Unsaturates, radical reactions with

In this chapter, we have shown how the recent advances in the crossed molecular beam technique allow us to study complex polyatomic reactions of relevance in astrochemistry. The focus was on the CN radical reactions with simple alkynes, but the same approach has been also applied to the study of other CN radical reactions with unsaturated small organic molecules, such as ethylene, benzene, and allene, which are of relevance in astrochemistry as well [77,81,84]. 

Scheme 3 Product formation in the lipoxygenase-catalysed free radical reaction with unsaturated fatty acids and fatty acid hydroperoxides ...

The investigation into thiyl-catalyzed isomerization of olefins eventually resulted in the measurement of relative rate coefficients for several CH3S + olefin reactions. These early relative rate studies relied on the interpretation of complex chemical systems to arrive at rate coefficients for the elementary reactions involving CH3S attack on the olefin double bond. More recently, the characterization of the CH3S laser induced fluorescence (LIF) spectrum [90,91] has led to direct measurements of rate coefficients for methyl thiyl radical reactions with unsaturated hydrocarbons. Reported relative rate coefficients are tabulated along with the recent direct measurements of Balia et al. [92] (see Table 6). This has been done to allow for observation of any reactivity trends that may be present and to facilitate qualitative discussion of proposed CH3S+olefin reaction mechanisms. 

Cyanoisopropyl radicals generally show a high degree of specificity in reactions with unsaturated substrates. They react with most monomers (c.g. S, MMA) exclusively by tail addition (Scheme 3.4). However, Bcvington et al.11 indicated that cyanoisopropyl radicals give ca 10% head addition with VAc at 60 °C and that the proportion of head addition increases with increasing temperature. 

Hydroxy radical and sulfate radical anion, though they may sometimes give rise to similar products, show quite different selectivity in their reactions with unsaturated substrates. In particular, the sulfate radical anion has a somewhat lower propensity for hydrogen abstraction than the hydroxyl radical. For example, the sulfate radical anion shows little tendency to abstract hydrogen from mcthacrylic acid.232... 

Few CIDNP studies on free radical reactions with olefins and related unsaturated molecules have been reported, and relatively little chemically useful information seems to have been derived, despite the potential relevance in polymerizing systems. Thus CIDNP has been reported in the decomposition of benzoyl peroxide in the presence of styrene and... 

Sulfur dioxide does not homopolymerize but does participate in a rather wide variety of free radical copolymerizations with unsaturated monomers. The resulting polysulfones have been known for quite a long time. Solonina ( f ) obtained a white solid from the reaction of SO2 with allyl ethers in 1898, but such products were not recognized as copolymers until the work of Marvel and Staudinger in the 1930 s. 

It was concluded that most probably maleic anhydride combines with the primary macroradicals (Scheme 4). Secondary radicals may be found by reaction with unsaturated groups of the rubber to add more monomers units along the chain and to produce a network. 

Bleaching is a complex phenomenon, which comprises destruction of conjugated chromophores, reaction with unsaturated targets, and even fragmentation of large molecules. This may require the simultaneous presence of several active oxygen species (OH° radicals, the superoxide anion O2 0, 02, HOO, CIO etc) [22]. 

Boranes are generally less electron rich than corresponding aluminium analogues and electron transfer mechanisms are usually not considered for reduction reactions. However, an increase of the hydridic character in complex hydrides such as.LiBEt3H ( super hydride ) [68] or LiAlH4 [66] allows for the formation of well-characterized radical products in reactions with unsaturated acceptor heterocycles such as (3). Electron transfer mechanisms for the reduction by complex hydrides should be quite intricate because the coordinatively saturated donor moiety (MHn ) and the a acceptor part (e.g. Li" ) can now well separately interact with the coordinating n acceptor substrate. 

As an odd-electron molecule, many of the reactions of nitrogen dioxide are typical of free radicals. This applies particularly to its association with atoms such as, H, F, and Cl or other radicals such as, the OH, NO, alkyl, and alkoxy radicals its addition reactions with unsaturated systems such as, olefins, acetylenes, and aromatic compounds, and its hydrogen abstraction reactions. The ability of the NO2 free radical to abstract hydrogen from an organic molecule ... 

Nitrogen dioxide, NO2. This is also an odd-electron molecule, and some of its reactions resemble those of a free radical, for example, its dimerization, its power of removing hydrogen from saturated hydrocarbons, and its addition reactions with unsaturated and aromatic hydrocarbons. A microwave study gives N—O, 1-197 A and the 0—N—O angle, 134° 15, in agreement with the results of earlier electron diffraction and infrared studies. 

Kaiser, R.L Le, T.N. Nguyen, T.L. Mebel, A. Balucani, N. Lee, Y.T. Stahl, F. Schleyer, P.v.R. Schaefer III, H.F. A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons — pathways to hydrogen deficient radicals in combustion flames. Faraday Discuss. 2001, 119, 51-66. 

Clearly, whether or not ozone is formed depends also on the rate at which it is destroyed, for example, by reaction with unsaturated hydrocarbons. Rates of reactions with alkanes are, as noted above, much slower than for reaction with OFI radicals, and reactions with ozone are of the greatest significance with unsaturated aliphatic compounds. The pathways plausibly follow those involved in chemical ozonization (Hudlicky 1990), and some of these are noted later. 

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

The stereochemistry of certain thermal reactions involving atom addition processes and resultant radical intermediates can also be studied conveniently by recoil techniques. In this type of experiment the recoil atom used to initiate the process must be thermalized before reaction, by diluting the system with a large concentration of nonreactive bath molecules. Two recently reported results on geometric isomerization serve as examples of the type of processes that can be studied (55,56). Both involve recoil chlorine atom reactions with unsaturated substrates. 

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