Oxidation using [bis(trifluoroacetoxy)]iodobenzene: A new and potentially practical approach to detection of polychlorinated phenols

Article abstract of DOI:10.1039/a701319a

A novel oxidation for pentachlorophenol, 2,4,6-trichlorophenol and 2,3,5,6-tetrachlorophenol using [bis-(trifluoroacetoxy)]iodobenzene has been developed, and the oxidation products from pentachlorophenol and 2,3,5,6-tetrachlorophenol have been identified as tetrachloro-1,4-benzoquinone; this novel reaction can be applied in electrochemistry using glucose oxidase for sensitive determination and identification of PCP, one of the most toxic polychlorinated phenols.

Full text of DOI:10.1039/a701319a

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Oxidation using [bis(trifluoroacetoxy)]iodobenzene: a new and potentially
practical approach to detection of polychlorinated phenols
Coralie Saby and John H. T. Luong*
Biotechnology Research Institute, National Research Council Canada, Montreal, Quebec, Canada H4P 2R2
3
A novel oxidation for pentachlorophenol, 2,4,6-trichloro-
phenol
and
2,3,5,6-tetrachlorophenol
using
[bis-
2
1
(trifluoroacetoxy)]iodobenzene has been developed, and the
oxidation products from pentachlorophenol and
2,3,5,6-tetrachlorophenol have been identified as tetra-
chloro-1,4-benzoquinone; this novel reaction can be applied
in electrochemistry using glucose oxidase for sensitive
determination and identification of PCP, one of the most
toxic polychlorinated phenols.
0
–1
–2
–3
I
Considerable effort, including time-consuming immunoassay,
has been directed toward the detection and quantitation of
priority chlorinated pollutants such as pentachlorophenol
(PCP), 2,3,5,6-tetrachlorophenol and 2,4,6-trichlorophenol.1,2
Although electrochemical detection has frequently been used
because of its simplicity and sensitivity, the oxidation products,
generally believed to be quinones or radical products, are prone
to polymerization, which results in electrode surface fouling.3
Direct electrochemical oxidation of chlorophenols also requires
an overpotential of +700 to +900 mV, which is sufficiently
extreme so as to introduce the possibility of electroactive
interference. These drawbacks can be overcome through
enzymatic oxidation of chlorophenols, followed by electro-
chemical reduction of the chlorinated quinones. However, the
two enzymes most commonly applied to this assay, tyrosinase
and laccase, have been tested and found to be effective only for
catechol, hydroquinone, p-cresol, 4-chlorophenol and 4-amino-
4-chlorophenol.4,5
–0.6 –0.4 –0.2
0.0
0.2
0.4
0.6
0.8
1.0
E / V vs. Ag/AgCl
Fig. 2 Cyclic voltammograms (0.1 m acetic acid, pH 3, 100 mV s21) of (D)
pentachlorophenol (10 mm), (2) the oxidation product of PCP and (—) the
standard solutions of tetrachloro-1,4-benzoquinone (10 mm). Reaction was
carried out in 0.1
m acetic buffer, pH 3, for 1 h, with 10 mm
pentachlorophenol and 500 mm [bis(trifluoroacetoxy)]iodobenzene. Hydro-
gen peroxide (440 mm) was added to stop the reaction. Equipment: CV-1B
voltammograph (Bioanalytical Systems), platinum wire counter electrode,
Ag/AgCl reference electrode and glassy carbon working electrode.
complex mixture of products including quinones, diols and
triols, and polyhydroxybiphenyls.7,8
We report here a novel and simple reaction for the oxidation
of pentachlorophenol into tetrachloro-1,4-benzoquinone
(TCBQ) in aqueous media using [bis(trifluoroacetoxy)]-
iodobenzene (BTIB). We also report the ability of TCBQ to
oxidize reduced glucose oxidase. Although this hypervalent
iodine agent.9,10 has been used to oxidize various naphthols11
into the corresponding quinone, no attempt has been made to
oxidize chlorinated phenols with BTIB. Reaction of penta-
chlorophenol with BTIB was performed in 0.1 m acetate buffer,
pH 3, at ambient temperature. A large excess of BTIB (molar
ratio BTIB/PCP from 10 to 50) must be used to accelerate the
reaction (2 h to 30 min). Neutralization of excess BTIB with
hydrogen peroxide prevented further reaction of the initial
products. Moreover, BTIB reaction by-products (iodobenzene
and trifluoroacetate ion) are not electroactive species.
Several chlorophenols could be oxidized by ceric sulfate or
chloroperoxidase. The resulting quinones were shown to
efficiently recycle glucose oxidase to the oxidized form.6
However, the PCP oxidation with ceric sulfate or hydrogen
peroxide catalysed by chloroperoxidase undergoes polym-
erization of the oxidation product. Photochemical oxidation
with hydrogen peroxide or singlet oxygen often produces a
0.25
(b)
0.20
UV–VIS spectral, HPLC and cyclic voltammetric data
obtained from the reacted solution demonstrated that the
reaction product of pentachlorophenol with BTIB is tetrachloro-
1,4-benzoquinone. Over the course of the reaction, the penta-
chlorophenol peaks at l = 294 and 303 nm declined [Fig. 1,
curve (a)] and a more intense absorption peak at 290 nm
emerged and intensified [Fig. 1, curve (b)]. Both the absorbency
profile and maximum wavelength of the PCP oxidation product
were confirmed to be identical to those of standard tetrachloro-
1,4-benzoquinone [Fig. 1, curve (c)]. The HPLC data also
indicated that a significant portion of tetrachloro-1,4-benzo-
quinone was converted to tetrachloro-1,4-hydroquinone during
the course of separation [HPLC column; 3.9 mm i.d. 3 15 cm
stainless steel packed with 10 m C18 silica gel (mBondapak C18,
Waters, Milford, MA), mobile phase: 25% pH 3.0, 0.1 m tartrate
buffer + 25% water + 50% MeOH at a flow rate of 0.5 ml
min21]. To facilitate the sensitive detection of tetrachloro-
0.15
(c)
0.10
(d)
0.05
(a)
0.00
200
250
300
350
l / nm
Fig. 1 UV–Visible (Beckman DU 640 spectrometer) spectrum of (a)
pentachlorophenol (10 mm), (b) the oxidation product of PCP, (c) the
standard tetrachloro 1,4-benzoquinone (10 mm) and (d) [bis(trifluo-
roacetoxy)]iodobenzene (100 mm after hydrogen peroxide addition).
Measurements were performed in 0.1 m acetic buffer, pH 3. Reaction was
carried out in 0.1
pentachlorophenol and 100 mm [bis(trifluoroacetoxy)]iodobenzene. Hydro-
gen peroxide (88 mm) was added to stop the reaction.
m acetic buffer, pH 3 for 2 h, with 10 mm
Chem. Commun., 1997
1197

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PhI(CF3CO2)
OH
O
O
OH
O
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
(CF3CO2)2IPh
(CF3CO2)2IPh
H2O
(1)
Cl
Cl OH
Cl
Cl
OH
O
1
2
3
4
5
Glucose oxidaseox + Glucose
Glucose oxidasered + TCBQ
Glucose oxidasered
(2)
(3)
Glucose oxidaseox + TCBQred
TCBQ
at the electrode surface: TCBQred
(4)
1,4-benzoquinone, this oxidation product was reduced with zinc
to tetrachloro-1,4-hydroquinone, which was detected by a
glassy carbon electrode, +450 mV vs. Ag/AgCl (model LC-44
thin layer electrochemical cell connected to a model CV-1B
potentiostat, Bioanalytical Systems, Lafayette, IN). The cyclic
voltammogram of the product obtained from the oxidation of
PCP by BTIB is identical with that of the standard tetrachloro-
1,4-benzoquinone with respect to the oxidation and reduction
peaks and the peak to peak separation (Fig. 2).
Another interesting result reported here is the capability of
tetrachloro-1,4-benzoquinone to reoxidize glucose oxidase in
the presence of excess glucose. As shown in Fig. 3(a), glucose
oxidase immobilized on a glassy carbon electrode was readily
reduced by its substrate, glucose. TCBQ then recycled the
reduced enzyme to its original active form, i.e. mediating the
rate-limiting electron transfer from the enzyme to the electrode.
The oxidation product obtained from the PCP–BTIB reaction
was also able to mediate the glucose oxidase–glucose reaction
[Fig. 3(b)]. Based on the above findings, a mechanism for the
oxidation of PCP and the mediating capability of the corre-
sponding oxidation product can be proposed [eqns. (1)–(4)].
Oxidation of phenols often produces a mixture of isomers
(1,4- and 1,2-benzoquinone). However, steric hindrance by the
(CF3CO2)IPh species could be the main reason why dechlori-
nation occurs only in position 4. In addition to PCP, BTIB also
oxidized two other important chlorophenols: 2,4,6-trichloro-
phenol and 2,3,5,6-tetrachlorophenol. It should be noted that the
1,4-benzoquinone structure exhibited unique symmetry, and
dechlorination of chlorophenols had to occur at position 4 if it
was chlorinated. In view of this, both 2,3,5,6-tetrachlorophenol
and PCP must yield the same oxidation product, i.e. tetrachloro-
1,4-benzoquinone. We confirmed that the oxidation product of
2,3,5,6-tetrachlorophenol and pentachlorophenol with BTIB
exhibits identical cyclic voltammetric behaviour and similar
UV–visible peaks (not shown). The oxidation products of these
two chlorophenols also mediated the glucose oxidase–glucose
reaction, whereas BTIB and BTIB by-products did not react
with reduced glucose oxidase. The detection limit of penta-
chlorophenol and 2,3,5,6-tetrachlorophenol was determined to
be 4 nm, whereas that of 2,4,6-trichlorophenol was 8 nm; the
steady state signal was attained within 2–5 min.
0.30
(a)
(ii)
0.25
0.20
0.15
0.10
I
(i)
0.05
0.00
–0.05
–0.10
0.30
(b)
We have demonstrated that highly chlorinated phenols can
easily be oxidized to quinones. In combination with bioelec-
troanalytical chemistry, this reaction offers a sensitive method
for the determination of these important pollutants.
(ii)
(i)
0.25
0.20
0.15
Footnote
0.10
I
* luong@biotech.lan.nrc.ca
0.05
0.00
References
–0.05
–0.10
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0.5
0.0
0.2
0.3
0.4
0.1
E / V vs. Ag/AgCl
Fig. 3 (a) Cyclic voltammograms of the GOD electrode in 1 mm tetrachloro-
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Received in Cambridge, UK, 25th February 1997; Com.
7/01319A
1198
Chem. Commun., 1997