Radical chain reactions halogenation

Chloroform reacts readily with halogens or halogenating agents. Chlorination of the irradiated vapor is beUeved to occur by a free-radical chain reaction (7). 

Free radical chain reactions depend on an easily generated free radical to initiate the chain. One way to generate this radical is to irradiate halogens, such as Ch and Brj. Another way is to add a small amount of an initiator molecule to the reaction mixture, such as AIBN. This molecule, when heated, decomposes into free radicals that react with other molecules to initiate a chain reaction. 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

Irradiation of mixtures of halogen and hydrocarbons at suitable wavelengths can lead to halogenation of the hydrocarbon by a free-radical chain reaction. 

Halogenation reactions of alkanes provide good examples of radical processes, and may also be used to illustrate the steps constituting a radical chain reaction. Alkanes react with chlorine in the presence of light to give alkyl chlorides, e.g. for cyclohexane the product is cyclohexyl chloride. 

Halogen-mediated synthesis of cyclic peroxides through free radical chain reactions... 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

A free radical chain reaction is also called a radical substitution reaction, because radicals are involved as intermediates, and the end result is the substitution of a halogen atom for one of the hydrogen atoms of alkane. 

A general free-radical chain reaction with the thermal or photoinduced dissociation of halogen as the initiation step is operative in radical halogenation [Eqs. (10.12)—(10.14)]. 

Different product ratios are expected from Cl- and C1S02- for the same reason that Cl- and Br- lead to different product ratios (Section 4-5A). Other reagents that sometimes are useful halogenating agents in radical-chain reactions include... 

The foregoing discussion adds further to our understanding of the selectivity observed in the halogenation reactions discussed in Chapter 4. When propene is chlorinated in sunlight, the product is 3-chloropropene, and we may explain this on the basis that the radical-chain reaction involves propagation steps in which a chlorine atom attacks the hydrogen corresponding to the weakest C-H bond ... 

Know the meaning of substitution reaction, halogenation, chlorination, bromination, free-radical chain reaction, chain initiation, propagation, termination, combustion. 

Write all steps in the free-radical chain reaction between a halogen and an alkane, and identify the initiation, propagation, and termination steps. 

In a free-radical chain reaction, every propagation step must occur quickly, or the free radicals will undergo unproductive collisions and participate in termination steps. We can predict how quickly the various halogen atoms react with methane given relative rates based on the measured activation energies of the slowest steps ... 

The addition of halogenated compounds increased the conversion of cyclohexane, while the selectivities remained substantially unchanged. This indicates the presence of a radical-based mechanism for the activation of cyclohexane. In fact, it is known that the radical chain reaction is enhanced in the presence of halogens, owing to their tendency to give rise to X species. 

Fire is a chain process. It can be stopped by breaking this chain. Scavengers are used to stop the free radical chain reactions and subsequently fire is extinguished. Halogenated compounds are usually good chain-reaction inhibitors. 

The formation of cyclopropyl halide radical anion pairs as intermediates is also invoked in Sr I type substitution reactions by Rossi and Meijs . It seems that the photostimulated reaction of cyclopropyl bromides like 7-bromonorcarane (120) with Ph2P M to give 121 involves a radical chain, and halogen-containing radical anions as chain carrier. 

Much of the chemistry of alkanes involves free-radical chain reactions, which take place under vigorous conditions and usually yield mixtures of products. A reactive particle—typically an atom or free radical—is needed to begin the attack on an alkane molecule. It is the generation of this reactive particle that requires the vigorous conditions the dissociation of a halogen molecule into atoms, for example, or even (as in pyrolysis) dissociation of the alkane molecule itself. 

Allylic bromination by NBS is a radical-chain reaction occurring on the surface of the NBS crystals. A solution of NBS in tetrachloroethane or nitro-methane adds bromide to a C=C group 354 solvents are therefore used in which NBS and, if possible, also the succinimide formed, are difficultly soluble dry CC14 is usually chosen, but for reactive substances cyclohexane or benzene may also be used. The more polarized the N-halogen is, i.e.9 the more positive the Br is rendered, the more easily does the A-halogenated compound cause addition to the C=C bond NBS is particularly suitable for allylic bromination because of its steric structure and the almost apolar nature of its N-Br bond. 

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