OXIDATION OF SIMPLE PHENOLS BY PHENYL IODOSO ACETATE BY I. M. MATHAI AND K. RENGARAJAN (Department of Chemistry, Madras Christian College, Madras-59) Received August 6, 1969 (Communicated by Prof. S. V. Anantakrishnan, F.A.sc.)
ABSTRACT Kinetic studies on the oxidation of phenol and 4-nitro and 4-methyl phenols by phenyl iodoso acetate are reported here. The reaction rate depends on the dielectric constant of the medium. A probable mechanism for the oxidation is also suggested. The free energies of activation for the oxidation of the three substrates studied are in accordance with the mechanism proposed. INTRODUCTION
iodoso acetates and benzoates as oxidants are similar to lead tetra acetate and they oxidise a variety of substrates including phenols, 1 :2 diols, primary and secondary alcohols, aminesl etc. But many of these oxidations have not been investigated kinetically. In the oxidations of phenols by lead tetra acetate 2 o- and p-quinol acetates, p-quinones and also o- and p-quinone diacetates are formed depending on the number and position of substituents. Siegel and Antony 3 isolated 2- and 4-acetoxy phenols as the major products in the oxidations of phenols by phenyl iodoso acetate. But in presence of excess of oxidant o-benzoquinone was also obtained in the case of simple phenol. We felt that it was necessary to follow the oxidations kinetically to throw some light on the mechanism. The following preliminary report is confined to a study of a few simple phenols. ARYL
Most of the work was carried out in 100% acetic acid. Other solvents like benzene, dioxan and water were added to the reaction mixture only to correlate the behaviour of the reaction with dielectric constant of the medium. 47 A6
48
1. M. MATHAI AND K. RENGARAJAN EXPERIMENTAL
(a) Phenyl iodoso acetate was prepared by a modification of the method suggested by Boeseken and Schneider. 4 Iodometric estimation showed that the purity of the sample was over 99•5%, m.p. 157° C. (b) Phenol and 4- methyl phenol (p-cresol) were purified by distillation, the middle fraction being used for kinetic runs. b.p. of phenol 181° C. ;
b.p. of 4- methyl phenol 198° C.
(c) 4 -Nitro phenol was purified by recrystallisation from hot water, m.p. 114° C.
(d) Acetic acid was purified by the method of Orton and Bradfield 5 and freshly distilled before each kinetic run in order to minimise the absorption of water by the acid. (e) Benzene and dioxan were purified by the methods of Washburn and Read, 6 and Eigenberger respectively. The boiling points and the refractive indices of the solvents are given in Table 1. TABLE I
Solvent
Acetic acid Benzene Dioxan
Boiling point ° C. ..
118
Refractive Index 1.369
..
80.1
1.498
..
101.3
1.420
(f) 50 ml. of a 0.002 M solution of phenol and 50 ml. of a standard solution of oxidant were mixed after they had acquired thermal equilibrium. Aliquot portions of the reaction mixture were withdrawn at suitable intervals and titrated iodometrically. Under our experimental conditions the reactions were found to be second order and the rate constants were obtained graphically by plotting log (a — x)/(b — x) vs. time. RESULTS
All the three substrates, viz., phenol, 4- nitrophenol and 4- methyl_phenol gave good second order plots. A typical second-order plot in the case of
Oxidation of Simple Phenols by Phenyl Iodoso Acetate
49
simple phenol is given in Fig. 1. Some of our results are presented in Table II. The Arrhenius parameters were obtained in the usual manner and are given in Table III.
5l
4 W
a
Ovt9
03j
,.
0•35 to
0.0
0.47
O.5
b-x
FiG. 1 DISCUSSION
As seen from Table II the rate of the reaction with the phenol as the substrate increases on the addition of water and decreases on the addition of solvents of low dielectric constants. This suggests that the reaction is between two dipolar molecules. This is confirmed by a linear plot of log k vs. (D — 1) / (2D + 1) (Fig. 2). A probable mechanism for the oxidation of simple phenol ma.y be formulated involving an intermediate (I) of the type suggested by Pausacker 8 in his mechanism for the oxidation of isohydrobenzoin by phenyl iodoso acetate. The formation of (I) probably involves nucleophilic attack by
I. M. MATHAI AND K. RENGARAJAN
50
TABLE II Temgerature C' 35
Substrate
Solvent (% by volume) -Water Benzene Acetic acid
Phenol
30 30 35 40 30 35 40
„
4-Methyl phenol
..
..
..
99
1
..
..
12.630
98
2
..
..
26.160
95
..
5
..
5.663
90
..
10
..
4•667
75
..
25
..
3.468
95
..
..
5
5•525
90
..
..
10
4.628
75
..
..
25
3.048
..
..
..
100
..
..
..
100
..
..
..
100
..
..
..
100
..
..
..
280.500
100
..
..
..
0.014
1(0
..
..
..
0•033
100
..
..
..
0.078
„
4-Nitro phenol ,,
6•645
100
100
40
Dioxan
Rate constant X 10$ mole-' sec'
10.050 3.859 76.780 146•200
TABLE III
Substrate Phenol 4- Methyl phenol 4 -Nitro phenol
E K. cals
AF K. cals
AS e.u.
18.31 22.28 31.21
18.86 17.82 22•82
—4.55 14•22 27•68
the oxygen atom of the phenol. The removal of one molecule of acetic acid and nucleophilic attack are probably synchronous steps. (I) can produce by heterolysis the species (II) which is actually the resonance hybrid of structures (III) and (]V). The species .(III) and (IV) can undergo nucleo-
Oxidation of Simple Phenols by Phenyl lodoso Acetate
51
philic attack by acetate ion forming (V) and (VI) respectively. (V) and (VI) stabilize themselves by protonic shift forming 2- and 4-acetoxy phenols 0•q
0•s
%at
0.7
0
a.'
0.4
0-37
o-37l
0.37E
D —1
o•3FZ
o-386
a.3g0
2vt1 FIG. 2
respectively. The substitution of acetate ion in the ring proceeds at least partly by an intermolecular mechanism because the reaction rate increases with the addition of sodium acetate (Table IV). This would be expected to favour the formation of (V) and .(VI). TABLE IV Amount of Rate constant Substrate Temperature sodium acetate x102 °C. mole-1 sec-1 M Phenol Phenol Phenol
.. ..
35
0.025
6.902
35
0.05
35
0•1
8.269 9.717
52
1. M. MATHAI AND K. RENGARAJAN CH a
H kJ C `O
^CBH6 0 "O.CO.CH 3
S' ^O I C8H5 CO U
+ CH 3 .000H
1 II
CHs
®^ I
O0
O+
I
U
I
(
E--+
III
IV
I
—
II
/\H
II1
f Il "
II + C B H SI -I- CH 3 . COO
y 0
0
CHa.CO•v
E—^
^O.CO.CH,
H
VI
V
OH (`
OH )\ .O.CO.CH 3
O. CO.CH3
The formation of o-benzoquinone in the oxidation of simple phenol in presence of excess of oxidant may be explained as follows: 2-Acetoxy
Oxidation of Simple Phenols by Phenyl Iodoso Acetate
53
phenol would be expected to undergo hydrolysis to some extent forming catechol which then can form o-benzoquinone by further reaction. The mechanism is probably similar to the one suggested by Pausacker 9 for the oxidation of glycols by phenyl iodoso acetate. I
C B HS
O^ O•CO•CH,
OH
j\ / OH
I
II
C6H5I (O•CO•CH 3 )a
^
+ CH3 •COOH
I
l
y 0
/\ CGHSI
+ II
O—I—C6H5_
%
I I) L\/
I
j
The first step, viz., the formation of (I) appears to be the rate determining step since the reaction rate is increased by the addition of water and decreased by the addition of benzene or dioxan. The formation of (I) would be expected to be favoured by the presence of electron releasing groups and hindered by the presence of electron-attracting groups in the phenyl ring of the phenol. This is borne out by the results in Table II. The free energy of activation for the three substrates increases in the order 4-methyl phenol < phenol < 4-nitro phenol corresponding with a decrease in the reaction rate in the same order. The oxidations of 4-nitro and 4-methyl phenols appear to proceed by the same mechanism as the ap values for the three substrates fall on a good straight line when plotted against the corresponding log k values (Fig. 3). It is expected that a detailed product analysis (in progress) and further kinetic work using substituted phenols, dials, amines and alcohols as substrates will enable us to bring forth the finer aspects of the mechanism of oxidations using phenyl iodoso acetate.
54
I. M. MATHAI AND K. RENGARAJAN
0
ro v.
d
z^o
3.0
4.0 -0.3
06
0
o-p
Fio. 3
ACKNOWLEDGEMENTS
We are grateful to Professor S. V. Anantakrishnan, F.A.SC., F.N.I., for his helpful suggestions and to Mr. R. Madhavan for carrying out the preliminary work. We also thank the U.G.C. for the award of a grant under the " Summer Research Participation Programme for College Teachers" to work on this problem. REFERENCES 1. Criegee, R.
Oxidations in Organic Chemistry, Edited by K. B. Wiberg,
Stewart, R.
Oxidation Mechanisms by W. A. Benjamin, Inc., 1964,
Academic Press, New York, 1965, p. 405
p. 103. 2. Wessely, F. and Sinwel, F. . . Monatsch, Chem., 1950, 81, 1055. Angew. Chem., 1958, 70, 173. Grieg^.e, R. Monatsch. Chem., 1955, 86, 292. 3. Siegel, A. and Antony, F. J. Chem. Soc., 1953, p. 107. 4. Pausacker, K. H. Ibid., 1927, p. 983. 5. Orton, K. J. P. and Bradfield, A. E. J. Amer. Chem. Soc., 1919, 41, 729. 6. Washburn, E. W. and Read, J. W. Prakt. Chem., 1931, 130, 75. 7. Eigenberger, E. J. J. Chem. Soc., 1953, p. 107. 8. Pausacker, K. H. 9.
Ibid., 1953, p. 107.
2075-70. Printed at The Bangalore Press, Bangalore•18, by V. J. F. Jesudason, L.P.T., Superintendent. Published by, B. S. Venkatacbar, Editor, "Proceedings of the Indian Academy of Sciences", Bangalore