1182
Can. J. Chem. Vol. 79, 2001
C+
C+
Fig. 2. Quantification of the main photoproducts identified in an
air-saturated aqueous solution of 1,4-DCB (3.7 × 10–4 M) irradi-
ated at 254 nm (one germicidal lamp).
Product VI: MS m/z (%): 296 (M , 100), 298 (M , 33),
1
consistent with C18H13ClO2. H NMR (CD3COCD3) δ: 7.83
′′
(s, OH), 7.82 (s, OH), 7.72 (t, J = 1.9 Hz, H2 ), 7.70 (s, 4H,
′′ ′′
H2′, 3′, 5′, 6′), 7.67 (m, H6 ), 7.49 (t, J = 7.9 Hz, H5 ), 7.39
′′
(m, H4 ), 6.86 (d, J = 3.0 Hz, H6), 6.84 (d, J = 8.75 Hz,
H3), 6.69 (dd, J = 8.75, 3.0 Hz, H4).
It was deduced from the H–H COSY spectrum that the
four protons 2′, 3′, 5′, and 6′ are fortuitously equivalent and
generate a single absorption (δ = 7.70 ppm) without any ap-
parent coupling constant.
Three other photoproducts were identified as phenol
(m/z = 94), 4,4′-dichlorobiphenyl (m/z = 222, 224, 226) and
2,4′,5-trichlorobiphenyl (m/z = 256, 258, 260) from GC–MS
analysis of the crude product of phototransformation after
evaporation of water.
The formations of 2-HDBF and terphenyl derivative were
also observed in deoxygenated solutions of 1,4-DCB irradi-
ated between 275 and 350 nm. Some complementary experi-
ments were carried out to explain the formation of 2-HDBF
in deoxygenated solution. (i) A deoxygenated solution of
hydroquinone (6.7 × 10–4 M) and 1,4-DCB (c ≈ 4 × 10–3 M)
was irradiated in the range 290–350 nm. In these conditions
the excitation of 1,4-DCB is negligible compared to the ex-
citation of hydroquinone. It was observed that 2-HDBF ap-
pears as one of the main photoproducts. (ii) 4′-Chloro-2,5-
dihydroxybiphenyl (4′-ClPhQH2), synthesized as described
in the Experimental section, was irradiated in the range 290–
350 nm in various conditions. 2-HDBF was the main
photoproduct on HPLC chromatogram of a deoxygenated di-
lute solution (c ≈ 6 × 10–5 M) irradiated for a few minutes.
This reaction was not observed in the presence of oxygen
and it is a minor pathway when the concentration of 4′-
ClPhQH2 is as high as 1.4 × 10–3 M. In relatively concen-
trated solutions irradiation leads to the formation of an insol-
uble product identified as an adduct resulting from
elimination of HCl between two molecules (CI-MS m/z =
404, 405 [M + 1], and 433 [M + 29]).
initial mass balance. To prove that 4-chlorophenol cannot
accumulate in the solution, 4-chlorophenol (1.4 × 10–5 M)
was added to a solution of 1,4-DCB (8.9 × 10–5 M), and af-
ter 10 min irradiation the concentration of 4-chlorophenol
was only 1.2 × 10–6 M.
The phototransformation of 1,4-DCB was also observed in
solutions irradiated in the range 275–350 nm, along with the
formation both of hydroxybenzoquinone and trace amounts
of hydroquinone. 4-Chlorophenol and 2,5-dichlorophenol
are formed in a first stage as it appears in Fig. 2, but do not
accumulate in these conditions due to the fact they absorb at
longer wavelengths than 1,4-DCB.
Deoxygenated solutions
Solutions of 1,4-DCB (3.7 × 10–4 M), deoxygenated as re-
ported in Experimental section, were irradiated at 254 nm.
The formation of 2,5-dichlorophenol was not observed, but
two unexpected photoproducts V and VI appeared on the
HPLC chromatogram. These two products were not ob-
served in irradiated diluted solutions (6.7 × 10–5 M). They
were isolated as described in the Experimental section and
OH
OH
OH
Cl
hν
1
2
Cl
identified by MS and H NMR spectroscopy, respectively, as
[3]
′′
2-hydroxydibenzofuran (2-HDBF) and 3 -chloro-2,5-
HO
HO
OH
4'-ClPhQH2
dihydroxy-p-terphenyl (CDTP):
OH
2'
3'
5'
6''
2''
5''
9
6
1
4
3
OH
Reactions induced by excitation of nitrate ions or ferric
ions
8
7
4
4''
3
O
V
6
6'
Nitrate ions absorb at wavelengths shorter than 340 nm
with a maximum at 302 nm (molar absorption coefficient ε =
7.2 M–1 cm–1 at 302 nm), hence, when solutions are irradi-
ated in the range 290–350 nm, NO−3 is excited without exci-
tation of 1,4-DCB.
HO
Cl
VI
C+
1
Product V: MS m/z (%): 184 (M , 100), 128. H NMR
(CD3COCD3) δ: 8.37 (s, OH), 8.00 (dd, J = 7.0, 1.3 Hz, H6),
7.55 (dd, J = 7.9, 0.9 Hz, H9), 7.47 (dt, J = 7.9, 1.3 Hz, H8),
7.47 (d, J = 2.5 Hz, H1), 7.44 (d, J = 8.8 Hz, H4), 7.33 (dt,
J = 7.5, 0.9 Hz, H7), 7.01 (dd, J = 8.8, 2.5 Hz, H3).
An air-saturated solution of 1,4-DCB (3.3 × 10–4 M) and
KNO3 (1 × 10–2 M) was irradiated between 290 and 350 nm.
The incident photon flow was roughly 6 × 1015 photons cm–3 s–1
(see Experimental section), but the solution absorbs only a
fraction of this energy (probably less than 20%). The main
photoproduct detected on the HPLC chromatogram of a so-
lution irradiated for 165 min was 2,5-dichlorophenol. The
formation of 4-chlorophenol was also observed, but in such
minor amounts that it could not be quantified. Nitrate ions
The identification of V was corroborated by its typical UV
spectrum (maxima at 251, 289, 310–320 nm) and by com-
parison of its HPLC retention time with those of an authentic
sample of 2-HDBF obtained by photolysis of phenylbenzo-
quinone.
© 2001 NRC Canada