1
866
GALSTYAN et al.
Influence of the concentration of cobalt(II) acetate on the selec-
subsequently recombines [reaction (4)] to give the re-
action products:
–4
tivity of 4-AT oxidation at the methyl group ([O
3
] = 4.8 ×10 ,
–
3
–1
[
4-AT] = 0.4 M; vair = 8.3 × 10 l s ; T = 95°C)
АсОArCH
2
+ O
2
→ АсОArCH
→ products.
2
O
2
,
(3)
(4)
Selectivity with respect to
[
2
Co(OAc) ], M
CH
3
group, %
АсОArCH
2
O
2
0
0
0
0
0
.06
.10
.14
.18
.22
43.9
58.4
71.0
87.0
87.1
EXPERIMENTAL
In the experiments, we used glacial acetic acid
analytically pure grade) purified by vacuum distilla-
(
tion from potassium permanganate. Crystalline 4-HT
was purified by repeated recrystallization from water,
and liquid 4-AT, by repeated distillation. Cobalt(II)
acetate (analytically pure grade) was used without ad-
ditional purification.
The major product of 4-AT oxidation at the
methyl group is 4-acetoxybenzoic acid (4-ABA) (see
figure). In early steps of the oxidation, we also identi-
fied 4-acetoxybenzaldehyde (4-ABAl) whose concen-
–
4
tration in solution does not exceed 10 M.
Acylation was performed in a three-necked flask.
Acetyl chloride was added dropwise at room tempera-
ture over a period of 30 min to the calculated amount
of 4-HT dissolved in an equivalent amount of acetic
acid. Then the reaction mixture was carefully diluted
with water to a 1 : 1 ratio, with continuous stirring un-
til the HCl evolution fully ceased. After that, the mix-
ture was separated in a separating funnel, and the or-
ganic layer was washed with a 5% alkali solution and
distilled.
The selectivity of oxidation at the side chain in-
creases on introducing into the system an oxidation
catalyst, cobalt(II) acetate. Under the experimental
conditions, the reaction is fast, and its major product is
4
-ABA (see figure). The oxidation selectivity depends
on the catalyst concentration, and, at a cobalt(II) ace-
tate concentration of 0.18 M and temperature of 95°C,
the oxidation is complete in 1 h, and the selectivity
with respect to the methyl group is 87% (see table).
Selective oxidation at the methyl group in the pres-
ence of cobalt(II) acetate becomes possible as a result
of two-step oxidation by ozone. Under the experimental
The reactor (glass column with a porous partition
for dispersing the ozone–air mixture) was charged with
10 ml of glacial acetic acid, 0.4 M 4-AT, and a calcu-
lated amount of the catalyst, after which the installa-
tion was thermostated and, after the ozonizer attained
the steady-state operation mode, the ozone–oxygen
2
+
conditions, ozone predominantly reacts with Co [re-
2
–1 –1
action (1)] (k = 9.7 ×10 , k
= 0.74 l mol s )
1
AcOArCH + O3
3
3
+
to give active Co species which involve 4-AT at a high
rate [reaction (2)] in selective oxidation at the methyl
group [9]:
–
4
mixture containing 4.8 × 10 M ozone was passed at
–
1
a rate of 30 l h .
After the oxidation was complete, the mixture was
poured into water with ice (1 : 3 ratio). The precipitated
-ABA was filtered off and dried. If necessary, 4-ABA
2
+
+
3+
Со + O
3
+ H → Со + HO + О
2
,
(1)
(2)
3
+
2+
+
АсОArCH + Со → АсОArCH + Со + Н .
3
2
4
can be readily hydrolyzed to 4-HBA. For this purpose,
to 4-ABA we added 10 ml of concentrated HCl, 20 ml
of water, and 4 ml of ethanol, refluxed the solution for
To ensure the occurrence of reaction (1), ozone
should be continuously supplied to the oxidation sys-
tem. Interruption of its supply impedes the process to
the point of its complete cessation. This fact addition-
ally confirms the assumption that the active form of
the catalyst is mainly formed by reaction (1). Under
the experimental conditions, the concentration of mo-
lecular oxygen in the gas mixture is one and a half or-
ders of magnitude higher than the ozone concentration.
Therefore, the acetoxybenzyl radical generated in the
system [reaction (2)] reacts mainly with an oxygen
molecule [reaction (3)] to form a peroxy radical which
1
h, and then cooled it to 10°C. The 4-HBA precipi-
tate was filtered off and dried. Yield 79% based on
the loaded 4-AT.
The ozone content of the gas phase was deter-
mined spectrophotometrically from the absorption at
254–290 nm. The oxidation products in solution were
identified and quantitatively determined by GLC on
a chromatograph equipped with a flame ionization de-
tector and a column 2 m long and 4 mm in diameter,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 81 No. 10 2008
Galstyan
