M. Fagnoni et al.
tion upon irradiation decreased the pH and gave
almost neutral or slight acidic solutions. The reaction
quantum yield was slightly lower than that of 1, with
Fr (2) of about 0.65 for a 4ꢁ10À4 m solution in neat
water, in this case independent of the presence of
oxygen. The addition of any of the surfactants uni-
formly reduced Fr to about 0.45, but the main effect
under these conditions was on the product distribu-
tion. In neat water, a complex mixture was formed,
containing a small amount of 2a (under nitrogen) or
2b (under air; Table 3 and Scheme 4), whereas a
much cleaner reaction course was observed in the
Scheme 3. Effect of surfactants on the photochemistry of compounds 1 and 3
presence of any of the surfactants 4–7, with cresol
2a as the major product both in aerated and deaer-
ated solutions. The highest amount of 2a (up to
60%) was detected when ammonium salt 5 was used, whereas
the presence of the other surfactants allowed the formation of
2a (at least 40%). Irradiation in neat water led to the formation
of a species highly absorbing around 260 nm, particularly
under oxygen, while this was not the case with any of the sur-
factants, which gave almost superimposable UV spectra (see
the Supporting Information, Figures S1 and S2).
2-Benzyl-4-chlorophenol (3, 2ꢁ10À4 m) was likewise decom-
posed with liberation of hydrochloric acid with a quantum
yield of approximately 0.6 in neat water and about 0.45 in the
presence of surfactants with virtually no oxygen effect. Howev-
er, a dramatic change occurred in product distribution. Thus, a
complex mixture was obtained upon irradiation in neat water
with 3a (15% under nitrogen, 9% under air) as the main prod-
uct accompanied by traces of 3c; however, the photoreaction
showed a high mass balance (up to 90%) in the presence of
surfactants, with o-benzylphenol (3a) as the exclusive product
(Scheme 3, Table 4). The addition of surfactants led again to a
reduction of the 260 nm absorption (see the Supporting
Information, Figures S3 and S4), although the effect was less
uniform in this case.
Figure 1. Absorption spectra of a 4ꢁ10À4 m water solution of 1 (solid line);
same after 10 min irradiation under deaerated (dashed line) and air-
equilibrated conditions (dotted line).
HPLC analysis (see Experimental Section); a trace of benzo-
quinone (1c) was also detected. Under nitrogen, phenol 1a
was the main product, with a 1a/1b ratio of 1.45.
Whereas in neat water the irradiated solution was slightly
yellow, in the presence of any of the surfactants, it remained
colorless. Under these conditions, the formation of 1b was
inhibited (compare Figure 2 and Figure 1) and made 1a the
main product independent of the presence of oxygen, with a
1a/1b ratio >5 and in most cases >10.
An aerated solution of 1 containing 4 (3ꢁ10À2 m) was like-
wise irradiated in a SolarBox equipped with a xenon lamp to
mimic the solar irradiation. After 4 h irradiation, 4-chlorophenol
was partially consumed (37%) with the same product distribu-
tion as when an UV lamp was used. When 4 was added in
10À3 m amount (eight times lower than its cmc),[18] some
phenol was formed (ca. 15%) but 1b was by far the main
product.
In the experiments above, the amount of surfactants added
has been above the critical micelle concentration, under condi-
tions that may deeply affect chemical reactions with respect to
neat (aqueous) solvent.[20,21] Thus, the solubility of nonpolar
substrates is enhanced, but because these are located in a
nonpolar and highly viscous environment within the micelles,
the conditions differ from both protic and aprotic media and
major effects on the reaction rate, the product distribution, the
mechanism, and the overall selectivity intervene.[20a] The affinity
of chlorophenols for surfactants is well established.[22] Notice
further that zeolites[23] and montmorillonite[24] have been
shown to be effective sorbents for the removal of chloro-
phenol contaminants in aqueous systems when the surface is
modified by a cationic surfactant.
No significant effects of surfactants 4–7 (that are transparent
in the l interval used) on the absorption spectrum of phenols
1–3 were observed (Figures 2; see the Supporting Information,
Figures S2 and S4) and the quantum yield of the reaction
underwent only a limited decrease. These pieces of evidence
support that the primary photochemical reaction remains the
same in micelles as in neat water, despite the very large
4-Chloro-3-methyl sodium phenate (2) was present as the
anion, but the photochemical behavior was quite similar to
that of 1. The starting pH was slightly basic (ca. 9, one unit
below in the presence of the cationic surfactant). Acidity libera-
100
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ChemSusChem 2011, 4, 98 – 103