Stable free radicals examples

There also exist systems in which a coloured species is decolourized in an imagewise manner. These positive working processes rely on the imagewise destruction of highly coloured, stable free radicals, examples of which include the verdazyls (158), the bipyridyls (159) and the nitroxide radical (160) (B-69MI11402). 

A few free radicals are indefinitely stable. Entries 1, 4, and 6 in Scheme 12.1 are examples. These molecules are just as stable under ordinary conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2 is somewhat less stable to oxygen, although it can exist indefinitely in the absence of oxygen. The structures shown in entries 1, 2, and 4 all permit extensive delocalization of the unpaired electron into aromatic rings. These highly delocalized radicals show no tendency toward dimerization or disproportionation. Radicals that have long lifetimes and are resistant to dimerization or other routes for bimolecular self-annihilation are called stable free radicals. The term inert free radical has been suggested for species such as entry 4, which is unreactive under ordinary conditions and is thermally stable even at 300°C. ... 

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Radical traps belong to the class of stable free radicals, for example, of the nitroxyl or phenoxyl type. Interacting with radicals prodnced by a reaction, radical traps give diamagnetic compounds. One can follow the progress of the reaction by a decreasing intensity of the ESR spectrum of the radical trap. 

If you recall that combustion is a free radical process, we can easily see why cyclic and branched alkanes bum more easily (and more smoothly) than straight-chain alkanes. The reason is that more stable free radicals are formed. This results in less knocking and a higher octane rating. Examples of free radical stability are the following ... 

Another example is paraquat, which can accept an electron from donors such as NADPH, becoming a stable free radical, which is not chemically reactive. However, it will generate reactive oxygen species by donating an electron to available oxygen (see chap. 7). 

The first workable capping agents for controlled radical polymerization were discovered by Rizzardo et al. [77, 78] who used nitroxides. The nitroxide reacts reversibly with radical chain ends but itself does not initiate the monomer. They called their new system Stable Free Radical Polymerization (SFRP). Scheme 32a depicts an example of SFRP using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy). SFRP was developed independently by Georges at Xerox for the synthesis of styrene block polymer as dispersing agents [79]. 

The donor ability (nucleophilicity) of R2M , including the stability of complexes being formed, increases as M becomes heavier. The acceptor properties of M in R2M (such as the ability to form the adducts with Lewis bases, for example, with pyridine and piperidine at — 30 °C) are determined by the low-lying unoccupied atomic d- and pz-orbitals160. Stable free radicals R3M (M = Ge, Sn) are obtained by a photochemical disproportionation reaction of R2M in a hydrocarbon solvent medium160 see equation 26, R = (Me3Si)2CH. 

In a stable free-radical polymerization (SFRP), the initiated polymer chains are reversibly capped by a stable radical, for example, the 2,2,6,6-tetra-methylpyridin-l-oxyl radical (TEMPO). Stable PS dispersions via miniemulsion polymerization were prepared by MacLeod et al. with an optimized ratio... 

TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) is a stable free radical that catalyzes many types of oxidations. For example, a catalytic amount of TEMPO added to a hypochlorite oxidation of an alcohol increases the rate by enabling lower-energy reaction mechanisms involving reversible oxidation of the N—0 bond. 

Fig. 3. Autoxidation of polyunsaturated fatty acids in phospholipid membranes. Addition of oxygen to lipid free radicals is extremely fast. It yields peroxyl radicals ROO which will tend to capture labile hydrogen atoms of neighbouring polyunsaturated lipids. Accidentally produced free radicals will therefore initiate a chain reaction of lipid peroxidation which will propagate along membranes. This process can result in several dozen propagation steps before it is stopped by a termination reaction. Examples of such termination reactions are the recombination of peroxyl radicals and the formation of a stable free radical from a free radical scavenger (scavH). Termination through recombination of low steady-state concentration of alkyl radicals is unlikely in aerobic medium.

It is important to note that even certain phase-transfer catalysts can be carbonylated to carboxylic acids, not by cobalt tetracarbonyl anion catalysis, but by acetylcobalt tetracarbonyl. This is a slow but high-yield reaction that occurs for quaternary ammonium salts that are capable of forming reasonably stable free radicals. For example, phenylacetic acid is formed in 95% yield from benzyltriethylammonium chloride (benzyl radi-... 

Bistrifluoromethyl nitroxide, (CF3)2NO, is a stable purple gas and is of particular interest because, of course, nitroxides are stable free radicals. The nitroxide is prepared from bistrifluoromethylhydroxylamine [283] by reaction with, for example, silver oxide [284] (Figure 8.111) or potassium permanganate [285]. 

The 1,4-dihydrobenzoic acids derived from reductive alkylation may undergo facile rearomatization with either loss of the carboxylic acid group or the alkyl group. The gibberellin synthesis intermediate (82), for example, was found to be especially labile, forming (83) simply on exposure to air." Oxidative decarboxylation may be deliberately achieved with lead tetraacetate or electrochemically." Loss of the 1-alkyl group is likely to be a problem when the alkyl moiety can form a reasonably stable free radical, since a chain reaction may then be sustained." ... 

Chlorine dioxide is a relatively stable free radical it is paramagnetic because it contains an unpaired electron. It can be prepared by oxidizing chlorite, 0102", for example, by peroxosulfate or CI2. The reaction with CI2 is... 

Now, by selectivity we mean here the differences in rate at which the various classes of free radicals are formed a more stable free radical is formed faster, we said, because the factor that stabilizes it—delocalization of the odd electron (Sec. 6.28)—also stabilizes the incipient radical in the transition state. If this is so, then the more fully developed the radical character in the transition state, the more effective delocalization will be in stabilizing the transition state. The isopropyl radical, for example, is 3 kcal more stable than the Ai-propyl radical if the radicals were completely formed in the transition state, the difference in act would be 3 kcal. Actually, in bromination the difference in act is 3 kcal equal, within the limits of experimental error, to the maximum potential stabilization, indicating, as we expected, a great deal of radical character. In chlorination, by contrast, the difference in is only 0.5 kcal, indicating only very slight radical character. 

The thermal conversion of polynuclear aromatic compounds to carbon can be considered a free-radical process because it involves a series of bond cleavage reactions. Many of the radical intermediates are expected to be unstable and cannot be detected by conventional spectroscopic techniques. However, the carbonaceous residues from pyrolysis do contain significant amounts of stable free radicals as apparent from EPR experiments. For example, Figure 8 shows the results of EPR measurements of the free-radical content for anthracene and naphthalene pitches after heating from 400 to 700 °C. The concentration of stable free radicals increases with heat treatment temperature as the molecular size grows. The more highly condensed... 

Odd-Alternate Polynuclear Aromatic Hydrocarbon Radicals. Substantial evidence supports the contention that the stable free radicals formed during the pyrolysis of polynuclear aromatic compounds are odd-alternate hydrocarbon radicals. As an example, the phenalenyl radical (5) is formed during pyrolysis of a number of organic compounds including acenaphthylene (3) and dihydronaphthalene (4) (24) (see Scheme III). The... 

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