Reaction types
Brief definition
Where they occur
Reduction ¼ gain of electrons.
Alkene, alkyne, aldehydes,
ketones, alkyl halides,
nitriles, carboxylic acid
and its derivatives, and
benzene and its derivatives.
Pericyclic reactions
Concerted reaction that takes
Conjugated dienes and
place as a result of a cyclic
a,b-unsaturated carbonyl
rearrangement of electrons.
compounds.
5.2 Radical reactions: free radical chain reactions
A radical, often called a free radical, is a highly reactive and short lived
species with an unpaired electron. Free radicals are electron-deficient
species, but usually uncharged. So their chemistry is very different from
the chemistry of even-electron and electron-deficient species, e.g. carbocations and carbenes. A radical behaves like an electrophile, as it
requires only a single electron to complete its octet.
Radical reactions are often called chain reactions. All chain reactions
have three steps: chain initiation, chain propagation and chain termination.
For example, the halogenation of alkane is a free radical chain reaction.
5.2.1 Preparation of alkyl halides
Chlorine or bromine reacts with alkanes in the presence of light (hn) or high
temperatures to give alkyl halides. Usually, this method gives mixtures of
halogenated compounds containing mono-, di-, tri- and tetra-halides. However, this reaction is an important reaction of alkanes as it is the only way to
convert inert alkanes to reactive alkyl halides. The simplest example is the
reaction of methane with Cl 2 to yield a mixture of chlorinated methane
derivatives.
CH 4
Methane
þ Cl 2 À!
hn CH 3 Cl
Methyl
chloride
þ CH 2 Cl 2
Dichloro
methane
þ CHCl 3
Chloroform
þ CCl 4
Carbon
tetrachloride
To maximize the formation of monohalogenated product, a radical substitution reaction must be carried out in the presence of excess alkane. For
example, when a large excess of methane is used, the product is almost
completely methyl chloride (chloromethane).
CH 4
Methane
ðLarge excessÞ
þ Cl 2 !
hn CH 3 Cl þ HCl
Methyl
chloride
192
CH5 ORGANIC REACTIONS
Brief definition
Where they occur
Reduction ¼ gain of electrons.
Alkene, alkyne, aldehydes,
ketones, alkyl halides,
nitriles, carboxylic acid
and its derivatives, and
benzene and its derivatives.
Pericyclic reactions
Concerted reaction that takes
Conjugated dienes and
place as a result of a cyclic
a,b-unsaturated carbonyl
rearrangement of electrons.
compounds.
5.2 Radical reactions: free radical chain reactions
A radical, often called a free radical, is a highly reactive and short lived
species with an unpaired electron. Free radicals are electron-deficient
species, but usually uncharged. So their chemistry is very different from
the chemistry of even-electron and electron-deficient species, e.g. carbocations and carbenes. A radical behaves like an electrophile, as it
requires only a single electron to complete its octet.
Radical reactions are often called chain reactions. All chain reactions
have three steps: chain initiation, chain propagation and chain termination.
For example, the halogenation of alkane is a free radical chain reaction.
5.2.1 Preparation of alkyl halides
Chlorine or bromine reacts with alkanes in the presence of light (hn) or high
temperatures to give alkyl halides. Usually, this method gives mixtures of
halogenated compounds containing mono-, di-, tri- and tetra-halides. However, this reaction is an important reaction of alkanes as it is the only way to
convert inert alkanes to reactive alkyl halides. The simplest example is the
reaction of methane with Cl 2 to yield a mixture of chlorinated methane
derivatives.
CH 4
Methane
þ Cl 2 À!
hn CH 3 Cl
Methyl
chloride
þ CH 2 Cl 2
Dichloro
methane
þ CHCl 3
Chloroform
þ CCl 4
Carbon
tetrachloride
To maximize the formation of monohalogenated product, a radical substitution reaction must be carried out in the presence of excess alkane. For
example, when a large excess of methane is used, the product is almost
completely methyl chloride (chloromethane).
CH 4
Methane
ðLarge excessÞ
þ Cl 2 !
hn CH 3 Cl þ HCl
Methyl
chloride
192
CH5 ORGANIC REACTIONS