Radical substitution reactions halogenation

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. 

The initiation step is the light-induced formation of chlorine atoms as the radicals. Only a few chlorine molecules will suffer this fate, but these highly reactive radicals then rapidly interact with the predominant molecules in the system, namely cyclohexane. 

Finally, when we are running out of cyclohexane, the process terminates by the interaction of two radical species, e.g. two chlorine atoms, two cyclohexyl radicals, or one of each species. The combination of two chlorine atoms is probably the least likely of the termination steps, since the Cl-Cl bond would be the weakest of those possible, and it was light-induced fission of this bond that started off the radical reaction. Of course, once we have formed cyclohexyl chloride, there is no reason why this should not itself get drawn into the radical propagation steps, resulting in various dichlorocyclohexane products, or indeed polychlorinated compounds. Chlorination of an alkane will give many different products, even when the amount of chlorine used is limited to molar ratios, and in the laboratory it is not going to be a particularly useful process. 

However, it is instructive to consider radical chlorination of alkanes just a little further, to appreciate the mechanistic concepts. If we carry out light-induced chlorination of propane, then we obtain 

Bromine will also halogenate alkanes, but in this case we find that bromine is considerably less reactive than chlorine. As a result, the reaction becomes much more selective, and the product ratios are more distinctive. In fact, bromination of alkanes is so selective that it is a feasible laboratory process to make alkyl bromides from alkanes. 

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Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

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. 

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

Radical substitution reactions involving allylic tin derivatives could be accompanied by a photoinduced 1,3-rearrangement54,55. A photostationary mixture of cinnamyl(tri-phenyl)stannane with its regioisomer l-phenylprop-2-enyl(triphenyl)stannane has been shown to form in the photolysis of ( )-cinnamyl(triphenyl)stannane in benzene under aerobic conditions, or in the presence of halogenated organic compounds or radicaltrapping reagents (equation 21). 

Radical substitution reactions can also be used to remove functional groups from molecules. A useful reagent for this (and, as you will see, for other radical reactions too) is tributyltin hydride, Bu3SnH. The Sn-H bond is weak and B SnH will react with alkyl halides to replace the halogen atom with H, producing BL SnHal as a by-product. 

In the presence of light or heat, alkanes react with halogens to form alkyl halides. Haiogenation is a radical substitution reaction, because a halogen atom X replaces a hydrogen via a mechanism that involves radical intermediates. 

In order to maximize the amount of monohalogenated product obtained, a radical substitution reaction should be carried out in the presence of excess alkane. Excess alkane in the reaction mixture increases the probability that the halogen radical will collide with a molecule of alkane rather than with a molecule of alkyl halide—even toward the end of the reaction, by which time a considerable amount of alkyl halide will have been formed. If the halogen radical abstracts a hydrogen from a molecule of alkyl halide rather than from a molecule of alkane, a dihalogenated product will be obtained. 

If a reactant does not have an asymmetric carbon, and a radical substitution reaction forms a product with an asymmetric carbon, a racemic mixture will be obtained. A racemic mixture is also obtained if a hydrogen bonded to an asymmetric carbon is substituted by a halogen. If a radical substitution reaction creates an asymmetric carbon in a reactant that already has an asymmetric carbon, a pair of diastereomers will be obtained in unequal amounts. 

Alkenes can undergo free radical substitution reactions with halogens. Which of the following best represents a chain propagation step during the free radical chlorination of methane ... 

Allylic hydrogens are especially reactive in radical substitution reactions. We can synthesize allylic halides by substitution of allylic hydrogens. For example, when propene reacts with bromine or chlorine at high temperatures or under radical conditions where the concentration of the halogen is small, the result is allylic substitution. 

Similar free radical substitution reactions can be effected with bromine (using, e.g., A-bromosuccinimide [NBS]) and, once the halogen is in place, nucleophilic substitution at carbon a to silicon (or indeed, elsewhere) occurs with relative ease. ... 

In Summary Increased reactivity goes hand in hand with reduced selectivity in radical substitution reactions. Huorine and chlorine, the more reactive halogens, discriminate between the various types of C-H bonds much less than does the less reactive bromine (Table 3-6). 

Control of addition vs substitution by free radicals can be effected by the reaction conditions, ie, radical concentration, temperature, and phase. Using halogens as propylene reactants, high temperatures and the gas phase favor high radical concentrations and substitution reactions cold, Hquid-phase conditions favor addition reactions. 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

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