Section Five - Radicals
Question 5.1
a.)
Chlorination of cyclopentane gives a single major product while chlorination of
methylcyclopentane gives a ghastly mixture. Why the difference?
b.) If you used bromine instead of chlorine in the halogenation of methylcyclopentane, what would
the major product be?
1°
H3 C
H
3°
h!, Cl2
H3 C
Cl
ClH2 C
H
2°
1° alkyl
halide
H3 C
Cl
H
H3 C
Cl
3° alkyl
halide
H3 C
H
Cl
Diastereomers, each racemic
H3 C
H
H
Cl
Diastereomers, each racemic
2° alkyl
halides
Answer 5.1
a) All of the 10 hydrogens in cyclopentane are identical - substitution of any one of them gives a
single product. Methylcyclopentane, on the other hand, has many different types of hydrogen:
b.) Bromine is much more selective in its radical reactions. The principal product will be that
formed from the most stable (i.e. 3°) radical.
1 x 3° Hs
H
CH3
Br
Br 2, h !
1
1
CH3
Question 5.2
On a past mid-term students were asked to show the elements of a radical chain reaction using an
example we did in class: either alkene polymerization or the halogenation of alkanes. For whatever
reason, one student started with the initiation step corresponding to the halogenation of alkanes but
used alkenes in the propagation steps instead of alkanes. The overall reaction he proposed was:
Cl2 + H2C
h!
CH2
ClCH2
CH2Cl
He came up with a consistent free-radical mechanism for this reaction. Can you? (By the way, the
product is an important industrial chemical because it is used to make vinyl chloride for PVC
polymers. 1,2-Dichloroethane is indeed formed via the free-radical mechanism proposed by this
student.)
Answer 5.2
Cl
h!
Cl
Cl + H2C
2 Cl
CH2
Initiation
Cl CH2 CH2
Propagation
Cl
CH2 CH2
Cl
Cl
Cl
CH2 CH2
Cl CH2 CH2 Cl
Cl
Cl
Cl CH2 CH2 Cl
Termination
2 Cl
CH2 CH2
ClCH2CH2CH 2CH2Cl
etc.
Question 5.3
Give a radical chain mechanism for the following bromination of propylene.
H
H
H 2C
C
CH 3
B r2, h!
C
H 2C
2
2
CH 2
Br
Answer 5.3
Br
Br
h!
2 Br
H
H2C
Initiation
H
H
C
HBr + H2C
H2C
C
C
CH2
CH2
H
Br
Stable allylic radical
H
H2C
Propagation
CH2
H
C
CH2
Br
Br
H2C
C
Br
C Br
H2
Termination via radical coupling.
Question 5.4
In class we looked at the free radical polymerization of ethylene and several substituted derivatives.
Styrene for example:
H2C
C
H
H2C
C
H
R•
Ph
Ph
Ph
Ph
Ph
Polystyrene
Styrene
Notice that in the structure of the polymer there is a regular pattern of alternating of CH2 groups and
CHPh groups. This is called head-to-tail polymerization.
Occasionally, the following substructure occurs in the polymer chain
Ph
Ph
Ph
Ph
in which a "head-to-head" defect occurs. This is rare.
Beginning with a radical, R•, to get your polymerization initiated, show the mechanism of
this polymerization process and give an example of a termination step.
Answer the following questions:
a.) Why is the vast majority of the polystyrene a "head-to-tail" polymer?
b.) How are "head-to-head" defects formed?
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c.) Why are "head-to-head" defects rare?
Answer 5.4
R is derived from initiator.
H
R
H2C
R
C
Ph
H
H
C C
H2
Ph
H2C
C
Ph
Radicals add to the double bond such
that the most stable radical is formed as
a result. In this case, the most stable
radical is the 2° benzylic radical - the R
odd electron can be resonance
stabilized by the aromatic ring (Ph).
H
H
C C C C
H2
H2
Ph
Ph
etc.
Since the polymer is always propagating via the benzylic radical, addition almost always occurs to
the CH2 end of the monomer (styrene). So the radical "head" is always adding to the CH2 tail of
monomer.
b.) Head-to-head defects are formed either by addition to the substitued end of the double bond, or
by radical coupling of two polymer units (termination).
R
R
H
H
C C
H2
Ph
H
C C
H2
Ph
C
R
CH2
Ph
H
C
Ph
C
H2
R
R
H
H
C C
H2
Ph
C
H
H
C C
H2
Ph
C
Ph
CH2
1° Radical
Ph
C
H2
etc.
R
Termination via
radical coupling.
Of these processes, the first is rare because it involves the formation of a relatively unstable 1°
radical (a high-energy process). The second is rare because the concentrations of the propagating
radicals is very low compared to the concentration of monomer. The chances of two radicals
encountering each other is relatively slim.
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