A radical abstracts a hydrogen atom from methane, leaving a primary methyl radical. The methyl radical then abstracts Cl• from Cl2 to give the desired product and another chlorine radical. Methane chlorination: propagationMethane chlorination: propagation The radical will then participate in another propagation reaction: the radical chain. Other products such as CH2Cl2 may also form.
Two free radicals (chlorine and chlorine, chlorine and methyl, or methyl and methyl) combine: Methane chlorination: terminationMethane chlorination: termination The last possibility generates in an impurity in the final mixture (notably, an organic molecule with a longer carbon chain than the reactants).
The relative rates at which different halogens react vary considerably:Script error: No such module "Unsubst".
fluorine (108) > chlorine (1) > bromine (Script error: No such module "val".) > iodine (Script error: No such module "val".).
Radical fluorination with the pure element is difficult to control and highly exothermic; care must be taken to prevent an explosion or a runaway reaction. With chlorine the reaction is moderate to fast; with bromine, slow and requires intense UV irradiation; and with iodine, it is practically nonexistent and thermodynamically unfavored. However, radical iodination can be completed with other iodine sources (see Template:Slink).[2]
Bond dissociation energies strongly influence any radical process and in a few unusual cases, free-radical halogenation can regioselect. Phenylic hydrogens have extremely strong bonds and are rarely displaced by halogens. Non-enolizable aldehydes oxidize to the acyl halide, but enolizable aldehydes typically halogenate at the α position instead. Indeed, allylic and benzylic hydrogens have bonds much weaker than alkanes, and are selectively replaced in the Wohl-Ziegler reaction. Generally, N-haloamines in sulfuric acid (but not other haloradical sources) halogenate alkane chains at penultimate carbons (e.g. pentane to 2-halopentane), chains terminating in only carboxylic acids at the center, and bridged compounds at the bridgehead. As of 2020[update]Script error: No such module "Check for unknown parameters"., the reasons for the latter N-haloimide selectivities remained unclear.[2]
Aside from those few exceptions, free-radical halogenation is notoriously unselective. Chlorination rarely stops at monosubstitution:[2] depending on reaction conditions, methane chlorination yields varying proportions of chloromethane, dichloromethane, chloroform and carbon tetrachloride.
For asymmetric substrates, the reaction produces all possible isomers, but not equally. Radical halogenations are generally indifferent amongst equi-substituted potential radicals and effect a so-called statistical product distribution. Butane (CH3−CH2−CH2−CH3), for example, can be chlorinated at the "1" position to give 1-chlorobutane (CH3−CH2−CH2−CH2Cl) or at the "2" position to give 2-chlorobutane (CH3−CH2−CHCl−CH3). The latter occurs faster, and is the major product.
Thus any single chlorination step slightly favors substitution at the carbon already most substituted. The rates are generally constant across reactions and predict product distributions with relatively high accuracy.[3][4] For example, 2-methyl butane ((CH3)2CHCH2CH3) exhibits the following results:
Note that the sole tertiary hydrogen is nearly as likely to chlorinate as the 6 hydrogens terminating the branches, despite their much greater abundance.
Variations
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Many mixtures of radical initiators, oxidants, and halogen compounds can generate the necessary halogen radicals. For example, consider radical bromination of toluene:[5]
