Chemistry Reference
In-Depth Information
Table 7.1.
Rate of decay of some acyloxyl radicals. (Hilborn and Pincock 1991)
k / 10
9
s
−
1
Substituent
CH
3
<1.3
CH
2
CH
3
2.0
(CH
3
)
2
CH
6.5
(CH
3
)
3
C
11
Table 7. 2 .
Activation parameters and rate constants (at 295 K) of the
β
-scission of the
tert
-butoxyl radical
log(A/s
−1
)
E
a
/kJ mol
−1
k/10
3
s
−1
Solvent
Reference
Gas phase
14.04
62.5
1.63
Batt et al. (1998)
CCl
4
13.4
±
0.5
53
±
4
10
±
2
Tsentalovich et al. (1998)
C
6
H
6
13.4
±
0.6
52
±
4
14
±
3
Tsentalovich et al. (1998)
CH
3
CN
13. 2
±
0.4
48
±
3
64
±
11
Tsentalovich et al. (1998)
(CH
3
)
2
COH
12.4
±
0.6
40
±
5
190
±
40
Tsentalovich et al. (1998)
CH
3
CO
2
H
12.1
±
0.3
37
±
3
340
±
60
Tsentalovich et al. (1998)
H
2
O
140 0
±
150
Erben-Russ et al. (1987)
Adams et al. 1982; Saebo et al. 1983; Sosa and Schlegel 1987). The resulting hy-
droxyalkyl radical is of lower energy, and this is the driving force of this re-
action (for the opposite situation in sulfur free-radical chemistry see below).
In secondary alkoxyl radicals the 1,2-H-shift reaction may be even faster than
the also very rapid
-fragmentation. For example, this ratio is near 3-4 in the
case of CH
3
CH(O
•
)OCH
2
CH
3
while the corresponding oxyl radical derived
from poly(ethylene glycol) only undergoes
β
-fragmentation (Schuchmann and
von Sonntag 1982; cf. also Gröllmann and Schnabel 1980). In the poly(ethylene
glycol) system,
β
β
-fragmentation is speeded up due to the formation a stabilized
−
OCH
2
•
radical, while in the former case a less stabilized
•
CH
3
radical has to be
eliminated.
Similar to
•
OH, alkoxyl radicals are also good H-abstractors [e.g., reaction
(7)], although they react with much lower rates (e.g.,
k
7
= 2.6
10
5
dm
3
mol
−
1
s
−
1
; Ellison et al. 1972; the tertiary butoxyl has a similar rate constant; Paul et
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