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Radical Substitution Reactions

Radical Substitution Reactions

Radical substitution reactions are fundamentally different from polar substitutions, such as electrophilic and nucleophilic substitution reactions.

Reaction types at carbon atoms.

Comparison between an electrophilic (red), a nucleophilic (blue), and a radical (green) attack.

Radicals attack the periphery of molecules - usually the hydrogen atom of a C-H bond. In contrast, the active agents in polar substitution reactions attack electrophilic or nucleophilic positions, respectively, in the molecule. The radical chlorination of methane is a good example of radical substitution reactions and is illustrated below. The differences between the chlorination of methane and longer-chain alkanes, as well as other halogenations (fluorine, bromine, iodine) are discussed.

Radical chlorination of methane

When methane and chlorine gas are mixed at room temperature in the dark, no reaction takes place. However, if this mixture is heated to about 300 °C or irradiated with ultraviolet light, the result is a vigorous reaction, which yields a product mixture of chlorinated C1 compounds and hydrogen chloride.

Radical chlorination of methane.

The product composition of the reaction depends on the amount of chlorine gas that was applied and the amount of time for which the energy acted. By determining the product composition as function of time, it may be proven that monochloromethane (CH3Cl) is first formed, as is also expected. If the reaction time is long enough and a sufficient amount of chlorine is applied, methane is completely converted into carbon tetrachloride (CCl4). Dichloromethane (CH2Cl2) and chloroform (CHCl3) are intermediate products of this reaction. In order to get a better understanding of the course and the results of the reaction, let us first take a look at the enthalpies of formation and the dissociation energies of the starting products and products that are involved.

The reaction enthalpy ΔH0 can be calculated using the enthalpies of formation ΔH0f of the starting products and the products, which are shown in the illustration under the respective compounds. As a result, the reaction enthalpy of the formation of monochloromethane and hydrogen chloride from methane and chlorine is -99.4 kJ/mol. That is, the reaction is very exothermic.

Enthalpies of formation and dissociation energies in the chlorination of methane.

Approximately the same result is obtained when the reaction enthalpy is calculated using the dissociation energies ΔHDiss of the bonds involved in the reaction, namely -104.5 kJ/mol.

Though the reaction is very exothermic, at room temperature it virtually never occurs. Obviously, the activation energy EA of the reaction is particularly high, as the reaction rate is controlled by the activation energy and not by the reaction enthalpy. The reaction mechanism of the chlorination of methane is further explained below.

Exercise: Radical substitution reactions

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