20.16
Radical Substitution: Halogenation of Alkanes and Alkyl Substituents
In the presence of heat or light, alkanes react with molecular halogens to form alkyl halides by a substitution reaction called radical halogenation.…
Alkanes react with molecular halogens in the presence of light or heat to form halogenated alkanes.
Such substitution reactions are called radical halogenations, and proceed via initiation, propagation, and termination steps.
Consider the chlorination of methane to produce methyl chloride.
During the initiation step, the chlorine molecule undergoes homolytic cleavage to form two highly reactive chlorine radicals.
Next, in the first propagation step, the chlorine radical abstracts hydrogen from a methane molecule to produce a methyl radical and a hydrogen chloride molecule.
In the second propagation step, the methyl radical abstracts a chlorine atom from a chlorine molecule, forming methyl chloride and regenerating the chlorine radical.
Finally, coupling of any two radicals depletes the reactive intermediates and terminates the reaction.
Alkane chlorination often results in multiple substitutions or polychlorination. An excess of alkane ensures monochlorination.
Interestingly, chlorination of higher alkanes produces a mixture of isomeric mono-chlorinated alkanes and polychlorinated products.
Notably, only fluorine, chlorine, and bromine react with alkanes via radical halogenation; iodine is unreactive.
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Q1: What are the three main steps of radical halogenation?
Radical halogenation proceeds through initiation, propagation, and termination. During initiation, the halogen molecule undergoes homolytic cleavage under light or heat to form reactive radicals. Propagation involves two steps where radicals abstract atoms and regenerate themselves. Termination occurs when two radicals couple, depleting reactive intermediates and stopping the reaction.
Q2: How does the initiation step begin in chlorination of methane?
In the initiation step, a chlorine molecule undergoes homolytic cleavage in the presence of light or heat, forming two highly reactive chlorine radicals. These radicals then enter the propagation phase where they abstract hydrogen atoms from alkane molecules, starting the chain reaction that produces halogenated products.
Q3: What happens during the propagation steps of methane chlorination?
Propagation occurs in two steps. First, a chlorine radical abstracts hydrogen from methane, producing a methyl radical and hydrogen chloride. Second, the methyl radical abstracts a chlorine atom from another chlorine molecule, forming methyl chloride and regenerating the chlorine radical. This cycle repeats, sustaining the chain reaction.
Q4: Why does excess alkane favor monochlorination over polychlorination?
Excess alkane ensures monochlorination because the high concentration of unreacted alkane molecules makes it more likely that radicals will encounter and react with alkane rather than the more reactive halogenated product. When chlorine is limited and alkane is abundant, the primary product methyl chloride undergoes fewer subsequent chlorination reactions.
Q5: Which halogens react with alkanes through radical halogenation?
Only fluorine, chlorine, and bromine react with alkanes via radical halogenation. Fluorine is the most reactive, chlorine is moderately reactive, and bromine is less reactive than chlorine. Iodine is generally unreactive towards alkanes through this mechanism, making it unsuitable for radical halogenation reactions.
Q6: What products form when chlorinating higher alkanes?
Chlorination of higher alkanes produces a mixture of isomeric mono-chlorinated alkanes and polychlorinated products. Because chlorine is unselective, it attacks multiple positions on the alkane and can substitute multiple hydrogens. This lack of selectivity makes radical halogenation unsuitable as a synthetic method for preparing specific halogenated products.
Q7: How does the termination step stop the radical halogenation reaction?
Termination occurs when any two radicals couple together, forming a stable bond and destroying the reactive intermediates. This coupling depletes the radical species needed to propagate the chain reaction, causing the halogenation to stop. Common termination products include chlorine-chlorine or radical-radical combinations that are no longer reactive.