The Mechanism of Halogenation

Problem 15.4 Draw all constitutional isomers formed by monochlorination of each alkane. 

Problem 15.5 Compounds A and B are isomers having molecular formula C5Hi2- Heating A with CI2 gives a 

The chlorination of ethane illustrates the three distinct parts of radical halogenation (Mechanism 15.1)  

Initiation Two radicals are formed by homolysis of a o bond and this begins the reaction. 

Propagation A radical reacts with another reactant to form a new a bond and another radical. 

Light or heat provides the energy needed for homoiytic bond cieavage to form radicais. Breaking the weak 0-0 bond of peroxides initiates radical reactions as well. 

The diradical O2 removes radicals from a reaction mixture, thus preventing reaction. 

Initiation Two radicals are formed by homolysis of a a bond and this begins the reaction. Propagation A radical reacts with another reactant to form a new o bond and another radical. 

Termination Two radicals combine to form a stable bond. Removing radicals from the reaction mixture without generating any new radicals stops the reaction. 

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These observations must be taken into account when considering the mechanism of halogen addition They force the conclusion that a simple one step bond switching process of the following type cannot be correct A process of this type requires syn addi tion It IS not consistent with the anti addition that we actually see... 

Steinmetz, H., and R. M. Noyes The Mechanism of Halogen-atom Catalyzed... 

Larsen, E. R. Ludwing, R. B. On the mechanism of halogen s flame suppressing properties, Journal of Fire and Flammability/Fire Retardant Chemistry, 1979, 10, 69-77. 

The mechanisms of halogenation in acid and base are somewhat different. 

Although the mechanism of halogen-metal exchange reactions is not clear, three mechanisms are supposed - reaction via four-centered transition states, via radical intermediates, and via halogen ate complexes. 

The mechanisms of halogen abstraction by "C CO CsRs were investigated for a range of organic halides.69 Some of the most interesting results were for organoha-lides with (3-H-atoms such as PhCHMeBr. Experimental kinetic data were fit to the proposed mechanism in reactions 10.47-10.53. 

No complete study on the mechanism of halogenation of metal acetyl-acetonates has been carried out. Although the halogenating agents used in these reactions can form the 3-halo compounds via a radical or ionic mechanism, it seems reasonable to assume that these reactions are electrophilic substitution reactions. The probable mechanism is as follows (49, 112) the chelate ring undergoes Sn2 attack by the electrophile ... 

For the mechanism of halogenation of heterocycles and the influence of the medium (solvent, addition of acid to convert A-heterocycles into their conjugated acids, and addition of base for production of the anions from acidic heterocycles such as imidazole and pyrrole), as well as the influence of temperature a review by Eisch717 should be consulted. 

These investigations by Grovenstein and Bourns settled an ambiguity with respect to the mechanisms of halogenations with molecular bromine and iodine. Painter and Soper s (1947) thorough kinetic study... 

The mechanism of halogenation of the dimers [Fe(/t -C6H4R)(CO)2]2, where R = H or Me, involves attack of halogen at one of the iron atoms, followed by asymmetric or symmetric cleavage. Points of similarity between addition of halogens to alkenes and to these iron dimers are stressed. ... 

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