Free-radical halogenation

In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform (CHCl₃), dichloromethane (CH₂Cl₂), and hexachlorobutadiene. It proceeds by a free-radical chain mechanism.

General mechanism

The chain mechanism is as follows, using the chlorination of methane as an example:

; Initiation: Ultraviolet radiation splits (homolyzes) a chlorine molecule to two chlorine atom radicals. center|Methane chlorination: initiation

; Chain propagation (two steps): 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. center|Methane chlorination: propagation The radical will then participate in another propagation reaction: the radical chain. Other products such as CH₂Cl₂ may also form.

; Chain termination: Two free radicals (chlorine and chlorine, chlorine and methyl, or methyl and methyl) combine: center|Methane 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 net reaction is: center|Methane chlorination overall reaction

The steady-state approximation implies that this process has rate law k[CH₄][Cl₂].

As a radical reaction, the process is halted or severely slowed by radical traps, such as oxygen.

Control

The relative rates at which different halogens react vary considerably:

:fluorine (108) > chlorine (1) > bromine () > iodine ().

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 ).

Aside from those few exceptions, free-radical halogenation is notoriously unselective. Chlorination rarely stops at monosubstitution:)

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. For example, 2-methyl butane ((CH₃)₂CHCH₂CH₃) exhibits the following results:

{| class=wikitable style="margin-left: auto; margin-right: auto; border: none;"

! Moiety !! Type !! # Hydrogens !! || Relative rate !! !! Quantity !! Proportion

|-

| || Primary || 6 || × || 1 || = || 6 || 28%

|-

| || Tertiary || 1 || × || 5 || = || 5 || 23%

|-

| || Secondary || 2 || × || 3.8 || = || 7.6 || 35%

|-

| || Primary || 3 || × || 1 || = || 3 || 14%

|-

| colspan=2 | Total || colspan=4 | || 21.6 || 100%

|}

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

Many mixtures of radical initiators, oxidants, and halogen compounds can generate the necessary halogen radicals. For example, consider radical bromination of toluene:

frameless|center|upright=2|bromination of toluene with hydrobromic acid and hydrogen peroxide in water

This reaction takes place on water instead of an organic solvent and the bromine is obtained from oxidation of hydrobromic acid with hydrogen peroxide. An incandescent light bulb suffices to radicalize.

Other sources include alkyl hypohalites or single-electron oxidation-capable transition metals. In particular, tert-butyl hypoiodite is a common iodine source for radical iodination.