Liquid-phase reactions of isomeric methylphenols with ozone

Article abstract of DOI:10.1134/S1070363210010020

Ozonation of isomeric methylphenols in acetic anhydride was studied. Here, acetic anhydride acts simultaneously as solvent and acylating agent. In the presence of a mineral acid methylphenols were converted into methylphenyl acetates during preparation of solutions for ozonation. The major products in the oxidation of isomeric methylphenyl acetates with ozone were aliphatic peroxides (80-90%); oxidation of the methyl group gave rise to the corresponding acetoxybenzyl acetates (7-14%) and acetoxybenzylidene diacetates (3-6%). A probable scheme for the liquid-phase oxidation of methylphenyl acetates with ozone in acetic anhydride was proposed.

Full text of DOI:10.1134/S1070363210010020

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 1, pp. 15–19. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.G. Galstyan, 2010, published in Zhurnal Obshchei Khimii, 2010, Vol. 80, No. 1, pp. 19–23.
Liquid-Phase Reactions of Isomeric Methylphenols with Ozone
A. G. Galstyan
Institute of Chemical Technologies, Dal’East Ukraine National University,
ul. Lenina 31, Rubezhnoe, Lugansk Region, 93000 Ukraine
e-mail: ozon@megabit.com.ua
Received December 8, 2008
Abstract—Ozonation of isomeric methylphenols in acetic anhydride was studied. Here, acetic anhydride acts
simultaneously as solvent and acylating agent. In the presence of a mineral acid methylphenols were converted
into methylphenyl acetates during preparation of solutions for ozonation. The major products in the oxidation
of isomeric methylphenyl acetates with ozone were aliphatic peroxides (80–90%); oxidation of the methyl
group gave rise to the corresponding acetoxybenzyl acetates (7–14%) and acetoxybenzylidene diacetates (3–
6%). A probable scheme for the liquid-phase oxidation of methylphenyl acetates with ozone in acetic anhydride
was proposed.
DOI: 10.1134/S1070363210010020
Reactions of methylphenols with ozone in carbon
tetrachloride were studied [1]. It was shown that ozone
reacts at the H–O bond to produce phenoxyl radicals
and that subsequent decomposition of the aromatic
ring yields ozonides and carbonyl compounds [1]
(Scheme 1).
Scheme 1.
H
O
OH
O
O
O
O
O3
O3
ozonides.
+ O2
+ OH
No oxidation products at the methyl group with
unchanged aromatic system were detected.
then equilibrium establishes in the system [3]; The
acylation is terminated when the conversion of 4-
methylphenol into 4-methylphenyl acetate reaches
51% (Fig. 1; curves 3, 3'). These findings are very
consistent with those reported in [4]; according to [4],
the acylation process follows the mechanism shown in
Scheme 2.
The present communication reports on the results of
studying ozonation of isomeric methylphenols in acetic
anhydride which exhibits a strong acylating ability.
The experimental conditions and procedures for the
preparation of reactants and analysis of the reaction
mixtures were reported previously [2].
Under the given conditions, the acylation with
acetic anhydride in the presence of mineral acid is
complete during dissolution of the substrate, so that the
ozonation involves acylated derivatives of methyl-
phenols, and ozone reacts mainly (as with toluene [4])
at double bonds in the aromatic ring and (to a lesser
extent) at the methyl group. Acetoxy group is fairly
inert toward ozone.
In these reactions, acetic anhydride acts simulta-
neously as solvent and acylating agent. The acylation
at 20°C is complete in 40 min (yield quantitative). The
rate of acylation increases in the presence of sulfuric
acid (which is known to be a typical acylation
catalyst), and the process is complete in 8–10 min
(Fig. 1). As follows from the data in Fig. 1, methyl-
phenols also undergo acylation with acetic acid; the
reaction in the presence of sulfuric acid is
characterized by high rate in the initial moment, and
The main products of oxidative decomposition of
isomeric tolyl acetates by the action of ozone at 5°C
were the corresponding aliphatic peroxy compounds;
15

16
GALSTYAN
Scheme 2.
O
O
CH3
CH3
C
C
CH3
CH3
C
+
+
+ H+
O H
C
CH3
C
O
O + CH3COOH
O
O
OH
OH
+
CH3COOH + H+
CH3
C
O
OH
CH3
CH3
C
C
+
+
CH3
+
O
CH3
H
C
O + 2 CH3COOH
C
OH
O
Me
Me
Me
+
OH + CH3
C
O
OCOCH3
OCOCH3
−H+
+
in addition, products resulting from oxidation of the
methyl group were detected: acetoxybenzyl acetates,
acetoxybenzylidene diacetates, and acetoxybenzoic
acids (Table 1). In the oxidation of 3- and 4-
methylphenyl acetates, the overall yield of oxidation
products at the methyl group was 19.3 and 20.4%,
respectively, whereas the selectivity of oxidation of 2-
methylphenyl acetate at the methyl group did not
exceed 10.8%.
major oxidation products were acylated aromatic
alcohols and aldehydes (Table 1). No acetoxybenzoic
acids were formed under these conditions. Interes-
tingly, the nature of the mineral acid almost did not
affect the selectivity of oxidation at the methyl gorup,
but the rate of the process changed appreciably. For
example, the reaction time shortened from 5 to 3 h
upon replacement of phosphoric acid by stronger
sulfuric acid (Fig. 2).
The selectivity of ozonation at the methyl group did
not change in the presence of sulfuric acid, but the
The formation of oxidation products at the methyl
group is likely to be related to acylation of
0.4
5.0
0.4
1
2
2
2'
0.3
0.2
0.3
0.2
0.1
2.5
3
3'
0.1
1
1'
1'
10
2'
20
30
τ, min
40
50
60
1
2
3
4
5
τ, h
Fig. 1. Variation of the concentrations of (1, 2, 3) 4-methyl-
phenyl acetate and (1', 2', 3') 4-methylphenol in the acyla-
tion of the latter at 20°C in (1, 1') acetic anhydride in the
presence of sulfuric acid, (2, 2') in acetic anhydride
containing no sulfuric acid, and (3, 3') in acetic acid in the
presence of sulfuric acid; [AcOC6H4CH3]0 = 0.4, [H2SO4]0 =
1.2 M; volume of the mixture 0.01 l.
Fig. 2. Effect of mineral acid on the rate and selectivity of
oxidation of 2-methylphenyl acetate with ozone in acetic
anhydride at 20°C: (1, 1') consumption of 2-methylphenyl
acetate, (2, 2') overall concentration of oxidation products at
the methyl group in the presence of (1, 2) H2SO4 and (1', 2')
H3PO4; [AcOC6H4CH3]0 = 0.4, [acid]0 = 1.2, [O3]0 = 4.0×
10–4 M; volume of the mixture 0.01 l.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 1 2010

LIQUID-PHASE REACTIONS OF ISOMERIC METHYLPHENOLS WITH OZONE
17
Table 1. Oxidation of methylphenyl acetates with ozone in acetic anhydride at 5°C; [AcOC6H4CH3]0 = 0.4, [O3]0 = 4.0×
10–4 M; ozone–air mixture flow rate 30 l/h.
Yield of oxidation products, %
[H2SO4]0,
Compound
M
peroxides acetoxybenzyl acetate
acetoxybenzylidene diacetate
acetoxybenzoic acid
2-Methylphenyl acetate
3-Methylphenyl acetate
4-Methylphenyl acetate
2-Methylphenyl acetate
3-Methylphenyl acetate
4-Methylphenyl acetate
–
–
87.7
79.7
78.6
88.2
79.1
78.0
4.3
7.5
1.9
3.2
3.4
3.5
7.3
7.5
4.6
8.6
9.2
–
–
7.8
1.2
1.2
1.2
7.4
12.6
13.0
–
–
acetoxybenzyl alcohols I and acetoxybenzaldehydes II
in statu nascendi with formation of acetoxybenzyl
acetates III and acetoxybenzylidene diacetates IV that
are less reactive toward ozone (Scheme 3). At least two
conditions should be met to ensure Scheme 3 to be
operative: (1) the rate of acylation of oxidation
products formed from compounds I and II should be
considerably higher than the rate of the their
ozonation; (2) the rate of ozonation of compounds III
and IV should not exceed the rate of ozonation of the
corresponding methylphenyl acetates. In order to
estimate on a qualitative level attainability of the first
conditions, the rate constants for ozonation and
acylation of unsubstituted benzyl alcohol were
determined. They were equal to 3.4 l mol–1 s–1 and
500 l2 mol–2 s–2, respectively (20°C), i e., the rate of
acylation of benzyl alcohol was higher by more than
two orders of magnitude than the rate of its ozonation.
By analogy, analogous relations may be presumed to
be typical of compounds I and II. The second
condition is also satisfied. The data in Table 2 show
that the rate of ozonation of acetoxy derivatives III
and IV is approximately twice as low as the rate of
ozonation of the corresponding methylphenyl acetates
and is considerably lower than the rate of ozonation of
non-acylated methylphenols.
Presumably, the formation of alcohols and
aldehydes in the system under study follows a scheme
proposed in [1, 6].
АсОАrМе + О3 → АсОАrСН2• + НО• + О2,
АсОАrСН2• + О2 → АсОАrCН2О2•,
АсОАrCН2О2• + АсОАrМе → АсОАrСН2О2Н + AсОАrСН2•,
АсОАrСН2О2Н → АсОАrCН2О• + НО•,
2АсОАrCН2О2• → Products.
Peroxy compounds resulting from ozonolysis of the
aromatic ring in methylphenols are fairly stable to
further oxidation with ozone. Removal of the solvent
Scheme 3.
OAc
OAc
OAc
OAc
HOCH2
AcOCH2
I
III
IV
Ac2O, H+
O3
OAc
Me
OCH
(AcO)2CH
II
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 1 2010

18
GALSTYAN
Table 2. Rate constants for the reactions of ozone with methylbenzenes and their oxidation products in acetic anhydride at 5°C
Compound
[O3]0 × 104, M
0.28–0.57
0.35–0.90
0.35–090
0.35–0.90
0.28–0.57
0.28–0.57
0.28–0.57
0.37–0.57
0.40–0.57
0.37–0.57
0.37–0.57
0.37–0.57
0.37–0.57
[ArH] × 102, M
7.7–28.3
9.7–38.3
9.3–37.1
9.9–31.9
20.1–38.9
15.1–35.9
15.1–35.9
0.8–0.9
k
app, l mol–1 s–1
Toluene
0.82±0.08 [5]
(0.96±0.01)×103
(2.15±0.02)×103
(2.26±0.02)×103
0.47±0.05
2-Methylphenol
3-Methylphenol
4-Methylphenol
2-Methylphenyl acetate
3-Methylphenyl acetate
4-Methylphenyl acetate
4-Acetoxybenzyl acetate
4-Acetoxybenzylidene diacetate
3-Acetoxybenzyl acetate
3-Acetoxybenzylidene diacetate
2-Acetoxybenzyl acetate
2-Acetoxybenzylidene diacetate
0.56±0.05
0.59±0.05
0.24±0.02
0.5–0.8
0.27±0.02
0.8–0.9
0.23±0.02
0.5–0.8
0.26±0.02
0.8–0.9
0.20±0.02
0.5–0.8
0.23±0.02
Table 3. Concentration of peroxy compounds derived from
under reduced pressure (5 mm) gave a light yellow
viscous oily liquid which was readily soluble in acetic
acid and acetic anhydride. The isolated peroxy
compounds reacted with potassium iodide at a high
rate; the reaction was complete in 1 h, and the amount
of liberated molecular iodine was equivalent to one
peroxy group. These data are consistent with those
reported in [7], according to which just hydroperoxy
moieties readily react with potassium iodide (Table 3).
This fact, as well as stoichiometric coefficients with
respect to ozone (n ≈ 1), suggests that the final
products of oxidation of methylphenyl acetates at the
aromatic ring have one hydroperoxy group and that
they are formed according to Scheme 4 [8].
3-methylphenyl acetate
Peroxide concentration × 102, M
Reaction time, min
after 1 h
after 24 h
020
040
060
080
100
06.6
13.3
19.5
26.6
33.2
06.7
13.3
19.7
26.5
33.5
120
33.5
33.7
Scheme 4.
OAc
OAc
OAc
OAc
OAc
CHO
O−
O
O
+
Ac2O
O3
O
+ O3
O
CH
OOH
O
Me
Me
Me
Me
Me
OAc
2. Galstyan, A.G., Sedikh, G.O., and Galstyan, G.A., Ukr.
Khim. Zh., 2007, vol. 73, no. 6, p. 104.
3. Okhrimenko, I.S. and Verkholantsev, V.V., Khimiya i
tekhnologiya plenkoobrazuyushchikh veshchestv (Chem-
istry and Technology of Film-Forming Substances),
Leningrad: Khimiya, 1978.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 1 2010

LIQUID-PHASE REACTIONS OF ISOMERIC METHYLPHENOLS WITH OZONE
19
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 80 No. 1 2010