Kinetics of conjugated oxidation of benzene with aldehydes in the presence of acetic anhydride

KINETICS OF CONJUGATED OXIDATION OF BENZENE WITH ALDEHYDES IN THE PRESENCE OF ACETIC ANHYDRIDE* D. I. METELITSA and L. V. SHIBAYEVA

IT WAS shown [1] that conjugated oxidation of benzaldehyde or acetaldehyde with aromatic compounds (benzene, toluene, chlorobenzene and naphthalene) at 115-166 ° and 50 atm air is accompanied b y hydroxylation into phenols. Conjugated oxidation of aromatic compounds and aldehydes in the presence of acetic anhydride as acetylating agent causes the concentration of phenol compounds and their rate of formation to phenylacetates to increase considerably. This paper seeks to examine in detail the kinetics of conjugated oxidation of benzaldehyde and acetaldehyde with benzene in the presence of acetic anhydride. EXPERIMENTAL

A 70 ml volume mixture of benzaldehyde, benzene and acetic anhydride was oxidized at 36-70 ° in a glass reactor. Oxygen flow was not less than 151./hr. A 120 ml volume mixture of acetaldehyde, benzene and acetic anhydride was oxidized at a temperature 90-166 ° and pressure 50 atm. The mixture was placed in a glass cylinder, which was placed in an autoclave and stirred with argon passed through while heating and then air, at a rate of 30-40 1./hr using 120 ml solution. Some of the experiments on conjugated oxidation of benzaldehyde with benzene were also carried out in an autoclave at temperature 50-140 ° and air pressure of 50 atm. During the experiments samples were taken for analysis from the autoclave using a special device. The raw materials and most products of oxidation were determined b y gas-liquid chromatography using an LKhM-7A apparatus. To separate the mixtures a column 2 m long, 2 mm diameter was used, containing polyethylene glycol adipate (20%) on eelite; the temperature of the evaporator was 250 °, the detector was of the flame-ionization type, helium being the carrier gas. Acetaldehyde and methanol were determined at a column temperature of 50 °, benzaldehyde and phenylacetate at 125 °, phenol and diphenyl at 154 °. Peracids and peroxides were determined iodometrically. From the autoclave carbon dioxide was absorbed b y means of baryta water and determined gravimetrieally. Acetic anhydride and benzaldehyde used in the study were distilled in argon at a pressure of 10 mmHg and at temperatures of 38 and 61.7 °, respec* Neftekhimiya 12, No. 5, 740-747, 1972. 185

186

D. I. M~TELITSA and L. V. SmBAYEVA

tively. "Cryoscopic" benzene was used. Phenyl acetate was prepared by acetylation of phenol with acetic anhydride in benzene at 52 °. At temperatures of 50 to 140° and an air pressure of 50 arm mixtures containing 95 ml benzene (8.9 mole/1.), 10 ml benzald'ehyde (0.82 mole/1.) and 15 mI acetic anhydride (AA) (1.32 mole/1.) were oxidized. Kinetic curves showing the consumption of benzaldehyde and formation of phcny] acetate, diphenyl (DP), phenol and peroxide compounds, are indicated by Fig. 1. mole/l.

I0 °, ~

see

[t;6Nst;NuJo

!

0.08 ~-

~.0

8 ~ 1.0 ¸

1.0

0

~/0

80 lOO

I ~O

0

I

I~

I

I

~0

[CMa~,],,,o/e/l.

min

Fz(~. 1

F~G. 2

FIG. 1. Kinetic curves of the consumption of benzaldehyde (11 and formation of peroxides [2], diphenyl (31, phcnyl acetate (4) and phenol (5). Temperature 110°; air pressure 50 arm. FIG. 2. Effect of acetic acid on the rate of oxidation of benzaldchyde (1) and the rate of formation of phcnyl acetate (2).

Conjugated oxidation of benzaldehyde with benzene was characterized by initial rates of oxidation of benzaldehyde W~o*H~ cH° and the maximum rate of formation of diphenyl W~x and the maximum rate of formation of the overall amount of phenyl acetate and phenol WC,H,OH+C,~=°c°cHI Table 1 shows results of experiments on oxidation of mixtures of benzene, benzaldehyde and acetic anhydride. It can be seen from Table 1 that with an increase in temperature the rate of formation of diphenyl increases more rapidly than the rate of formation of phenol compounds and the maximum concentration of phenol compounds somewhat decreases. It is therefore advisable to carry out oxidation at lower temperature. During oxidation of benzaldehyde in the presence of acetonitrile, instead of benzene (140°) a certain amount of phenol compounds is formed. ""

In&x

°

Kinetics of conjugated oxidation of benzene with aldehydes

187

TABLE 1. R~.S~rL~S O~ CO~JUCATEI) OXIDATIO~ OF BE~ZALDE~rrDE wrrH B~.NZE~E Pressure 50 a t m air

(WoC 6 H s C H O ) x

X104

50]H+C6~t ~O C O C H s (W.C.61~I .. )x

X104

mole/1..see

50 80 110 140 140"

(w£~x)x

mole/l..sec

4-3±0.8 6.4±0.8 8 . 3 ±0 . 8 10.5±0.8

lo~

mole/l.-sec

0.4±0.3 0.6±0.3 0.7±0.3 0-8±0.3 0.2±0.3

0 .3 ± 0 .3 0.8±0.3 1.3±0.3 2.3±0.3

CeHsOH ÷ CeHsOCORm=x mole/1.

0"078 0.074 0-070 0.070 0.012

* The experiment was carried out in acetonitrile.

Oxygen concentration has a considerable effect on the formation of phenol. The experiments were carried out in a glass reactor with mixtures of CeHsCHO (0.70 mole]l.)+C~H e (7.62 mole/1.)+(CHaCO)20 (1-13 mole/k) at a t e m p e r a t u r e of 51.7 ° and oxygen contents of 20, 50 and 100% vol. As a result it was shown t h a t the rate of formation of phenyl acetate increases in a linear manner with an increase in oxygen content in the mixture oxidized and reaches 1.08× 10 -s mole/l..sec when using pure oxygen at a pressure of 1 arm.

[CsHsCHO],mo/e/l. •~

0

04/

° 0

0.8

~2

~ I'0

1-6

Il 2"0

3"0

[CHsCHO ,mole/L ]

l/0

FIG. 3. Effect of the concentration of benzaldehyde (1) and acetaldehyde (2) on the rate of formation of phenyl acetate. Temperature 51.6°; oxygen pressure 1 a t m and 166 °, 50 a t m air, respectively. 1-70 ml mixture; 2-120 ml mixture, acetic anhydride 1.77 mole/1, and benzene with acetaldehyde in equal proportions.

Oxidation of benzaldehyde and formation of phenyl acetate are influenced by the amo u n t of acetic acid in the mixture. At a temperature of 51-7 ° and a pressure of 1 arm oxygen mixtures with an overall volume of 70 ml which contained 0.70 mole/l, benzaldehyde, 1.13 mole/1, acetic anhydride and benzene mixed with acetic acid in different proportions were oxidized. With an increase in acetic acid concentration to 2.48 mole/1, the rate of formation of

D. ~[. METELITSA a n d L. V. SHIBAYEVA

188

phenyl acetate and the rate of oxidation of benzaldehyde increase. A further increase in the ratio of [CHaCOOH]0/[CeHe]o reduces both values. Figure 2 shows the dependence of WC,HsOCOCH81r¢~ ~ 1 on the concentration of "max /tx~6±a63o CH3COOH. For practical purposes it is important to select a [C6HsCHO]o/[C6H6]o ratio for which rates of formation of phcnyl acetate arc maximum. Therefore, at a temperature of 51-7 ° mixtures of 70 ml containing 2.48 mole/1. [CH3COOH] o, 1.13 mole/1. [VA]o and various proportions of C6H.~CHO and CsH6, were oxidized at an oxygen pressure of 1 arm. Figure 3 (curve 1) shows the dependence of WC~n~°c°cH' on [C6H~CHO]o, from which it follows that 0.705 mole/1. is the optimum concentration of C6HsCHO, [C6He]0 in this case being 7.62 mole/1. Using the reagent ratio of the mixture the temperature dependence of oxidation of the mixture was examined: 36.3

42.1

51-7

60.2

69.6

wC~EsOCOCHs y ](b~ mole/1..sec

T e m p e r a t u r e , °C

0.7

0.7

1.0

1.3

1.3

WOCC~HsHOX 10~, mole/l..sec

0.7

1-0

1-6

1.8

2.0

m a x

~

~

~

From the data given activation energies corresponding to the temperature course of the initial rate of benzaldehyde oxidation and maximum rate of phenyl acetate formation, were calculated. For Wc'HscH°w0 E----(5.0~=l.0) kcal/mole; for WCg~x~°c°cy2 E----(4.3! 1.0) kcal/molc. To establish the type of hydroxylation of benzene in conjugated oxidation with benzaldehyde at a temperature of 51.7 ° and an oxygen pressure of 1 arm 30 hr after the beginning of the reaction, 2,4,6-tri-tert-butylphenol as inhibitor in a concentration of 0.020 mole/1. (Fig. 4), was added to the oxidizing mixture. The Figure indicates that the inhibitor added to the reaction inhibits considerably the oxidation of benzaldehyde and the formation of peroxide compounds, b u t does not affect in practice the rate of formation and the maximum concentration of phenyl acetate, which indicates a non-radical type of formation under these conditions. The addition of 0.005 mole/1. Co(CH~COO)~.4H20 at the very beginning of conjugated oxidation of benzaldehydc with benzene under the same conditions, by decomposing effectively the peroxide compounds, markedly accelerates benzaldehyde oxidation and no phenyl acetate is formed at all. Mixtures of a volume of 120 ml containing acetic anhydride in a proportion of 1-77 mole/1, and acetaldehyde with benzene in various proportions, were oxidized at 166 ° and 50 atm air. Figure 5 illustrates kinetic curves of acetaldehyde consumption and the formation of phenyl acetate, peroxide compounds and CO,. The formation of phenyl acetate is autocatalytic. I f acetaldehyde is completely absent, no phenyl acetate is formed and an increase in concentration of CH3CHO to 3-68 mole/1, increases the value of wC, 1 m aH,oeoe,,,/ro x / L"-"6 H 6JO rr

Kinetics of conjugated oxidation of benzene with aldehydes

189

(Fig. 3, curve 2). I n conjugated oxidation of a c e t a l d e h y d e with benzene the a m o u n t of d i p h e n y l is a b o u t 20% of the p r o p o r t i o n of p h e n y l acetate and m e t h a n o l is f o r m e d in low concentrations.

mole/l. 2.0~

rnole/L

mole/l.

O'J~l

0.7 ~ , , ~ f

0

/40

80 I00

0"03

°°o.o,tp t . # I

O

/40

0"05

80

120

~

0"01 I

I

¢0

I

I

80fO0 m/n

I

0

FIG. 4

I

110

I

80

1

I

.

m/r/

I20

FIG. 5

Fro. 4. Kinetic curves of consumption of benzaldehyde (1; 1') and formation of peroxides (2; 2') and phenyl acetate (3; Experiments without inhibitor; 3'--2,4,6tri-tert-butylphenol 0.02 mole/1.; mixture, mole/1.; benzene 7.62; benzaldehyde---0.705; acetic anhydride 1.13; acetic acid--2.48.

3').1,2,3

1";2';

FZG. 5. Kinetic curves of the consumption of acetaldehyde (1) and formation of peroxides (2), COs (3) and phenyl acetate (4) without inhibitor; 4' with inhibitor. Mixture: acetaldehyde---2.0; benzene--7.95; acetic anhydride 1.77 mole/l. Mixtures of benzene (8.42 mole/1.), a c e t a l d e h y d e (1.47 mole/l.) and acetic a n h y d r i d e (1.77 mole/1.) were oxidized at different t e m p e r a t u r e s and a pressure of 50 a r m air. The results o f these experiments were: . Temperature, °C 106 x "ma~WC6HsOeOCH'mole/1..sec , [C6HsOCOCHs]raax, mole/1.

140 3-5 0.017

150 6.4 0-025

166 11 0.042

Addition of 0.02 mole/1. 2 , 4 , 6 4 r i - t e r t - b u t y l p h e n o l to the m i x t u r e oxidized a t a t e m p e r a t u r e of 166 ° m a r k e d l y reduces the r a t e of f o r m a t i o n of p h e n y l acetate (Fig. 5, curves which indicates a partially radical t y p e of p h e n y l acetate f o r m a t i o n u n d e r these conditions. F o r practical purposes it is i m p o r t a n t to k n o w the p r o d u c t ratio of conjug a t e d oxidation, the degree of reagent conversion and main p r o d u c t yield in percentage of the reacted r a w materials. A special balance e x p e r i m e n t

4, 4'),

D. I. METELITSA and L. V. SHIBAYEVA

190

was carried out for this purpose at a temperature of 166 °, air pressure of 50 arm using a mixture which consisted of 2.0 mole/1, acetaldehyde, 7.95 mole/1. benzene and 1.77 mole/1, acetic anhydride. The main difficulty in carrying out this experiment was in determining the reacted acetaldehyde since owing to high vapour pressure it is entrained from the autoclave with the escaping TABLE 2. RESULTS OF A BALANCE EXPERIMENT Temperature 166 °, air pressure 50 atm Time, min 20 40 60 80 I00

i

CH3CHO COs reacted, formed mole/1, mole/1. I % 1.64 1.80 1.85 1.90 1.98

0"161 0.28! 0.36 0.45 0.50[

Benzene reacted

I I

I m°le/l" i

9'7 15.5 19.4 23.6 25.2

i[ %

0.0046 0.06 0.023 0-29 0.o35 i 0-45 0'047 0.60 0-053 0 . 6 7

[C6HsOCOCH3]+ [C6H,OH] +

I mole/1. i 0.0046 0.0180 0.0295 0.0412 i 0.0456

! I !

% 100.0 78"4 84'4 86-2 85"8

_ [Diphenvl] [

moh tl. 0.0, 0.0, 0-0

I

0.0

gas in spite of efficient operation of the cooler. In a balanced experiment acetaldehyde is recovered at the outlet from the autoclave with ethanol at a temperature of --78 °. The amount of acetaldehyde entrained was calculated from the balance. Results of the balance experiment are shown in Table 2. The overall amount of phenyl acetate and phenol is given in percentage of the benzene reacted and the amount of CO2, as a percentage of the acetaldehyde reacted, the remaining reacted aldehyde being converted to acetic acid. As shown by Table 2, during 100 min reaction benzene is converted to the extent of 0.67% tbrming a total oi 85.8% of phenol and phenyl acetate, acetaldehyde is oxidized completely, giving ~ 75% of acetic acid and ~ 2 5 % COs. RESULTS

The mechanism of oxidation of aldehydes with oxygen is now reliably established and is described b y the system [2]: O RCH0+0~

-, RC /

+H0;,

0

RC /

(i)

0

+0, -+ RC/2 %

,

(I)

00"

O RC j

\oo"

O + RCH0

-+ RC J

\00H

O +RC

"j ,

(2)

Kinetics of conjugated oxidation of benzene with aldehydes

191

O RC~

\

-~RCH0 ~ RCO--OO--CH(OH)R -* 2RCOOH,

(3)

OOH O 2RCJ

~ products

(4)

\oo During oxidation of aldehydes with acetic anhydride per-acids can easily react with the latter, forming diacyl peroxides 0

0

RCO3H~- (CH,C0)~O ~ RC J

\

\ C--CHs+CH,COOH O--O

/

(5)

Conjugated oxidation of benzene with aldehydes is accompanied by the formation of phenol, which is readily converted to phenyl acetate by reaction with acetic anhydride C,H,0H+(CH,CO)20 ~ C,HsOCOCH,+CH,COOH.

(6)

During oxidation of aldehydes and benzene four methods are possible for the formation of phenol compounds. 1. Per-acid may be used as hydroxylating agent O RC ~

+C6H,

\

-* CeH,OH+RCOOH.

(7)

OOH The reaction of hydroxylation of aromatic compounds with trifluoracetic acid [3] is well known, this being the most electrophilic agent among peracids CFsCO3H, C,HsCOsH, HCOaH and CH3CO3H kccording to the Taft a values CFa-[-2-6; C,H5~-0.6; H-[-0.49 and CH 3 0.0. Reaction (7) takes place by a molecular mechanism [3]. Experiments with the addition of an inhibitor during conjugated oxidation of benzaldehyde with benzene at 30-70 ° showed t h a t radical hydroxylation is practically completely absent (Fig. 4). Therefore, under our conditions reaction (7) is the main means of hydroxylation of benzene. 2. An acyl peroxy radical may also be used as hydroxylating agent

0

Rej

\oo

H

o

+0,H. -~ ReJ

÷

~/0

-~ C,HsOH

\o" ~'%/~

(8)

The reaction, which is similar to (8) takes place between acyl peroxy radicals and olefins and results in epoxy formation [4].

D. I. METELITSA and L. V. SHIBAYEVA

192

3. At high temperatures per-acids may decompose at the O--O-bond giving HO" radicals capable of easily hydroxylating aromatic compounds. 4. Phenyl carboxylate may be formed directly by the interaction of benzene 0

S \

with RC

radicals, which are formed during oxidation of aldehydes and

O" during the breakdown of per-acids and diacyl peroxides. OCOR C~Hs÷RC

~-i

\o

. H

%/\

0, C6HsOCOR ÷ I-I0~'.

H

0

// The RC

\

radical may not only be decarboxylated, but also added

O" to the benzene nucleus to form a new radical which can be oxidized to phenyl carboxylate with an iodine atom: (C6He.OCOC~Hs)" ÷ I-~C6HsOCOCeHs+

HI

[5]

It may be concluded from the experimental results that the second, third and fourth method of hydroxylation is basically possible at high temperatures. At 166 ° the addition of an inhibitor considerably reduces the rate of phenyl acetate formation during conjugated oxidation of acetaldehyde with benzene (Fig. 5, curves 4, 4') which proves a partially radical type of hydroxylation. Comparison of kinetic curves of acetaldehyde consumption and formation of phenyl acetate (Fig. 5 curves 1, 4) indicates that the maximum rate of phenyl acetate formation takes place after aldehyde has been oxidized and it may then be assumed that phenyl acetate is formed either by the breakdown of per-acid or, what is more likely, by breakdown of the acetyl peroxide into which per-acid is converted by reaction with acetic anhydride. Indeed, a calculation of the rate of decomposition of acetyl peroxide at 166 °, when [peroxide]max 4 × 10 -3 mole/1., indicates that Wdecom~----k d × 4 X 10-3~0"645 >( x 4 x 10 -a mole/1..sec. The value of ka at 166 ° was calculated from the equation log kd:lS.91-32300/RT×2"3 [6]. The rate of decomposition of acetyl peroxide is higher than the maximum rate of formation of phenyl acetate which at 166 ° is 1.3× 10 -5 mole/1..sec. It is therefore possible that the peroxide subject to decomposition ensures the formation of phenyl acetate at high temperatures. It follows from experimental results that at high temperatures CO~. forms intensively. Comparison of kinetic curves of CO, formation and acetaldehyde

Kinetics of conjugated oxidation of benzene with aldehydes

193

consumption (Fig. 5) indicates that decarboxylation takes place not only during the oxidation of acetaldehyde but also after it has been oxidized: it is possible that acetic acid is deearboxylated by a reaction conjugated with oxidation. During conjugated oxidation of benzaldehyde with benzene diphenyl is formed in considerable quantities by the reactions O

H

~--%--/='~/ C6H5--C S - - - ~ P h ' + C e H , -* ~ = 7 7~/" \ -co, H O"

o.

* HO~-CeHs--C6H5 •

Replacement of benzaldehyde by acetaldehyde considerably reduces the amount of diphenyl. A certain amount of phenyl is formed at high temperatures (140 °) during oxidation of benzaldehyde in acetonitrile in the complete absence of benzene, apparently, via decarboxylation of per-benzoic acid. CeHsCOsH -* C6HsOH~-CO2.

During the oxidation of benzaldehyde with benzene, phenyl formate is formed in small amounts by one of the courses of the Bayer-Williger reaction O --* Ph0CHO-~ PhCOOH

CeHsCHO ~ P h C ~

\

[7]

O0H SUMMARY

1. A study was made of conjugated oxidation kinetics of acetaldehyde and benzaldehyde with benzene in the presence of acetic anhydride. Reaction conditions were found under which the rate of formation and the concentration of phenyl acetate are maximum. 2. During conjugated oxidation of benzaldehyde with benzene at temperatures of 30-70 ° and an oxygen pressure of 1 arm, perbenzoie acid is the main hydroxylating agent. Hydroxylation at high temperatures in the acetaldehydebenzene-acetic anhydride system takes place basically by a radical mechanism. REFERENCES 1. D. I. METELITSA, Izv. A N SSSR, Ser. khim., 702, 1972 2. N. M. EMANUEL', Ire. T. DENISOV and Z. K. MAIZUS, Tsepnye reaktsii okisloniya uglevodorodov v zhidkoi raze (Liquid-15hase Chain Reactions of Hydrocarbon Oxidation). Nauka~ Moscow, 1965 3. A. J. DAVIDSON and R. O. C. NORMAN, J. Chem. See., 5404, 1964 4. P. I. VALOV, E. A. BLYUMBERG and N. M. EMANUEL', Izv. A N SSSR, Set. khim., 791, ]969 5. P. KOVACIC, C. G. REID and M. J. BRITTAIN, J. Organ. Chem. 35, 2152, 1970 6. M. LEVY, M. STEINBERG and M. SZWARC, J. Amer. Chem. See. 76, 5978, 1954 7. Y. OGATA, J. TABUSHI and H. AKIMOTO, J. Organ. Chem. 26, 4803, 1961