Inhibition of Autoxidation

23 Inhibition of Autoxidation K. U . I N G O L D

Downloaded by COLUMBIA UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0075.ch023

Division of Applied Chemistry, National Research Council, Ottawa, Canada

Methods for inhibiting the autoxidation of organic substrates are reviewed. Antioxidants can be divided into two broad groups according to whether they reduce the rate of chain initiation (preventive antioxidants) or interfere with the normal propagation process by reacting with free radicals (chain-breaking antioxidants). Preventive antioxidants can be subdivided into peroxide decomposers, metal ion deac tivators, and ultraviolet light deactivators. Chain-breaking antioxidants can be subdivided according to whether they add, donate hydrogen, or donate an electron to the chain -propagating radicals. The detailed mechanisms by which these various classes of antioxidants inhibit oxidation are discussed.

'ηρίιβ autoxidation of most organic substances is a free radical chain process. The over-all oxidation rate depends on the rate of chain initiation, chain propagation, and chain termination. The rate can gen erally be reduced, and/or the length of the induction period can be increased by adding relatively low concentrations of compounds which contain certain specific functional groups. These compounds have been indiscriminately referred to as antioxidants (primary and secondary), inhibitors, retarders, deactivators, stabilizers, etc. The term "antioxidant" is probably most widely used and I employ it as a general term for all compounds which inhibit oxidation—i.e., for all compounds which reduce the rate of attack of oxygen on a substrate. In theory, at least, antioxidants may be divided into two broad groups according to whether they reduce the rate of chain initiation ( the so-called preventive antioxidants) or interfere with the normal propaga tion process by reacting with free radicals (the chain-breaking antioxi dants). Since the two groups of antioxidants interfere at different points in the oxidation process, they have a mutually reinforcing effect on one another. When they are used together, the over-all inhibiting effect is 296 In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

23.

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Inhibition of Autoxidation

297


generally greater than the sum of the individual effects from each antioxidant. This phenomenon is referred to as synergism. In practice, of course, many antioxidants have both preventive and chain-breaking antioxidant properties. Such compounds exhibit autosynergism, and they are generally very effective antioxidants. Autosynergism is usually achieved by incorporating into a single compound two or more functional groups, one of which is known to act predominantly by a preventive mechanism and the other by a chain-breaking mechanism. Considerable interest in antioxidants is currently centered about empirically discovered compounds such as the dialkyl dithiophosphates which were originally believed to function by a single mechanism but which, in fact, function by two mechanisms. Under many circumstances these compounds must owe much of their efficiency to autosynergism. Synergistic behavior by two antioxidants is not confined to compounds which inhibit by entirely different mechanisms—for example, two chain-breaking phenolic antioxidants may synergize one another. This homosynergism is caused by the suppression of the unfavorable chain propagation reactions of one phenoxy radical by a hydrogen atom transfer from the second phenol. It is considerably more difficult to inhibit oxidation in the gas phase than i n the liquid phase. A t the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are i n creased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful i n the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. Preventive Antioxidants Preventive antioxidants reduce the rate of chain initiation, and they can be subdivided into three main categories. Peroxide Decomposers. These convert hydroperoxides ( R O O H ) to nonradical products. R O O H + D —» molecular products (generally R O H + D O or R'CO + H 0 ) 2

This reduces the importance of the various hydroperoxide decomposition reactions which give free radicals. A peroxide decomposer may convert the hydroperoxide to a product which is itself an antioxidant—

In Oxidation of Organic Compounds; Mayo, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

298

OXIDATION

OF ORGANIC

COMPOUNDS

1

e.g., Lewis acids convert cumene hydroperoxide to phenol and acetone. H C H (CH ) COOH -» C H OH + (CH ) CO +

6

5

3

2

6

5

3

2

Metal Ion Deactivators. These generally chelate ions of copper and the transition metals which might otherwise catalyze the decomposition of hydroperoxides to free radicals, ROOH + M


ROOH + M

-> RO + O H " + M

n +

( n + 1 ) +

-> R 0

2

( n + 1 ) +

+ H + M +

n +

Ultraviolet Light Deactivators. These absorb short-wavelength radiation which might otherwise generate free radicals—e.g., Hy R CO -» R CO 2

2

hy R O O H - » RO' + O H The mechanisms of inhibition by peroxide decomposers, metal deactivators, and ultraviolet absorbers are fairly well understood in general terms, although many details of the individual reactions remain to be elucidated. Classifying a preventive antioxidant into one of the three categories above w i l l only rarely describe its entire function. The dual behavior of dialkyl dithiophosphates in the liquid phase has been mentioned. Many other phosphorus- and sulfur-containing antioxidants commonly classified as peroxide decomposers can also act as chain breakers. Similarly, the structure of many metal deactivators and ultraviolet absorbers indicates that they must also have some chain-breaking activity. Chain-Breaking Antioxidants Chain-breaking antioxidants which interfere with the normal propagation processes may react with peroxy radicals, R 0 ' , or, more rarely, with the carbon radical, R*. The antioxidant may react with the propagating radical by addition, by hydrogen transfer, or by electron transfer. The chain can be terminated directly, but more commonly a new radical is formed, which either continues the chain at a reduced rate or terminates a second chain. 2

Among chain-breaking antioxidants the stable dialkyl nitroxides such as di-teri-butyl nitroxide ( I ) and 2,2,6,6-tetramethyl-4-pyridone nitroxide (II) inhibit oxidation by the simplest mechanism and therefore exhibit


23.

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299

Inhibition of Autoxidation Ο


I

II

the simplest kinetics. These nitroxides react rapidly with R* radicals to give stable molecular products. R 'NO* + R- -> R ' NOR 2

2

They do not react with R 0 * radicals. The inhibited oxidation rate is proportional to the oxygen pressure and inversely proportional to the nitroxide concentration, but it is independent of the substrate concentra tion—i.e., rate oc [ 0 ] / [nitroxide]. The nitroxides react slowly with hydrocarbons and hydroperoxides and therefore have little tendency to initiate oxidation. However, they are of disappointingly little practical value compared with more conventional chain-breaking antioxidants because they must compete with molecular oxygen for the R' radicals, and the 2

2

R + 0

2

- » R0 " 2

reaction is extremely rapid. Stable diaryl nitroxides such as 4,4'-dimethoxydiphenyl nitroxide (III) are somewhat better antioxidants than the dialkyl nitroxides. The III inhibited rate depends on both the oxygen pressure and the substrate

(III)

ο concentration, which implies that III reacts with both R* and R 0 * radi cals. The reaction with R 0 ' is much slower than the reaction with R'. Except at rather low oxygen pressures the nitroxide III is a less efficient inhibitor than the corresponding diphenylamine ( I V ) and hydroxylamine ( V ) . 2

2


300

OXIDATION

CH 0

OCH

3

3

OF

ORGANIC

COMPOUNDS

CH 0

1

OCH,

^3

3

Ο

H

Η

IV

V


Carbon black and many polynuclear hydrocarbons are effective i n hibitors of oxidation. Their antioxidant properties are believed to arise from their ability to trap free radicals. The most useful and most thoroughly studied chain-breaking anti oxidants are phenols and aromatic amines. These compounds can gen erally transfer a hydrogen atom to a peroxy radical. R0

+ A H —> R O O H + A '

2

The rate of transfer is accelerated by electron-releasing substituents on the aromatic ring of the antioxidant and retarded by steric protection of the labile hydrogen or its replacement by deuterium. The subsequent fate of the radical A ' determines the over-all kinetics of the inhibited reaction and the practical usefulness of the antioxidant. If A* is a fairly stable phenoxy radical, it w i l l probably add a peroxy radical or dimerize. R0 * + A ' —» R O O A 2

A* + A ' - > A

2

In this case the kinetics are simple. The inhibited rate is proportional to the substrate concentration and inversely proportional to the antioxidant concentration—i.e., rate oc [ R H ] / [ A H ] . More generally, phenoxy radi cals which are not sterically hindered w i l l enter into chain transfer reactions, and the kinetics may become extremely complicated. Chain transfer with the substrate is usually slow, but it can be rapid with hydroperoxides. slow A ' + R H - > A H + R*

fast A

4- R O O H —» A H 4-

R0 ' 2

Aromatic primary and secondary amines behave generally like the phenols. However, the over-all process is more complex because an addi tional series of termination and transfer reactions is introduced by the rapid formation of nitroxides in the following reaction. R0 * + A · - > RO* 4- A O ' 2


23.

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e.g., R 0

2

301


+


Ο The amine antioxidants therefore give rise to both nitrogen and nitroxide radicals. The reactions of these two species are still not well known. Several other types of antioxidants can inhibit oxidation by donating a hydrogen atom to a peroxy radical. H o w a r d (15) has described the interesting case of inhibition by a second hydrocarbon or its hydroper oxide. In the particular case he discussed, the inhibition of the oxidation of cumene by Tetralin ( T H ) or Tetralin hydroperoxide ( T O O H ) occurs because the tetralylperoxy radical ( T 0 * ) reacts with a cumylperoxy radical ( C O 2 ) much more rapidly than two cumylperoxy radicals react with one another. The inhibition sequence can be represented by the following reactions. 2

o

2

C 0 * + T H -> C O O H + T 0 * 2

2

C 0 * + T O O H -> C O O H + T 0 * 2

2

C 0 * + Τ 0 · - » C O H + α-tetralone + 0 2

2

2

Triarylmethanes also inhibit oxidation by hydrogen transfer to a peroxy radical. In this case it is the triarylmethyl radical which traps the second peroxy radical. R0 * +

PI13CH

2

-» ROOH + P h C 3

R 0 ' + P h C - » Ph COOR 2

3

3

The inhibition efficiency of the triarylmethanes decreases as the oxygen partial pressure is increased because of a decrease i n the steady-state concentration of the triarylmethyl radicals. Ph C' + 0 3

2

Ph COO' 3

A n inhibition mechanism involving electron transfer between a chain-propagating radical and the antioxidant has frequently been sug gested but has rarely been identified with any certainty. This process remains one of the least understood of all inhibition mechanisms. Prob ably the most clear-cut example of inhibition by one electron transfer (either partial or complete) has come from studies of metal-catalyzed oxidations. Many workers have reported that under certain conditions transition metals may inhibit rather than catalyze oxidations. Cobalt, manganese, and copper are particularly prominent i n this respect. R 0 ' + Co 2

2+

-> R 0 " C o 2

8+

or R 0 5 - C o 2

( 2 +

5

) +


302

OXIDATION

R(V + M n

2 +

-> R 0 ' M n 2

OF ORGANIC

COMPOUNDS

1

or R O ^ M n ^

3 +

The inhibition of hydrocarbon oxidation by aromatic tertiary amines which contain no labile hydrogen, such as 2V,iV-dimethylaniline and Nj^N^N'-tetramethyl-p-phenylenediamine, has been assigned to an electron-transfer process. However, this seems rather unlikely as pyridine Table I.

Rate Constants for Some Reactions of Antioxidant (A and AH)

Reaction

a

Me -4-piperidone nitroxide (II) (MeO) -diphenyl nitroxide (III)

R' + A" -> RA

4


2

R 0 - + A - » ROOA

Phenoxy (MeO) -diphenyl nitroxide (III)

A' + A -> A

4-MeO-phenoxy Phenoxy Anilino (?)

2

2

2

R0

2

+ A -> R 0 A

R0

2

4- A H - » R O O H 4- A

2

Manganous stéarate Zinc diisopropyl dithiophosphate

+

Phenol 4-MeO-phenol 2,6-ter£-Bu -4-methylphenol 2,4,6-terf-Bu -phenol (VIH) N-Methylaniline Diphenylamine (MeO) -diphenylamine (IV) ( MeO ) -diphenylhydroxylamine ( V ) Zinc diisopropyl dithiophosphate 2

3

2

2

R0

2

4- R O O H -> R O O H 4- R O

A 4- R O O H -> A H 4- R 0 * 2

2

( Hydroperoxide ) Phenoxy 4-MeO-phenoxy ter£-Bu -phenoxy (VI) 3

A' 4- ΑΉ -> A H + A "

ter£-Bu -phenoxy 4- 4-MeO-phenol ter£-Bu -phenoxy + phenol ter£-Bu -phenoxy 4- 3-chloroaniline (MeO) -diphenyl nitroxide 4- 2,6-tertBu -phenol 3

3 3

2

2

4-MeO-phenoxy + 9,10-dihydroA ' + R H -> A H 4- R" anthracene Phenoxy + Tetralin ter£-Bu -phenoxy 4- ethylbenzene ( MeO ) -diphenylnitroxide 4* ethylbenzene "Addition products shown for radical-radical reactions, but disproportionation products may also be formed. 3

2


23.

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303


and triphenylamine which also contain no labile hydrogen do not inhibit oxidations under similar conditions. The radical addition and hydrogen transfer mechanisms of inhibition by chain-breaking antioxidants are now reasonably well understood in both qualitative and quantitative terms. The electron-transfer mechanism of inhibition deserves greater attention. Chain-Breaking Antioxidants in the Liquid Phase log k, (liters/mole/sec.)


10

Temp., °C.

Ref.

7-8 7-8

60, 65 65

(3,16) (3)

4.7

57 65

(23) (3)

^7.2 —9 --9

60 25 25

(ΐοη (7,17) (17)

5.3 1.6

60 60

(8,9) (5)

3.7 5.0 4.2 4.2 3.6 4.6 5.5 5.7 2.6

65 65 40 65 65 65 65 65 60

(2,11) (2 U) (1,10,13,14) (2,12) (3) (2,3) (3) (3) (5)

30-56

(15)

57 60 21

(23) (19, 20 ) (18,19, 20)

3.8 0.8 -1.2 -3.5

24 24 24 60

(6,19, 20) (6,19, 20) (18) (4)

A H + R' is the structure of R likely to have a very large effect on the rate constants. The most useful chain-breaking antioxidants react with peroxy radi cals. Their rate constants are i n the range 10 to 10 liters per mole per second for this reaction. The radicals formed from the antioxidants (if any) must be reactive towards other free radicals but unreactive i n chain transfer. 4

Literature (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

6

Cited

Berger, H., et al.,ADVAN.CHEM.SER.75, 346 (1968). Brownlie, I. T., Ingold, K. U . , Can. J. Chem. 44, 861 (1966). Ibid., 45, 2419, 2427 (1967). Buchachenko, A. L . , Sykhanova, O. P., Kaloshnikova, L . Α., Neiman, M . B., Kinetika i Kataliz 6, 601 (1965). Burn, A. J.,ADVAN.CHEM.SER.75, 323 (1968). DaRooge, M . Α., Mahoney, L . R., J. Org. Chem. 37, 1 (1967). Dobson, G., Grossweiner, L . I., Trans. Faraday Soc. 61, 708 (1965). Gol'dberg, V . M., Obukhova, L . K., Dokl. Akad. Nauk SSSR 165, 860 (1965). Gordberg, V. M . , Obukhova, L . K., Izv. Akad. Nauk SSSR, Ser. Khim. 1966 2217 Howard, J. Α., Ingold, K.U.,Can. J. Chem. 40, 1851 (1962). Ibid., 41, 1744 (1963). Ibid., 41,2800 (1963). Ibid., 42,2324 (1964). Ibid., 43, 2724 (1965).


23.

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305

(15) Howard, J. Α., Schwalm, W . J., Ingold, K. U . , ADVAN. CHEM. SER. 75, 6 (1968). (16) Khloplyankina, M . S., Buchachenko, A . L . , Neiman, M . B., Vasil'eva, A. G., Kinetika i Kataliz 6, 394 (1965). (17) Land, E. J., Porter, G., Trans. Faraday Soc. 59, 2016, 2027 (1963). (18) McGowan, J. C., Powell, T., J. Chem. Soc. 1960, 238. (19) Mahoney, L. R., J. Am. Chem. Soc. 89, 1895 (1967). (20) Mahoney, L. R., DaRooge, Μ. Α., J. Am. Chem. Soc. 89, 5619 (1967). (21) Mamedova, U . G., Buchachenko, A . L . , Neiman, M . B. Izv. Akad. Nauk SSSR, Ser. Khim. 1965, 911. (22) Neiman, M. B., Mamedova, U. G., Blenke, P., Buchachenko, A. L . , Dokl. Akad. Nauk SSSR 144, 392 (1962). (23) Thomas, J. R. J. Am. Chem. Soc. 86, 4807 (1964). October 9, 1967. Issued as N R C No. 9594.


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Inhibition of Autoxidation

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