectra of other dissolved species such as excess reactant and product molecules. 

Detection and characterization of enzyme-based free radicals... 

Electron-spin resonance (e.s.r.) spectroscopy is a technique for the study of species containing one or more unpaired electrons. The scope of the method includes the detection and characterization of some transition-metal ions, simple molecules and ions (e.g. O2, NO, NOg, COi"), and organic radicals, including biradicals and triplet states. 

Pascual E.C., Blank I., Goodman B.A. and Yerantzian C. (2000) The detection and characterization of free radicals generated during the decomposition of solutions of the coffee flavour compound furfuryl mercaptan. 18th Int. Colloq Chem Coffee (Helsinki, 2-6.8.1999) (ASIC, 2000), 50-7. 

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