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The reactions of alkanes with halogens
The halogenation of alcanes involves
substitution of a halogen atom for one of the alkane’s hydrogens. The
halogenation reaction takes place by a radical mechanism.
Methane, ethane, and other alkanes react with
fluorine, chlorine, and bromine. Alkanes do not react appreciably
with iodine.
Mechanism for the halogenation
reaction
The first step is the
fragmentation of a chlorine molecule, by heat or light, into two chlorine atoms,
called the initiation step.
In the second step a chlorine atom
abstracts a hydrogen atom from a methane molucule. This step produces a
molecule of hydrogen chloride and a methyl radical. A methyl radical
abstracts a chlorine atom from a chlorine molecule. This step produces a
molecule of methyl chloride and a cholrine atom.
The chain sequence is interrupted whenever two odd-electron
species combine to give an even-electron product. Reactions of this type
are called chain-terminating steps.
Reactivity
of the halogens
Fluorine is most
reactive, so reactive that without special precautions mixtures of fluorine and
methane explode.
Chlorine is the next most reactive,
chlorination of methane is easily controlled by the control of heat and light.
Bromine is much less reactive toward
methane than chlorine.
Iodine is so unreactive that the reaction
between it and methane does not take place for all practical purposes
The reactivity of the
halogens decreases in the order, F2>Cl2>Br2>I2
Structure and stability of free radicals
Free radicals are species that
contain unpaired electrons. The order of stability of alkyl radicals is
the same as for carbocations: Tertiary > Secondary > Primary > Methyl.
Free radicals, like carbocations, have an unfilled
2p orbital and are stabilized by substituents, such as alkyl groups, that
release electrons.
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