82 (1963) RECUEIL 51 -___-

547.461.6.07:547.593.21 I ~547.594.3 PREPARATION OF ADIPIC ACID BY OXIDATION OF

CYCLOHEXANOL AND CYCLOHEXANONE WITH NITRIC ACID Part I. Reaction mechanism

BY

W. 3 . VAN ASSELT and D. W. VAN KREVELEN (Central Laboratory Staatsmijnen, Geleen and Laboratory for

Chemical Technology, Technological University, Delft)

The oxidation of cyclohexanol with nitric acid to adipic acid proceeds via two stable intermediates known from the literature 1 viz. 6-hydroxyimino- 6-nitro hexanoic acid and the hemihydrate of 1,2-cyclohexanedionc, two sub- stances forming beside each other in a given ratio. This ratio can be calculated as a function of the temperature and of the nitric acid and nitrous acid con- centrations. The nitrous acid plays a very important part in the oxidation process. The oxidation of cyclohexanone proceeds in the same way as that of cyclohexanol, provided sufficient nitrous acid is present. In the absence of HNOZ the oxidation does not proceed at all a t low temperatures.

The catalysts used - ammonium vanadate and copper nitrate - have very different functions. Under the influence of the vanadate the hemihydrate of cyclohexanedione is rapidly converted to adipic acid, whereas in the absence of vanadate this substance is slowly broken down to glutaric acid, succinic acid and oxalic acid. Copper is effective only at higher tempcraturcs where it prevents the further break-down of unstable inter mediates.

.

A. Introduction The oxidation of cyclohexanol to adipic acid by means of nitric acid

proceeds via some stable and instable intermediates. In the light of the data and suggestions published by Lindsay1 and

Godt 2 the mechanism of the oxidation can be pictured as set forth below:

Cyclohexanol is first oxidized to cyclohexanone, in which process nitrous acid is released. Next, the cyclohexanone reacts with the nitrous acid to yield isonitrosocyclohexanone which, by reaction with nitric acid, gives 2-nitro-2-nitrosocyclohexanone.

- A . F. Lindsuy, Chem. Eng. Sci. 1, Suppl. 79 (1951). H . C. Godt and J. F. Quinn. J . Am. Chem. SOC. 78, 1461 (1956).

52 W. J, van Asselt and D. W. van Krevelen

With attendant ring opening and uptake of water the latter compound is transformed into 6-hydroxyimino-6-nitro hexanoic acid (hereinafter to be referred to as nitrolic acid), the greater part of which (over 90%) is con- verted to adipic acid and N20; in addition, slight amounts of glutaric acid are produced.

By-products may form by introduction of a second nitroso-group into the intermediately formed isonitrosocyclohexanone.

According to Godt cyclohexanone may moreover yield the hemihydrate of 1,Zcyclohexanedione (to be referred to as Dione), especially at lower nitric acid concentrations ; this Dione is converted to adipic acid and furthermore mainly to succinic acid and glutaric acid.

Nitrol ic acid

t I I

CSN 0 H ' 'NO2

I Succinic Glutaric Adipic acid acid acid

Preparation of adipic acid by oxidation of cyclohexanol, etc. 82 (1963) RECUEIL 53

The present study was undertaken to verify and elaborate the above diagram, to investigate the part played by the catalysts in the process and to measure the reaction kinetics of the various reactions.

B. Methods 1. Standard procedure of the oxidation experiments

The oxidation experiments were carried out in a cylindrical vessel (volume 120 ml) fitted with an electromagnetic stirrer and a cooling jacket. In this vessel 50 ml of nitric acid, (with or without addition of catalyst) are heated up to the desired reaction temperature, after which appr. 1 g of cyclohexanol is added dropwise. The temperature was kept within 0.5" throughout the oxidation.

In the oxidation of cyclohexanone either HNO2 is dissolved in the nitric acid beforehand, or, after the addition of cyclohexanone, a KNOz solution is supplied to the nitric acid slowly and dropwise, or a stream of HNO2 gas is passed through it. Which of these methods has to be used for oxi- dizing mixtures of cyclohexanone and cyclohexanol depends on the per- centage of cyclohexanol in the mixture.

The following catalysts were used : ammonium metavanadate, NH4V03 , and copper nitrate, Cu(N03)~. 3 aq, mostly in amounts of 0.02 mol/l. The reaction time was taken so long that the yield of adipic acid and nitrolic acid (Cs-yield) did not further increase. The amounts of glutaric and succinic acid, on the other hand, do increase gradually until complete conversion of the intermediates is reached, but this takes a very long time at low temperatures.

2. Analysis of the reaction products Upon completion of the oxidation the reaction mixture is diluted with

water and analysed by the method of liquid-liquid partition chromato- graphy. The column packing in this case consists of 20 g of silicagel (particle size 0.2-0.4 mm) impregnated with 7.50 ml of water. 1 g of silicagel, im- pregnated with 0.37 ml of sample, is placed on top of the column after which the mixture is eluted with chloroform-butanol mixtures (0-20 % butanol). The collected fractions are titrated with 0.02 M alcoholic potassium hydroxide, phenolphthalein being used as indicator. Nitrolic acid, adipic acid, glutaric acid and succinic acid can be readily separated in this way.

During the experiments it also appeared that beside the above-mentioned products a substance X is produced which could afterwards be identified as an intermediate of the nitrolic acid-adipic acid conversion.

The nitric acid is retained on the column and does not interfere with the determination. Normally, a column can be used several times provided the sample is removed after each analysis.

54 W. .I. van Asselt and D. W. van Kreveleri

The accuracy of this analysis varies from 1 to 3 % for nitrolic acid and adipic acid and from 5 to 20 % for glutaric acid and succinic acid, the latter two being present in small amounts.

3. Preparation and determination of some intermediates The intermediates nitrolic acid and Dione were prepared in the way

indicated by Godt 2 and purified by washing and recrystallisation. Elenien- tary analysis and examination in the mass spectrometer yielded results consistent with the structure suggested by Godt. It was also shown that nitrolic acid can be readily separated from the other dicarboxylic acids on the chromatographic column and, in the form of a dibasic acid, be quan- titatively determined in chloroform by titration with alcoholic sodium hydroxide solution.

C . Results and Discussion 1. Oxidation of’ cyclohexanol

a) Oxidation experiments according to the standard procedure. The oxidations were carried out with 30-67% by weight of nitric acid.

The results obtained with 40% nitric acid are illustrated in fig. 1, table 1 showing some complete analytical results. *

By C S is here to be understood the total yield of products with 6 C-atoms,

100.

80 -

Yield 60- ( . / . C g )

4 0 -

I I I I I I 0 ;O 2 b 30 40 50 60 10 80 90 - Temp.(*C)

Fig. I . Oxidation of cyclohexanol (1 g) in 40% nitric acid (50 ml). Influence of catalysts

0 without a catalyst with 1 mmol HNIVOB

A with 1 mmol Cu(N03)~ 0 with 1 mmol Cu + 1 mmol V

Table I Some complete analytical results obtained in the oxidation of cyclohexanol (1 g) in 40% nitric acid (50 ml)

Temp. (“C)

25 25 25 25

Reaction Nitrolic Adipic Glutanc Succinic Sum An01 c6

Catalyst time acid 1 acid acid 1 acid 1 11 added yield (mmol/50 ml) -

(min) (mmol/50 ml) ( %)

- 60 4.69 1.42 0.50 0.37 10.13 60 1 cU(NOS)2 60 4.60 1.40 0.28 0.34 10.14 59 1 NH4VOs 60 4.50 4.80 0.70 0.12 10.12 9.93 93 lcU+lV 60 4.36 4.34 0.56 0.06 9.32 9.53 92

56 W. J. van Asselt and D. W. van Ki-evelen

so : adipic acid, nitrolic acid and a not further identified intermediate formed during decomposition of nitrolic acid into adipic acid. The portion of the oxidation products not shown in the graph (100- % c6) consists of glutaric acid, succinic acid, lower decomposition products and, as will be demon- strated further, Dione.

The lines found at all other acid concentrations show essentially the same trend as those in fig. 1. At higher acid concentration the lines shift, diver- gently, towards the upper 1.h. corner.

At lower concentrations they shift, slightly, convergently, towards the opposite corner.

At higher temperatures, normally above 40-50", the sum of the products is indeed appr. 100 %; at lower temperatures and in the absence of vanadate glutaric acid and succinic acid are formed only in small amounts which continue to increase with duration of the experiment. The c6 %, however, remains constant. At low acid concentrations and low temperatures the reaction is often retarded. In consequence, it was often essential under these conditions to add a small amount of KNOz to the nitric acid beforehand so as to make the reaction proceed at a moderate rate.

Fig. 1 clearly shows that copper and vanadate, used as catalysts, produce quite different effects. Whereas at low temperatures vanadate highly in- creases the yield, this effect vanishes at elevated temperatures where the copper begins to play a part. Go& demonstrated that at low temperatures the hemihydrate of cyclohexanedione is formed ; the yield agrees approxi- mately with the percentage not accounted for by the analysis of the oxida- tion products obtained without a catalyst at low temperatures.

Consequently, a possible explanation of the effect of vanadate would be that vanadate converts Dione to adipic acid, whereas in the absence of a catalyst and under the same conditions this substance is transformed very slowly into by-products. The fact that vanadate loses its effect at high temperatures can be ascribed to two causes : 1) Dione is not formed anymore. 2) The further oxidation of this Dione by the nitric acid proceeds more rapidly than the conversion to adipic acid under the influence ofthe vanadate.

As to the effect of copper at elevated temperatures the following may be observed: In addition to an improvement of the yield a change is noted in the distribution of the by-products. Whereas at higher temperatures the glutaric acid/succinic acid ratio obtained without a catalyst, or with a vanadate catalyst, is usually 3-4 : 1, the ratio found in experiments using a copper catalyst is appr. 2 : 1. The absolute amount of succinic acid, however does not show any appreciable change. This demonstrates that copper impedes the formation of glutaric acid.

So, whereas vanadate converts a by-product at an increasing rate to adipic acid, i.e. acts as a true catalyst, copper seems to have an inhibiting effect on side reactions.

Preparatiori ofadipic acid by oxidation of cyclohexanol, etc. 82 (1963) RECUEIL 57

b. Some special experiments.

tests of a special character.

on a given aspect of the reaction mechanism.

Beside the experiments by the standard procedure we performed some

These will be discussed separately because each experiment throws light

1. To 50 ml of 51 % nitric acid 0.8 mmol of hydrazine sulphate were added first, to remove dissolved nitrous gases, and subsequently 1 g of cyclohexanol. Whether or not use was made of a vanadate catalyst, no oxidation products could be demonstrated in the reaction mixture after 1 hour at 15". If subsequently the hydrazine sulphate was removed with a small excess of KNOz, the reaction proceeded to completion within appr. 40 min and the yield was equal to those obtained with the standard pro- cedure.

I t must be concluded therefore that under the abose reaction conditions cyclohexanol is not attacked by nitric acid alone.

2. In the oxidation of cyclohexanol an equimolecular amount of HNOz is formed first, a portion of which is afterwards consumed during the formation of nitrolic acid. Experiments in which, upon completion of the reaction, the amount of HNOz was roughly determined, have shown the remainder to be approximately equal to the difference between the amount of cyclohexanol added and the amount of nitrolic acid formed (= yield of CG without addition of catalyst). In the presence of NH4V03 under the same conditions the remaining amount of HNOz was even slightly greater; in consequence there is good reason to state that in the extra-formation of adipic acid under the infruence of vanadate no HNOz is consumed. It even looks as if nitrous acid is formed in this conversion process.

3. If during the oxidation at room temperature part of the excess of HNOz is removed by addition of small amounts of urea, the yield of CG obtained without application of a catalyst shows a slight rise. If an addi- tional amount of HNO2 is previously dissolved in the nitric acid, the yield of C G is very much lower. So, an excess of HNOz lowers the yield of c6 $ no vanadate catalyst is used.

4. If, upon completion of the oxidation reaction carried out without a catalyst (at low temperatures) vanadate is added, the degree of conversion rapidly goes up to 100%. The added percentage is accounted for almost exclusively by the larger amount of adipic acid formed and not by the increased nitrolic acid concentration. In this stage much less heat is released than in the conversion of a corresponding amount of cyclohexanol to adipic acid.

58 W. J. van Asselt and D. W. van Krevelett

80

2. Oxidation of cyclohexanone Cyclohexanone is not oxidized by nitric acid if no nitrous acid is present.

Upon addition of small amounts of nitrite, the ultimate amount of conver- sion products proves to be equimolecular with the amount of nitrite added. The way in which the experiments are conducted appears to be very impor- tant now. If first 1 g of cyclohexanone is brought into 50 ml of nitric acid and afterwards a KNOz solution, or HN02 gas is added to it slowly, the results strongly deviate from those obtained in all other oxidation tests. (Fig. 2). Low yields are obtained and copper increases the yield under these conditions, even at low temperatures.

- 0

d 0

/ v ,

Y

Y 1 '

1 I I I I I I 1 0 10 20 30 4 0 >O 60 7 0 - Ternp.PC)

Fig. 2. Oxidation of cyclohexanone by addition of a KNOa soluttion dropwise to a mixture of cyclohexanone (1 g) and nitric acid (50 ml)

A 40% HN03 with 1 mmol C~(N0a)a 0, 40% HNO3 with I mmol Cu(NO3)z + I mmol NH4V0.3 0 40% HNO3 with 1 mmol NHdV03 9 40% HN03 without a catalyst 0 51 % HN03 without a catalyst 0 51 % HN03 with 1 mmol NH4VOy

We must assume here that side reactions take place; these may be due to the combined effect of NO and NO2 formed from the KNOz added, and the excess of cyclohexanone present. The effect of copper therefore is based probably on the suppression of these side reactions by nitrogen oxides.

However, if first HNOz gas is introduced into the nitric acid and sub- sequently cyclohexanone is added, the yields show qualitatively the same trend as those obtained with cyclohexanol. The experimental results are illustrated in figs. 3, 4 and 5.

Preparatiorr of adipic acid by oxidation of cyclohexanol, etc. 82 (1963) RECUEIL 59

20

I I I I I I 1

10 20 30 4 0 30 €0 70 - Temp.(.C)

Fig. 3. Oxidation of cyclohexanone (1 g) in 51 ”/, nitric acid (50 ml) containing dissolved HNOZ

Numbers in figure = mmol of dissolved HNOz 0 without a catalyst 0 with 1 mmol NH4VO3

-

I I I I I I , I I

Fig. 4. Oxidation of cyclohexanone ( I g) in 40% nitric acid (50 ml) containing dissolved HNOZ

Influence of catalysts

0 without a catalyst A with 1 mmol Cu(N03)~ 0 with 1 mmol NHaV03 0 with 1 Cu + 1 V

( x Experiments with cyclohexanol + HNO2)

60 W. J. van Asselt and D. W. van Kreveien

I 1:: 0'

10

/

I I I I I I I

The products formed are essentially the same, but the yield of c6 obtained without a catalyst is lower than that obtained with cyclohexanol, because the average HN02 concentration during the test is lower.

The difference in c6 is also larger according as more HNO2 has been dissolved a t the start.

The drawn lines in the figures 3, 4 and 5 give the yields obtained with equimolecular amounts of cyclohexanone and HNO2 (10 mmol). Com- parison of these three lines shows that equal yields are obtained at those temperatures where the partial pressures of nitric acid (PHNOJ are equal.

This is demonstrated also in fig. 7 where, starting from the line for 40% HN03, the points with equal PHNO, values have been plotted for 51% and 30% nitric acid.

In the oxidation of mixtures of cyclohexanol and cyclohexanone HNO2 is formed only in the first conversion of cyclohexanol. This nitrous acid is consumed again during the formation of nitrolic acid; so, if no catalyst is present, the c6 yield will be equal to the percentage of cyclohexanol in the mixture. When vanadate is used, the yield is higher, because then also the Dione formed beside the nitrolic acid is converted to adipic acid without consumption of HNOz.

These observations have been confirmed experimentally. It should be borne in mind though that initiation with HNO2 is necessary in all cases, while sometimes (e.g. at 25 % cyclohexanol) a fairly large amount of HNO2 fails to set the reaction going.

Preparation of adipic acid by oxidation of cyclohexanol, etc. 82 (1963) RECUEIL 61

80

(*I. Yield c,) 60- 40,

t 20-

0

- ///~~:~~

I 0 1 I I I I I I I I I I

I I I I

1 1 I I I I I I I

Apparently, this is in contradiction to the assumption that cyclohexanol is rapidly oxidized to cyclohexanone plus HN02. This assumption, how- ever, cannot but be correct because kinetic measurements * have demon- strated that cyclohexanol and cyclohexanone are oxidized at equal veloci- ties. Therefore the first step of the oxidation of cyclohexanol should be considered as a chain reaction proceeding under the influence of HN02. If this HN02 is rapidly taken away by the excess of cyclohexanone, the reaction dies away before all of the cyclohexanol has been converted. The effect of this phenomenon becomes more pronounced as the nitric acid concentration is lower. 30 % nitric acid is not capable of oxidizing a mixture containing an excess of cyclohexanol under these conditions; the amount of product obtained then corresponds to the quantity of HN02 added.

3. Oxidation of the hemihydrate of 1,2-cyclohexanedione (Dione) Purified Dione has been subjected to an oxidation test in the same way

as cyclohexanone. The results of the oxidation experiments are represented in table 11.

The figures clearly show that complete conversion is obtained and 80-90% adipic acid is formed if vanadate is used in the process. Nitrolic acid has never been demonstrated in the experiments, showing that the

* An article on this subject is in preparation.

HN03 ( %I

40 40 40 40 40 42 42 42 42 42 42

50 50 52 52

30 30 30 30 30 30

~~ ~

Temp. ("C)

25 35

45-46 46

55-57 25

35-36 37

44-47 55-61 55-62

34-36 44-48 35-40 45-51

34 45

55-56 35 45

55-57

Table I1 Oxidation of hemihydrate of cyclohexanedione (1.00 g) in nitric acid (50 ml)

Reaction time (min)

55 35 20 60 45 55 35 35 20 10 10

75 70 20 10

105 80 75 60 30 25

I Sum of the products

determined chromatographically

Catalyst (rnmol/l)

- - - - -

20 v 20 v 20 Vf Cu 20 v 20 v 10 v - -

20 v 20 v - - -

20 v 20 v 20 v

1.62 3.45 4.55 5.92 5.92 8.21 8.21 8.20 8.13 8.23 8.17

5.72 5.49 8.20 8.15

2.46 4.96 5.92 8.34 8.29 8.26

Yield

Adipic acid ( %)

2.7 1.9

89.1 86.0 83.5 81.8 78.0 64.2

0.7 0.7

86.0 82.4

1.2 81.6 77.7 73.6

Glutaric acid ( %)

10.8 7.3 8.5

12.2 15.0 17.0 19.8 31.9

4.2 3.7

12.2 15.2

11.6 15.4 19.1 22.2

Succinic acid ( %)

58.6 63.0 2.4 1.8 1.5 1.2 2.2 3.7

64.7 62.5 2.0 2.0

59.4 3.0 3.1 4.2

Preparation of adipic acid by oxidation of cyclohexanol, efc. 82 (1963) RECUEIL 63

adipic acid cannot have formed via nitrolic acid. It also appears that Cu has no effect on the oxidation of this intermediate. At lower vanadate concentrations the amount of glutaric acid increases.

In some cases no complete conversion can be achieved (table 11) without a catalyst owing to the reaction time being too short then; in other cases, however, the temperature rise during the reaction suggests that the conver- sion is indeed complete but that lower decomposition products are formed (e.g. oxalic acid). In such cases the sum of the products found approaches 5.9 mmol (= 72%).

4. Influence of nitrous acid on the distribution of the reaction products If, after an oxidation executed without a catalyst, vanadate is added, the

degree of conversion rapidly goes up to 100%; the only product formed during this stage is adipic acid. We have also seen that under the influence of vanadate Dione is rapidly converted to adipic acid. We may take it therefore that the portion of the products which after a low-temperature oxidation without vanadate is lacking in the analysis (100-% CS), must have consisted primarily of Dione and also that this Dione forms simul- taneously and along with the nitrolic acid.

The influence of nitrous acid is such that the ratio in which nitrolic acid and Dione are formed (Dione/N.A. ratio) increases with the HNOz-con- centration. To make a further study of this influence some oxidation tests were carried out on cyclohexanone and cyclohexanol with 40 % nitric acid ;

. I , ,

0 5 0.70.8 1.0 l.5 '2.0 a0 4.0 5.0 7 0 1' + D i o n e l N A r o t l o

Fig. 6. Influence of HNOz and temperature on the Dione/N.A. ratio during oxidation of cyclohexanol and cyclohexanone in 40% nitric acid.

64 W. J. van Asselt and D. W. van Krevelen

Preparation of adipic acid by oxiddon of cyclohexanol, etc. 82 (1963) RECUEIL 65

in these tests the nitrous acid concentration was varied by dissolving more or less HNOz in the nitric acid prior to the experiment. The HNOz-con- centration i n a sample of the liquid was measured before, during, and after the test. In all cases the final concentration of the HNOz proved to be appr. 10 % below the theoretical value : initial concentration - nitrolic acid concentration. The results are represented in fig. 6.

The nitrous acid concentration in each test was taken equal to the average value between the initial and final concentrations.

The points found at 10, 15 and 20" lie on straight lines with a gradient = 1.5. Through the points found at 5" and 25" lines have been drawn running parallel to the former.

Since the relation between log (Dione/N.A.) - 1/T proves to be also linear, it is possible to extrapolate towards higher temperatures. Further- more it appears from fig. 7 that in oxidations without a catalyst a given P H N O ~ is needed for reaching a given yield (or a given Di0nejN.A. ratio).

As the relation between log (PHNO~) and 1/T is also linear, a plot of log (PHNOJ versus log (Dione/N.A.) may also be expected to yield straight lines, with the nitrous acid concentration as parameter.

This has been elaborated further in fig. 8, so that the Dione/N.A. ratio can be read from the graph for any reaction condition. The Dione/N.A. ratio can also be calculated by means of the

empirical formula derived from fig. 8 viz. Dione/N.A. = 0.69

where: (HN02) = conc. of nitrous acid in gmol/l P H N O ~ = partial pressure of nitric acid in mm Hg.

Both in the chromatographic determination and in the measurement of the HNOz-concentration fairly large errors can be made. It has appeared, however, that the calculated Dione/N.A. ratio (fig. S), averaged over a large number of experiments is only appr. 10% below the value found chro- matographically; the difference can be partly accounted for by the fact that the actual HNO3-concentration was lower than the value taken for the calculation.

We may assume therefore that the Dione/N.A. ratio calculated from fig. 8 is a fair approximation of the actual value.

D. Conclusions concerning the reaction mechanism By means of the test results we shall first determine the substances formed

in the oxidation and then examine the way in which they decompose further. In this way we expect to arrive at a diagram of the reaction mechanism that can be used in interpreting the kinetic measurement 3 *.

* An article on this subject is in preparation. W. J. van Asselt, Thesis, Delft (1960).

66 W. J. van Asselt and D. W. van Krevelen

CYCLOHEXANONE CYCLOHEXANOL

0-

High temp. /' HN03 I koX I N O , N 0 2 - + ---Via intermediates

I

i.a.

NOH, 1

1

*NOH OH

H *

k ~ . ~ . '

1 k H

ADlPlC A C I D

Hcmihydratc o f

1.2- cyclohexanedione

(Dione 1

\ \ = O

/ =0

Vanadate I

I / ' . / '

' i

I GLUTARIC ACID

/

SUCClNlC ACID

LOWER DE COM PO SI T ION PRODUCTS

3lYS

Fig. 9. Kinetic model

Preparation of adipic acid by oxidation of cyclohexanol, erc. 82 (1963) RECUEIL 67

1. Cyclohexanol is oxidized with nitric acid to cyclohexanone + nitrous acid, probably by a chain reaction in which HNO2 plays a part. The reaction is very fast in most cases.

2. Starting from cyclohexanol and H N 0 3 , we succeeded in stopping the reaction by removal of the nitrous acid, and in isolating cyclohexanone in the reaction mixture, so that the conversion of cyclohexanone must be considered as the rate-determining step. The oxidation products formed are nitrolic acid and Dione. Also in this reaction HNO2 plays an important part: cyclohexanone is not attacked by pure nitric acid.

The reaction undoubtedly proceeds via some intermediate products which, however, are so unstable that they could not be demonstrated.

3. Under the influence of nitric acid the decomposition of nitrolic acid into adipic acid proceeds at a rate lower than the rate of oxidation. The reaction also yields a product X of unknown composition which, finally, is converted again to adipic acid.

4. Under the influence of vanadate the Dione is rapidly converted to adipic acid whereas glutaric acid is produced as a by-product. If no catalyst is present, mainly succinic acid and lower decomposition products are obtained.

5. Cyclohexanone, or the intermediates forming during the transition of cyclohexanone into nitrolic acid and Dione, may, via side reactions, yield products which are converted further to glutaric acid and succinic acid. This suggests introduction of nitro or nitroso groups in the 3 or 4 position or in the 2 and 6 position.

These reactions, which take place notably at high temperatures in the presence of large amounts of nitrous gases, are suppressed by addition of copper.

A diagram of the reaction mechanism is illustrated in fig. 9.

The results of the extensive study by Lubyantshii et al. 4 9 59 6 on the oxidation of cyclohexanol and cyclohexanone with nitric acid under pres- sure, are difficult to compare with the results presented in this paper because our experiments were conducted under much more moderate conditions. Moreover L. did not add extra HNO2 during the oxidation of cyclohexa- none, so that the yields of oxidation products differ appreciably. According to L. the catalyst (a mixture of Cu++ and vanadate) should accelerate the formation of nitrolic acid. However this can account only for the influence of copper at high temperature but certainly not for the influence of vanadate.

(Received July 10th. 1962).

Y. A. Lubyantshii, R. V. Minati and M. S. Furnian, Khim. Prom. 453 (1960). Y. A. Lubyantshii, R. V. Minati and M. S. Furman, Khim. Prom. 529 (1960). Y. A. Lubyantshii. C . I . Kostylev, M. S. Furman, Khim. Prom. 533 (1960).