Elimination involving free radicals

Another common type of mechanism found to operate in pyrolytic eliminations involves free radicals. Initiation occurs by pyrolytic cleavage. A schematic example of this type of reaction is 

Free radical eliminations are frequent during pyrolytic reactions, and they are common for linear chain polymers. At higher temperatures (600° C-900° C) this type of reaction is also common for small molecules and explains the formation of unsaturated or aromatic hydrocarbons from aliphatic ones. As an example, butane decomposition may take place as follows  

More examples of this type of reaction will be given in Section 2.6. 

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The free radical mechanism responsible for the polymeric chain scission is basically not different from elimination involving free radicals described in Section 2.2. However, the process can be more complicated and some particularities are described below. 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

The addition reactions take place at a carbon-carbon multiple bond, or carbon-hetero atom multiple bond. Because of this peculiarity, the addition reactions are not common as the first step in pyrolysis. The generation of double bonds during pyrolysis can, however, continue with addition reactions. The additions can be electrophilic, nucleophilic, involving free radicals, with a cyclic mechanism, or additions to conjugated systems such as Diels-Alder reaction. This type of reaction may explain, for example, the formation of benzene (or other aromatic hydrocarbons) following the radicalic elimination during the pyrolysis of alkanes. In these reactions, after the first step with the formation of unsaturated hydrocarbons, a Diels-Alder reaction may occur, followed by further hydrogen elimination ... 

The -elimination with two groups lost from adjacent atoms is another common reaction in pyrolysis, usually taking place with an Ei mechanism and not involving free radicals. An a-atom is the atom bound to a specific group or bond, and any atom adjacent to it is indicated as a p-atom. p-Eliminations or 1,2-eliminations involve, for example, the elimination of a group from a-atom and a hydrogen from the p-atom. For polymers where the pyrolysis takes place in condensed phase, E2 and Ei mechanisms are not excluded. The Ej mechanism involves a cyclic transition state, which may be four-, five- or six-membered [4]. No discrete intermediate is known in this mechanism (concerted mechanism). Two examples of reactions with E mechanism involving different sizes of cyclic transition state are shown below [3] ... 

Some reactions taking place during pyrolysis occur through an E, mechanism (see Section 2.1). These reactions are not slowed down by free radical inhibitors, and therefore they do not involve free radicals. Typical reactions of this type are eliminations of small molecules such as HX (X = F, Cl, Br), water, methanol, ammonia, CO, CO2, etc. from the polymer side groups, A typical example of elimination reaction can be represented schematically by the equation ... 

The other type of pyrolytic elimination reaction involves free radicals. The steps are similar to those that we studied in the free radical substitution reactions, i.e. there is an initiation step, followed by several propagation steps, and then there are some termination steps. Free radical elimination is found in polyhalides and primary monohalides. For the general primary monohalide, R2CHCH2X,... 

The other type of pyrolytic elimination reaction involves free radicals, for example in polyhalides and primary monohalides. These reactions involve the normal sequence for radical reactions, namely initiation, propagation and termination steps. 

Halogenated flame retardants work by generating hydrogen halides on heating. They reduce or eliminate the free radical branching reactions involved in flame propagation ... 

The surface modification of polymethylsiloxane particles involved free radical chlorination. Carbon tetrachloride was employed to eliminate possible chain transfer by a solvent and small amounts of AIBN added to the reaction resulted in higher... 

Figure 56.1 shows that irradiation of a-iodoketones involves light-induced homolytic cleavage of the Cl bond proceeding to the radical pair (1) in solvents. When hexane is used, o,P-unsaturated ketone 6f and reduced product 6e are obtained by elimination and reduction. These processes involve free radicals. However, in the presence of a small amount of water in hexane, the radical pair (I) can undergo subsequent electron transfer to afford the ion pair (11) ultimately, the substituted product 2-hydroxycyclododecanone is formed. ... 

The trimethylsilyl ethers 212 of four-membered 1-alkenyl-1-cyclobutanols rearrange to the ring-expanded 0-mercuriocyclopentanones 213. These can be converted into the a-methylenecyclopentanones 214 through elimination or further expanded by one-carbon atom into cyclohexanones 215 via the Bu3SnH-mediated free radical chain reactions [116]. A similar radical intermediate is suggested to be involved in the ring expansion of a-bromomethyl-fi-keto esters [117]. (Scheme 84)... 

Russell and coworkers have made an extensive study of the photolytically initiated substitution reactions of a variety of reagents with 1-alkenyl derivatives of SnBu333,34, the general reaction being as shown in reaction 26. The process is thought to involve addition-elimination in a free radical chain mechanism, illustrated in Scheme 3. 

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