Halogenation free-radical chain mechanism

The elementary steps (1) through (3) describe a free-radical chain mechanism for the reaction of an alkane with a halogen. 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

Rather than using the halogens themselves, other halo n radical donors are more commonly used in laboratory scale synthesis. One of the simplest of these is CCU, which can chlorinate alkanes by a free radical chain mechanism.The chain lengths are not very long (equations 76-78), because of their slightly endothermic nature and in part because the reaction is also kinetically rather slow. Elevated temperatures are therefore normally required. Nitrosylchloride at 1(X) C has also been used for these reactions. ... 

Halogenation proceeds by the free-radical chain mechanism shown in Mechanism 10.2. 

Reduction by hydrogen-atom donors necessarily involves intermediates with unpaired electrons. Tri-n -butyltin hydride is the most important example of this type of reducing agent. It is able to reductively replace halogen by hydrogen in many types of halogen compounds. Mechanistic studies have indicated a free-radical chain mechanism. The ability of trisubstituted stannanes to function effectively in these... 

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

Iodine accelerates the decomposition of acetaldehyde In the steady-state range, the order is approximately 1.0 and 0.5 in aldehyde and iodine, respectively. The experimental results of Rollefson and Faull have been reinterpreted and added to by O Neal and Benson . The iodine-catalysed reaction is a free radical chain process initiated by the attack of an iodine atom on the acetaldehyde molecule. The proposed mechanism fits the experimental data very well. The thermal decomposition of acetaldehyde is catalysed also by other halogens and halogen compounds . 

Halogenated systems in the gas phase act by interrupting the radical chain mechanism that sustains the flame, whereby they are highly efficient. The different halogen-free systems, on the other hand, work like this ... 

A reaction mechanism is a step-by-step description of the bond-breaking and bond-making processes that occur when reagents react to form products. In the case of halogenation, various experiments show that this reaction occurs in several steps, and not in one magical step. Indeed, halogenation occurs via a free-radical chain of reactions. 

In the preceding section we saw that nucleophilic substitutions on R-X reactants can, under some circumstances, occur via a radical chain mechanism. A much more common substitution pathway involves substitution of a hydrogen on an R-H reactant by a halogen. This involves conventional free radicals and a chain mechanism. The term given to these kinds of substitutions is Sh2 (substitution, homolytic, bimolecular). The most synthetically useful free radical substitution involves halogenation, and N-bromosuccinimide (NBS) is a common reagent (covered in the Connections highlight at the end of this section). 

Most halogenation reactions of hydrocarbons proceed via a free-radical chain reaction mechanism. Of the halogens CI2, Btj, and Ij, which initiation reaction should proceed most easily Explain your answer. 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Chain reaction (Section 4.17) Reaction mechanism in which a sequence of individual steps repeats itself many times, usually because a reactive intermediate consumed in one step is regenerated in a subsequent step. The halogenation of alkanes is a chain reaction proceeding via free-radical intermediates. 

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

---