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Lesson Seven-

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Seven Free radical: structure and stability A free radical (often simply called radical may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2 as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radical trapped in this manner. Even under these conditions, the methyl radical decomposes with a halt-time of 10—15min in a methanol lattice at 77K. Since the lifetime of a radical depends not only its inherently stability, but also on the conditions under which it is generated, the terms persistent or stable are usually used for the different senses. A stable radical is inherently stable; a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. Associated with the spin of an electron is a magnetic moment, which can be expressed by a quantum number of +1/2 or –1/2. According to the Pauli principle, any two electrons occupying the same orbital must have opposite spins, so the total magnetic moment is zero for any species in which all the electrons are paired. In radical, however, one or more electrons are unpaired, so there is a net magnetic moment and the species is paramagnetic. Radical can therefore be detected by magnetic susceptibility measurement, but for this technique a relatively high concentration of radicals is required. A much more important technique is electron spin resonance (ESR, also called electron paramagnetic resonance (EPR. The principle of ESR is similar to that of NMR (nuclear magnetic resonance, except that electron spin is involved rather than nuclear spin. The two-electron spin states (ms=+1/2 and ms=-1/2 are ordinarily of equal energy, but in a magnetic field, the energy is different. As in NMR, a strong external field is applied and electrons are caused to flip from the lower state to the higher by the application of an appropriate radio frequency signal. In as much as two electrons paired in one orbital must have opposite spins that cancel each other, an ESR spectrum arises only from species that have one or more unpaired electrons, that is, free radicals. Since only free radicals give an ESR spectrum, the method can be used to detect the presence of radical and can 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 (ESR peaks are split by nearby protons. Fortunately (for the existence of most free radicals is very short , it is not necessary for a radical to be persistent for an ESR spectrum to be obtained. Electron spin resonance spectra have been observed for radicals with lifetime considerably<1s. Failure to observe an 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 very reactive radicals to produce more persistent radicals, the new radicals can be observed by ESR. Azulenyl nitrones have been developed as chromotropic spin trapping agents. The most important spin—trapping compounds are nitroso compounds, which react with radicals to give fairly stable nitroxide radicals. An N-oxide spin trap has been developed, and upon trapping a reactive free radical, 13P NMR can be used to identify it. This technique is effective, and short-lived species such as the oxiranylmethyl radical has been detected by spin trapping. Other molecules have been used to probe the intermediacy of radicals via SET processes. They are called SET probes. Because there is an equal probability that a given unpaired electron will have a quantum number of +1/2 or -1/2, radicals cause two lines or group of lines to appear on an electronic spectrum, and are sometimes referred to as doublets. Another magnetic technique for detection of free radicals uses an ordinary NMR instrument. It was discovered that if an NMR spectrum is taken during the course of a reaction, certain signals may be enhanced, either in a positive or negative direction, other may be reduced. When this type of behavior, called chemically induced dynamic nuclear polarization (CIDNP, is found in the NMR spectrum of the product of a reaction, it means that at least a portion of that product was formed via the intermediacy of a free radical.

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