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more stable; Lundqvist and Eriksson 2000) phenoxyl radical [e.g., reaction (69)].
Interestingly, the
para
-
•
OH adduct undergoes water elimination much faster
than the
ortho
-
•
OH adduct [
k
=
10
8
dm
3
mol
−1
s
−1
; Raghavan and Steenken 1980; Roder et al. 1999; Mvula et al. 2001; see also
Raghavan and Steenken 1980]. Phosphate buffer catalyses the water elimination
in weakly basic solutions with a rate constant of 5
10
9
dm
3
mol
−1
s
−1
vs
∼
1.7
×
∼
1.1
×
10
7
dm
3
mol
−1
s
−1
. At neu-
×
10
3
s
−1
(Raghavan and Steenken
tral pH, water elimination occurs with
k
= 1.8
×
10
3
s
−1
(Land and Ebert 1967).
In neutral solutions and the absence of buffer, this reaction is slow compared
to the reactions of these
•
OH adducts with O
2
[e.g., reactions (60) and (61);
k
=
1.2
1980; Roder et al. 1999; Mvula et al. 2001)
∼
5
×
10
9
dm
3
mol
−1
s
−1
; with little if any reversibility, in contrast to other hy-
droxycyclohexadienyl radicals (Fang et al. 1996); for details see Chap. 8] and the
subsequent elimination of HO
2
•
[e.g., reactions (62) and (63);
k
= 1.3
×
10
5
s
−1
;
Raghavan and Steenken 1980; Roder et al. 1999; Mvula et al. 2001]. This reaction
is more than 100 times faster than an HO
2
•
elimination from the peroxyl radicals
of other (nonphenolic) aromatic compounds. This remarkable increase in rate
is possibly due to additional routes such as reaction (62). This high rate of HO
2
•
elimination renders cyclization and subsequent fragmentation reactions [cf. re-
actions (49) and (50)] less likely, and in the presence of O
2
, the final yields of cat-
echol and hydroquinone are only slightly lower than in the presence of another
oxidant (Raghavan and Steenken 1980; Roder et al. 1999; Mvula et al. 2001).
×
The simple phenol system has been discussed here at some length, because ma-
terial balance is obtained and mechanistic details are fairly well understood.
However, according to the data in Tables 3.5 and 3.6, there is a very noticeable
gap in the material balance in the case of the hydroxylated benzoic acids, al-
though some aspects such as the acid-catalyzed water elimination, in salicylate
also more pronounced in the case of the
para
-OH-adduct radical, are very simi-
lar (Mark and von Sonntag, unpubl.). Interestingly, addition of Fe(III) to oxidize
the intermediates also did not improve the material balance (Tables 3.5 and 3.6).
From this, it follows that the underlying chemistry of the salicylate and the other
hydroxybenzoate systems are at present not yet adequately understood, and the
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