Radical stability unreactive radicals

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

The mechanism of this transformation is outlined in Scheme 38 and each step has important features. In step 1, the tributyltin radical abstracts the radical precursor X. A possible side reaction, the addition of the tributyltin radical to the allylstannane, is much slower than comparable additions to activated alkenes. Even if this addition occurs, the stannyl radical is simply eliminated to regenerate the starting materials. Thus, for symmetric allylstannanes, this reaction is of no consequence. As a result, the range of precursors X that can be used in allylation is more extensive than in the tin hydride method. Even relatively unreactive precursors like chlorides and phenyl sulfides can be used if they are activated by adjacent radical-stabilizing groups. 

Transfer agents that lead to production of unreactive radicals may be added to limit molecular weight [48-51]. Best suited are agents whose radicals are stabilized by adjacent groups or by resonance. The effectiveness of a transfer agent is characterized by its transfer constant, defined as the ratio of the rate coefficients of chain transfer and propagation ... 

The most widely used antioxidants are free radical scavengers that remove reactive radicals formed in the initiation and propagation steps of autoxidation. A number of natural or synthetic phenols can compete, even at low concentrations, with lipid molecules as hydrogen donors to hydroperoxy and alkoxy radicals, producing hydroperoxides and alcohols and an unreactive radical. (3-carotene reacts with per-oxy radicals, producing a less-reactive radical. These stabilized radicals do not initiate or propagate the chain reaction. 

In the attack by the comparatively unreactive bromine atom, we have said (Sec. 2.23), the transit.bn state is reached late in the reaction process the carbon-hydrogen bond is largely broken, and the organic group has acquired a great deal of free-radical character. The factors that stabilize the benzyl free radical stabilize the incipient benzyl free radical in the transition state. 

The second reason that vitamin C is used as an electron donor is that the reaction product is fairly stable and unreactive. When vitamin C gives up an electron, it becomes a free radical called the ascorbyl radical. By free-radical standards, the ascorbyl radical is not very reactive. Its structure is stabilized by electron delocalization — the resonance effect first described by Linus Pauling in the late 1920s. This means that vitamin C can block free-radical chain reactions by donating an electron, while the reaction product, the ascorbyl radical, does not perpetuate the chain reaction itself. 

When a free radical reacts, it usually snatches an electron from the reactant, turning it into a free radical. This in turn will steal a single electron from another nearby molecule. A chain reaction ensues until two free radicals react together, effectively neutralizing each other, or alternatively, until an unreactive free-radical product is formed. Free radicals are said to be quenched by vitamin C, because the free-radical product — the ascorbyl radical — is so unreactive. As a result, free-radical chain reactions are terminated. Lipid-soluble vitamin E (a-tocopherol) works in the same way, in membranes rather than in solution, often in cooperation with vitamin C at the interface between membranes and the cytosol (the watery ground substance of the cytoplasm that surrounds the intracellular organelles). When vitamin E reacts with a free radical, it too produces a poorly reactive (resonance-stabilized) free-radical product, called the a-tocopheryl radical. Tocopheryl radicals can be reconverted into vitamin E using electrons from vitamin C. 

Radical chlorination shows a substantial polar effect. Positions substituted by EWG are relatively unreactive toward chlorination, even though the substituents are capable of stabilizing the radical intermediate.For example, butanonitrile is chlorinated at C(3) and C(4), but not at C(2), despite the greater stability of the C(2) radical. 

Unwanted radicals in biological systems must be destroyed before they have an opportunity to cause damage to cells. Cell membranes, for example, are susceptible to the same kind of radical reactions that cause butter to become rancid (Section 26.3). Imagine the state of your cell membranes if radical reactions could occur readily. Radical reactions in biological systems also have been implicated in the aging process. Unwanted radical reactions are prevented by radical inhibitors—compounds that destroy reactive radicals by creating unreactive radicals or compounds with only paired electrons. Hydroquinone is an example of a radical inhibitor. When hydroquinone traps a radical, it forms semiquinone, which is stabilized by electron delocalization and is, therefore, less reactive than other radicals. Furthermore, semiquinone can trap another radical and form quinone, a compound whose electrons are all paired. 

The reactivity of free radicals is linked to their stability. Reactivity is a function of spin density of the atom and the type of orbital occupied by the unpaired electrons. Increasing the number of atoms in a radical generally decreases reactivity, making monoatomic radicals such as Li and F very reactive. Conjugation decreases the spin density and therefore the reactivity. The resultant delocalization does not necessarily, however, lead to a very stable radical. Steric effects play a significant role in radical reactivity. "If the bulkiness of the surrounding substituents exceeds 12-fold that of the central atom containing a radical site", the radical is unreactive and fails to react at all in many cases. Triphenylsilyl radical (45), for example, is very reactive but perchlorobenzyl radical (46) is not. 

Most acrylonitrile monomer pairs fall into the nonideal category. One such nonideal monomer pair is acrylonitrile-vinyl acetate, with R =4.05 and f 2 = 0.061 at 60°C. This is an example of a nonideality sometimes referred to as kinetic incompatibility. Acrylonitrile, because of the potential resonance stabilization offered by the nitrile group, is a reactive monomer but a relatively unreactive radical. On the other hand, vinyl acetate offers little possibility for resonance stabilization, so it can be categorized as a relatively unreactive monomer but highly reactive radical. The effect, shown in Table 12.6, is that the reaction between the very reactive acrylonitrile monomer with the highly reactive vinyl acetate radical has an extremely high rate constant. 

Lacking resonance stabilization, the chain radicals doubtless are very reactive, but owing to the corresponding lack of resonance structures in the transition state allyl acetate is a relatively unreactive monomer. These factors are conducive to the occurrence of the competitive reaction... 

This is sufficiently stable (the reasons for its stability are discussed below, p.312) to be recrystallised from various solvents, and obtained as violet prisms that may be kept more or less indefinitely. It is relatively unreactive towards other neutral molecules, but reacts readily with other radicals it is indeed used as a trap , forming stable products, e.g. (12), with almost any other radical ... 

Radicals derived from hydrofluorocarbons (HFCs) as well as hydrofluo-roethers (HFE) are often destabilized with respect to the methyl radical [51, 57,68,70,79-82], The low stability of these radicals implies that the C-H bonds in the corresponding closed shell parent compounds are comparatively strong and thus rather unreactive towards attack of oxidizing reagents. This latter property is of outstanding importance for the use of these compounds in a variety of technical applications, in which thermally stable, non-oxidizable, non-flammable compounds are needed. However, with respect to the environmental fate of these compounds high C-H bond energies... 

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