Free radical addition reactions

Reaction (13) is typical of aliphatics and alcohols, whereas reaction (14) is common for double bonds, especially in conjugated and aromatic systems. To form the final products, the radicals R and ROH undergo additional reactions. Free radical scavengers are a very important component of Fenton systems, and their importance in pollutant degradation will be discussed further in Secs. Ill and IV. 

In the creation of network structures by chemical bonding (covalent bonding), there is a method of (1) crosslinking at the same time as polymerization or (2) crosslinking by chemical reaction after linear polymer chains have been synthesized. The latter method can be further divided into the addition polymerization in the presence of divinyl conqjounds (radical polymerization, anionic polymerization, ionic polymerization, etc.) or the formation of crosslinked structures by polycondensation of multifunctional compoimds. In the addition reaction, free radical polymerization is generally utilized. In this free radical polymerization method, initiators are usually used, but light, radiation, and plasmas can also be used. 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

The first three chapters discuss fundamental bonding theory, stereochemistry, and conformation, respectively. Chapter 4 discusses the means of study and description of reaction mechanisms. Chapter 9 focuses on aromaticity and aromatic stabilization and can be used at an earlier stage of a course if an instructor desires to do so. The other chapters discuss specific mechanistic types, including nucleophilic substitution, polar additions and eliminations, carbon acids and enolates, carbonyl chemistry, aromatic substitution, concerted reactions, free-radical reactions, and photochemistry. 

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. 

The mechanism of free-radical addition follows the pattern discussed in Chapter 14 (pp. 894-895). The method of principal component analysis has been used to analyze polar and enthalpic effect in radical addition reactions. A radical is generated by... 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

Aromatic rings are moderately reactive toward addition of free radicals (see Part A, Section 12.2) and certain synthetically useful substitution reactions involve free radical substitution. One example is the synthesis of biaryls.175... 

Although thiolacetic acid additions are free-radical reactions (60), it was found recently that the addition to electron-poor olefins can be base catalyzed (61) (eqs. [14], [15]). Thus the (S)-(-) adduct is obtained with an e.e. of 54% when cyclohexenone is treated with thiolacetic acid in benzene in the presence of catalytic amounts of cinchonine. The reaction appears to be quite general, although very high e.e. s (>80%) have not yet been achieved. 

In a study aimed at the identification of products of free radical reactions with polystyrene- and aromatic-based PEMs using model compounds, Hiibner and Roduner observed the addition of free radicals to the aromatic rings, preferentially in the ortho position to alkyl- and RO-substituents (in polystyrene- and aromatic-based PEMs, the para position is blocked by the presence of the sulfonic acid group). This study demonstrated the combined ortho-activation by these substituents and the meta-directing effect... 

The detailed mechanism for these Co AlPO-18- and Mn ALPO-18-cata-lyzed oxidations are unknown, but as previously pointed out vide supra) and by analogy to other metal-mediated oxidations a free-radical chain auto-oxidation (a type IIaRH reaction) is anticipated [63], This speculation is supported by several experimental observations that include (1) an induction period for product formation in the oxidation of n-hexane in CoAlPO-36, (2) the reduction of the induction period by the addition of free-radical initiators, (3) the ability to inhibit the reaction with addition of free-radical scavengers, and (4) the direct observation of cyclohexyl hydroperoxide in the oxidation of cyclohexane [62],... 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

Cis-Trans Isomerization via Radicals. The reversible addition of free radicals to olefins can induce cis-trans isomerization of the double bonds.131-134 This reaction can be quite clean, converting the olefin to a thermodynamic cis-trans equilibrium mixture with very little double bond migration.131,134 cis,cw-Cyclotetradecane-l,8-diene (28), for example, is... 

We have seen that the polarization forces cause very large collision cross sections between ions and molecules, and it is experimentally observed that a chemical reaction occurs on almost every collision. Some observed ion molecule reactions and their observed rate constants and cross sections are tabulated in Table IV. In addition, several free radical reactions are shown for comparison purposes. Let us look at the reactions at the top of Table IV and try to understand why the reaction occurs. 

SCHEME 7. Examples of Organic Reactions. Free Radical Addition... 

The more effective a termination step is, the shorter will be the propagation cycle and the less product will be produced per initiation event. In the limiting case, if each initiation event was terminated, then no product would be produced. This is the role of antioxidants added to many products and most processed food. These additives scavenge free radicals produced by the reaction of oxygen with C-H bonds and prevent them from participating in oxidation propagation cycles—thus oxidative degradation is stopped or slowed markedly. 

Photopolymerization of 7 at 25 °C resulted in approximately 80% conversion of double bonds and a Tg of 70 °C. Apparently, the Tg of the material increased as crosslinking proceeded, but the reaction stopped at only partial conversion when vitrification occurred. The result was a Tg only 20-40 °C higher than the sample temperature. In addition, trapped free radicals were detected in crosslinked films of 7 by electron spin resonance (ESR) (28). 

The carboxylic acid function plays no part in this reaction free-radical addition of hydrogen bromide to the carbon-carbon double bond occurs. 

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

The composition of the reaction products (1 1 adducts) needs further clarification. In the case of terminal olefins the anti-Markovnikov 1 1 addition product is almost the only 1 1 adduct, whereas the isomeric amide is formed in minute amounts only. Markovnikov-additions of free radicals to olefins have been observed in other cases too as side products (28). The point of initial attack in the free radical addition to an olefin of the type RCH=CH2 is at the terminal carbon. The intermediate radical (I) produced by this process (anti-Markovnikov) has a higher degree of resonance stabilization than the alternative radical (II) (4, 78). This means that in the present reaction,... 

Although the addition of free radicals to metal centers, leading to one-equivalent oxidation of the metal [see Eq. (288)] is an oxidative addition, we use the term here to describe those additions of substrates to metal centers that involve overall two-equivalent changes.381 386 The reactions of alkyl halides with cobalt(II) or with iridium(I) provide examples of one-equivalent and two-equivalent oxidations of the metal center, respectively ... 

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