Exercise 9: - Radical reactions
- tert-Butyl peroxide is a source of
free radicals:
A mixture of methylpropane and CCl4
is stable at 130°C. If a small amount
of tert-butyl peroxide is added, a
reaction occurs that yields 2-chloro-2-methylpropane and chloroform. A small amount of 2-methyl-2-propanol, (CH3)3COH,
equivalent to the peroxide used, is also formed. Propose a mechanism which explains the
formation of these products.
Under the influence of heat
or light a molecule of peroxide dissociates:
To
determine the fate of the (CH3)3CO. we must
look at the products and the sentence “A small amount of 2-methyl-2-propanol, (CH3)3COH,
equivalent to the peroxide used, is also formed” provides the answer. We can start step 2 by writing
the following:
To
determine the source of the hydrogen we must look at the reactants. CCl4 has no hydrogen atoms. Thus the hydrogen of the alcohol must come
from the methylpropane:
To
determine which hydrogen in abstracted, look at the products. 2-Chloro-2-methylpropane can only be formed
by abstraction of the tertiary hydrogen.
Step 2 is therefore:
The new intermediate
(CH3)3C. starts step
3. What is its fate? Look at the products. It must form (CH3)3CCl. The chlorine must come from the CCl4
hence step 3 is:
The new intermediate
.CCl3 starts step 4.
What is its fate? Look at the
products. It must form HCCl3. The hydrogen must come from the (CH3)3CH
hence step 4 is:
Steps 3 and
4 constitute a chain reaction.
The overall
mechanism is:
2. A mixture of H2 and Cl2
does not react at room temperature. At
high temperature or under the influence of light (of wavelength absorbed by
chlorine) a violent reaction takes place and HCl is formed. The photochemical reaction yields as many as
a million molecules of HCl for each photon absorbed. The presence of a trace of oxygen slows down
the reaction. Propose a possible
mechanism for this reaction.
3. When a mixture of methane and chlorine
is irradiated, reaction begins at once.
However, when irradiation is stopped, the reaction gradually slows down;
it does not stop immediately. Why?
It is a chain reaction and
will continue until all reactive species are consumed.
4. Bromination of methane is slowed down
by the addition of fairly large amounts of HBr.
Suggest an explanation.
