Section 9.2 Chlorination and Bromination of Alkanes 3393-D Molecule:
Methyl radicalAny two radicals in the reaction mixture can combine to form a molecule in which all
the electrons are paired. The combination of two radicals is called a termination step
because it helps bring the reaction to an end by decreasing the number of radicals
available to propagate the reaction. The radical chlorination of alkanes other than
methane follows the same mechanism. A radical chain reaction, with its characteris-
tic initiation, propagation, and termination steps, was first described in Section 4.10.
The reaction of an alkane with chlorine or bromine to form an alkyl halide is
called a radical substitution reactionbecause radicals are involved as intermedi-
ates and the end result is the substitution of a halogen atom for one of the hydrogen
atoms of the alkane.
In order to maximize the amount of monohalogenated product obtained, a radical
substitution reaction should be carried out in the presence of excess alkane. Excess
alkane in the reaction mixture increases the probability that the halogen radical will
collide with a molecule of alkane rather than with a molecule of alkyl halide—even
toward the end of the reaction, by which time a considerable amount of alkyl halide
will have been formed. If the halogen radical abstracts a hydrogen from a molecule of
alkyl halide rather than from a molecule of alkane, a dihalogenated product will be
obtained.
Bromination of alkanes follows the same mechanism as chlorination.
2 Brpropagation stepsinitiation stepBrmechanism for the monobromination of ethane
+ CH 2 CH 3 CH 3 CH 2 + HBrCH 3 CH 2CH 3 CH 2 CH 3 CH 2CH 3 CH 2Br BrBr+ CH 3 CH 2 Br + Br+ Br 2++CH 3 CH 2 CH 2 CH 3CH 3 CH 2 Brtermination steps∆HBr BrBr Bror
hCl HClCH 2 Cl + Cl 2++CH 3 Cl CH 2 ClCH 2 Cl 2 + Cla dihalogenated compoundCl Cl 2 Cl initiation stepClor
hCl + H HCl + CH 3
a methyl radical propagation stepsCH 3CH 3mechanism for the monochlorination of methane+ CH 3 Cl + ClCl ClCl+ Cl 2CH 3 + CH 3 CH 3 CH 3+ CH 3 CH 3 Cltermination stepsCl∆Tutorial:
Radical chain reaction3-D Molecule:
Ethyl radical