.
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decabromodiphenyl ether (deca-BDE), they are incapable of
adsorbing onto the TiO2 surface and do not have either
hydrogen atoms available for abstraction or sufficiently high
electron density for oxidation by an alcohol free-radical.
Although the dehalogenation of them has been well evi-
denced in the photocatalytic experiments,[9] questions still
remain to be solved: how do they undergo dehalogenation
while alcohol is used as both solvent and hole capture? What
is the KSIE for the dehalogenation of them in [D0]alcohol/
[Dn]alcohol?
In this study, we show that photocatalytic reactions for
these highly hydrophobic polyhalogenated aromatics proceed
through a protonation mechanism which induces subsequent
dehalogenation reactions, thus resulting in an inverse KSIE in
[D0]methanol/[D4]methanol solutions (Scheme 1, right). The
reaction of this substrate type for TiO2 photocatalysis does
not involve direct ET or a diffusive solvent free-radical
mechanism, but processes along a previously unknown path-
way. The reaction path for organic substrates in solution
mainly depends on the adsorption property of the substrate
onto the TiO2 surface.
Indeed, the conversion rate of deca-BDE in [D0]methanol
was slower than in [D4]methanol (Figure 1b), and the
observed H/D KSIE was significantly smaller than 1. In this
case, the alcohol functioned as both a solvent and a reactant,
with the latter donating both electrons and hydrogen atoms to
react with h+vb for formation of an aldehyde and protons. The
inverse KSIE differed from the normal KSIE of which the
general prediction is the exponential decay with an increasing
À
proton mass for the homogenous cleavage of the C H/D
bond.[10] Furthermore, we quantitatively detected the corre-
sponding dehalogenation products of nonabromodiphenyl
ether (nona-BDE), including BDE-206, DBE-207, and BDE-
208 (Scheme 2), using HPLC and MS (see Figure S1 in the
Scheme 2. TiO2 photocatalytic dehalogenation of deca-BDE in metha-
nol.
We chose deca-BDE, which is a widely used nonpolar, and
extremely hydrophobic model compound (logKo/w ꢁ 14) for
photocatalytic dehalogenation,[9a] to observe the KSIE during
TiO2 photocatalysis (l > 360 nm UV light) in anaerobic
[D0]methanol and [D4]methanol at 298 K (Figure 1a).
Supporting Information). Moreover, the kinetics of the
formation of nona-BDE were similar to those for the
dehalogenation of deca-BDE (i.e., the reaction proceeded
faster in [D4]methanol; Figure 1b), although further debro-
mination was inevitable (octa-BDE was also formed for
prolonged photoirradiation). The formation of bromide ions
was detected by IC (see Figure S2), and the approximate
stoichiometric yield also confirmed that the inverse KSIE was
about 0.7 and resulted from the reaction depicted in
Scheme 2. The hvb+-induced oxidation of an alcohol solvent
to form an aldehyde and protons never gives an inverse
KSIE.[11] Therefore, the inverse KSIE phenomenon was
attributed to the reduction of deca-BDE by eÀ
.
cb
Table S1 shows that the kinetics at each stage of the
dehalogenation could be described by the inverse KSIE, as
calculated from either the deca-BDE conversion or the nona-
BDE yield. No significant difference was observed between
the values of these two types of inverse KSIEs, thus indicating
that the main reaction was consistent with that in Scheme 2.
The result suggests that the dehalogenation reaction of deca-
BDE did not proceed through the direct ET pathway or the
typical solvent free-radical mechanism with a normal KSIE.
Instead, the sp2 hybridization of carbon for deca-BDE may
have been affected by H+/D+-induced addition, which will be
discussed below.
To investigate the effect of the alcohol solvent, we used
[D0]methanol/[D1]methanol and [D0]methanol/[D3]methanol
as the solvent to perform the same experiments, and the
inverse KSIE also occurred in these two systems (see
Table S2). In addition, we replaced the methanol solvent
with ethanol and isopropanol to further confirm this inverse
KSIE. Both solvents exhibited clear inverse KSIEs
Figure 1. a) HPLC chromatograms obtained at various photoirradiation
times for the TiO2 photocatalytic dehalogenation of decabromodi-
phenyl ether (deca-BDE) in anaerobic methanol. b) Time-resolved
evolution of the photocatalytic dehalogenation of deca-BDE and
formation of nonabromodiphenyl ether (nona-BDE) in [D0]methanol/
[D4]methanol. Solid lines and dashed lines indicate that the reaction
was performed in [D0]methanol and [D4]methanol, respectively. Each
data point shown represents the average value of three experiments,
all of which were within a 5% uncertainty.
À
(Table S2). For the alcohols, oxidative dissociation of the C
+
À
H and O H bond by the h vb typically exhibits a normal H/D
KIE,[11] and the C O bond of the alcohols is converted into
À
=
the C O bond of an aldehyde or ketone, that is, conversion of
2
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
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