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 a typical example:

1. Initiation: Splitting or homolysis of a chlorine molecule to form two chlorine atoms, initiated by ultraviolet radiation or sunlight. A chlorine atom has an unpaired electron and acts as a free radical.
2. chain propagation (two steps): a hydrogen atom is pulled off from methane leaving a primary methyl radical. The methyl radical then pulls a Cl• from Cl2.

This results in the desired product plus another chlorine radical. This radical will then go on to take part in another propagation reaction causing a chain reaction. If there is sufficient chlorine, other products such as CH2Cl2 may be formed.
3. chain termination: recombination of two free radicals:
The last possibility in the termination step will result in an impurity in the final mixture; notably this results in an organic molecule with a longer carbon chain than the reactants.

The net reaction is:

In the case of methane or ethane, all the hydrogen atoms are equivalent and thus have an equal chance of being replaced. This leads to what is known as a statistical product distribution. For propane and higher alkanes, the hydrogen atoms which form part of CH2 (or CH) groups are preferentially replaced.

The reactivity of the different halogens varies considerably. The relative rates are: fluorine (108) > chlorine (1) > bromine (7 × 10−11) > iodine (2 × 10−22). Hence the reaction of alkanes with fluorine is difficult to control, that with chlorine is moderate to fast, that with bromine is slow and requires high levels of UV irradiation while the reaction with iodine is practically nonexistent and thermodynamically unfavorable.

A common method in organic synthesis employing is the Wohl–Ziegler reaction, which employs N-bromosuccinimide, which can undergo homolysis to yield a bromine radical and is a free-radical halogenation variant.

Control of halogenation

For clarity, the unique hydrogens will be labeled as follows:
a = (CH3)2 (primary), b = CH (tertiary), c = CH2 (secondary), d = CH3 (also primary)
Applying the aforementioned substitution ratios in the formula: ([number of hydrogens] × [ratio factor]) / [(primary hydrogens × 1) + (secondary hydrogens × 3.8) + (tertiary hydrogens × 5)]
a: 6 × 1 = 6           a = 6/21.6 = 28%
b: 1 × 5 = 5           b = 5/21.6 = 23%
c: 2 × 3.8 = 7.6     c = 7.6/21.6 = 35%
d: 3 × 1 = 3           d = 3/21.6 = 14%
Denominator total for this species = 6 + 5 + 7.6 + 3 = 21.6

Because all six "a" hydrogens, both "c" hydrogens, and the three "d" hydrogens are chemically equivalent with the others in their three classifications (i.e., any "a" hydrogen is equivalent with any other "a" hydrogen), these rates accurately reflect where a single chlorination may take place for 2-methylbutane. The single tertiary hydrogen "b" is nearly as susceptible as the six, primary "a" hydrogens, and almost doubly susceptible as any of the three, also primary "d" hydrogens, illustrating the radical stability differences between tertiary and primary hydrogens (the secondary "c" hydrogens also follow the radical stability order as previously mentioned).

Free-radical iodination is usually not possible because iodine is too unreactive to form a radical. For the other halogens, free-radical halogenation generally proceeds in the following order:

Oxygen is a halogenation inhibitor.

An example of radical bromination of toluene is given below:[3]

This reaction takes place on water instead of an organic solvent and the bromine is obtained by oxidation of hydrobromic acid with hydrogen peroxide. An incandescent light bulb is sufficient for bromine radical generation.

See also

References

  1. Carey, F.A. Organic Chemistry, Sixth Edition. New York, NY: McGraw Hill. 2006.
  2. Peters, W. Chemistry 3421 Lecture Notes. University of Colorado, Denver. Radical Halogenation, Fall 2006.
  3. Free radical bromination by the H2O2–HBr system on water Ajda Podgoršek, Stojan Stavber, Marko Zupana, and Jernej Iskraa Tetrahedron Letters 47 (2006) 7245–7247 doi:10.1016/j.tetlet.2006.07.109
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