Chemistry Reference
In-Depth Information
Bu-
t
Bu-
t
76
°
C
Bu-
t
C
Bu-
t
Bu-
t
•
(1.10)
Bu-
t
•
30
29
1.4 The persistent radical effect
The persistent radical effect
12
is an important phenomenon in radical chemistry that was first discovered
through studies of triphenylmethyl radicals. One manifestation of this effect is seen in the reaction of
carbon tetrachloride with phenyl radicals and triphenylmethyl radicals generated from phenylazotriphenyl-
methane
21
, which gives excellent yields of chlorobenzene and of 1,1,1-trichloro-2,2,2-triphenylethane
31
(Equation 1.11).
12
1
CCl
4
∆
Ph
Ph
3
C
•
+
•
PhCl
+
CCl
3
•
Ph
3
CCCl
3
Ph
3
CN
=
NPh
N
2
−
(1.11)
21
1
31
The azo compound
21
decomposes into phenyl and trityl radicals, together with a molecule of nitrogen.
The more reactive phenyl radical then abstracts chlorine from carbon tetrachloride and radical coupling
of the trichloromethyl radicals with the persistent trityl radicals
1
produces
31
. It is striking that only
unsymmetrical
coupling takes place, and no appreciable amount of hexachloroethane is formed. The
explanation is that formation of even trace amounts of hexachloroethane would also lead to formation of
the dimer
5
of triphenylmethyl in equilibrium with the monomeric radical. Therefore, the concentration
of triphenylmethyl will greatly exceed that of the transient trichloromethyl radical, and since the rates
of reactions of trichloromethyl with itself and with triphenylmethyl are both close to the diffusion limit,
unsymmetrical coupling will predominate. However, the concentration of triphenylmethyl will remain
essentially constant, since for every trichloromethyl formed another triphenylmethyl is also produced.
Experimental confirmation is provided by EPR examination of the reacting solution, which shows a high
concentration of the triphenylmethyl radical.
This phenomenon was correctly analyzed in 1936 by Gomberg's student Bachmann, who studied the
role of triphenylmethyl in the reaction of pentaphenylethane
32
solution at 100
◦
C (Equation 1.12).
12f
This
ethane dissociates rapidly and reversibly into triphenylmethyl and diphenylmethyl, but dimerization of the
diphenylmethyl radical
33
to form detectable quantities of tetraphenylethane (which is stable under the
reaction conditions) seldom occurs.
100
°
C
+
(1.12)
32
1
33
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