Free radical, mechanisms types

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

The sample labeled atactic in Fig. 7.10 was prepared by a free-radical mechanism and, hence, is expected to follow zero-order Markov statistics. As a test of this, we examine Fig. 7.9 to see whether the values of p, P, and Pj, which are given by the fractions in Table 7.9, agree with a single set of p values. When this is done, it is apparent that these proportions are consistent with this type... 

In tbe first attempt to prepare a two-dimensional crystalline polymer (45), Co y-radiation was used to initiate polymerization in monolayers of vinyl stearate (7). Polymerization at the air—water interface was possible but gave a rigid film. The monomeric monolayer was deposited to give X-type layers that could be polymerized in situ This polymerization reaction, quenched by oxygen, proceeds via a free-radical mechanism. 

The mechanism is of the Sn 1 type. That aryl cations are intermediates was shown by the following experiments aryl diazonium chlorides are known to arylate other aromatic rings by a free-radical mechanism (see 14-17). In radical arylation it does not matter whether the other ring contains electron-withdrawing or electron-... 

In this chapter, we discuss free-radical substitution reactions. Free-radical additions to unsaturated compounds and rearrangements are discussed in Chapters 15 and 18, respectively. In addition, many of the oxidation-reduction reactions considered in Chapter 19 involve free-radical mechanisms. Several important types of free-radical reactions do not usually lead to reasonable yields of pure products and are not generally treated in this book. Among these are polymerizations and high-temperature pyrolyses. 

Mechanisms of aldehyde oxidation are not firmly established, but there seem to be at least two main types—a free-radical mechanism and an ionic one. In the free-radical process, the aldehydic hydrogen is abstracted to leave an acyl radical, which obtains OH from the oxidizing agent. In the ionic process, the first step is addition of a species OZ to the carbonyl bond to give 16 in alkaline solution and 17 in acid or neutral solution. The aldehydic hydrogen of 16 or 17 is then lost as a proton to a base, while Z leaves with its electron pair. 

When thiols are added to substrates susceptible to nucleophilic attack, bases catalyze the reaction and the mechanism is nucleophilic. These substrates may be of the Michael type or may be polyhalo alkenes or alkynes. As with the free-radical mechanism, alkynes can give either vinylic thioethers or dithioacetals ... 

This was also accomplished with BaRu(0)2(OH)3. The same type of conversion, with lower yields (20-30%), has been achieved with the Gif system There are several variations. One consists of pyridine-acetic acid, with H2O2 as oxidizing agent and tris(picolinato)iron(III) as catalyst. Other Gif systems use O2 as oxidizing agent and zinc as a reductant. The selectivity of the Gif systems toward alkyl carbons is CH2 > CH > CH3, which is unusual, and shows that a simple free-radical mechanism (see p. 899) is not involved. ° Another reagent that can oxidize the CH2 of an alkane is methyl(trifluoromethyl)dioxirane, but this produces CH—OH more often than C=0 (see 14-4). ... 

With the evidence available at the moment, it is the author s opinion that Pines and Goetschel s free-radical mechanism is well founded, and thus this mechanism is quite distinct from the bond shift reaction occurring over metals. In order to preserve this distinction, we shall retain the term vinyl insertion for the type of isomerization exemplified in (24) and (25). 

That the desulfurizing action of Raney nickel is retained even in the absence of hydrogen has been demonstrated by Hauptmann, Wladislaw and Camargo16 who found that Raney nickel which had been deprived of its hydrogen through heating in vacuum at 200° converted, for example, benzaldehyde dibenzyl thioacetal into a mixture of stilbene and bibenzyl. That this type of reaction proceeds by a free-radical mechanism appears... 

Most examples of polymerization used to create nanoparticles occur by a free radical mechanism involving distinct initiation, propagation, and termination processes [39]. Polymerization occurs within a continuous liquid medium, which also comprises the monomer, initiator, and a surfactant. Four different polymerization techniques are described to polymerize vinyl type monomers, namely ... 

The stepwise dehydrocyclization of hydrocarbons with quaternary carbon atoms over chromia was interpreted by Pines 94). Here a skeletal isomerization step prior to cyclization was assumed. This is not of a cationic type reaction, and the results were explained by a free radical mechanism accompanied by vinyl migration (Scheme IXA). Attention is drawn to the fact that... 

It makes good sense to draw free-radical mechanisms in the manner shown by these examples. However, shorter versions may be encountered in which not all of the arrows are actually drawn. These versions bear considerable similarity to two-electron curly arrow mechanisms, in that a fishhook arrow is shown attacking an atom, and a second fishhook arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type of representation is quite clear if the complement of electrons around a particular atom is counted each time but, if in any doubt, use all the necessary fishhook arrows. 

S " ") and depolymerization (S " " - Sg + Sy -t- Sg 4-. ..). This free radical mechanism has often been discussed on the basis of the observed first order rate equations found for the formation of it-sulfur and the decomposition of organic polysulfides as well as on the basis of the experimentally determined apparent activation energies (50-150 kJ/mol ). However, this type of mechanism seems rather unlikely at least at moderate temperatures sinc (a) no free radicals have been detected in liquid sulfur below 170 (b) only the... 

The various types of polyacrylate polymers are manufactured from the relevant monomers by a free radical mechanism using peroxide initiators and can be block or random polymers depending on the degree of prepolymerization of the monomers used. 

According to Sato and Herring (39), ascorbic acid can function as an antioxidant by interfering with the free radical mechanism due to the presence of the ene-diol portion of the molecule. However, St. Angelo et al (5) reported that reductone-type compounds, such as maltol, kojic acid, 3-hydroxyflavone, etc., were effective antioxidants and suggested that the alpha ketol structure may also play an important role concerning free radical mechanisms. 

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

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