138 SALWAP : STUDIES OK THE OXIDATION OF UNSATURATED

XI.-Studies o n the Oxidation of Unsaturated P a t t y Oils and Unsaturated Fat ty Acids. Pcc9-t 1. The Formation of Am-olein by the Oxidation. oj‘ Linseed Oil and 1, inolenic Acid .

Ry ARTHUR HENRY SALWAY.

IT has long been known that fatty oils possessing a high degree of unsaturation undergo a profound change when exposed t o the air, being gradually converted into viscid liquids o r varnish-like solids. This phenomenon, commonly termed “ drying,” has been frequently studied with the object of determining the nature of the chemical reactions which accompany the change, but the results obtlained are insufficient to enable a satisfactory formulation of the process to be made. Hence the mechanism of the change is still a, matter for further investigation.

It was early recognised that the ‘( drying ” of oils is associated with the absorption of oxygen from the atmosphere, and quanti- tative experiments (Lippert, Zeitsch. angew. Chem., 1898, 11, 412 ; Weger, Chem. Rev. Fett-Hare-Ind., 1899, 4, 301; and others), in which thin films of linseed oil were exposed to air, show that the oil is able to absorb about 20 per cent. of its weight of oxygen. Kissling (Zeitsch. angew. Chem., 1891, 4, 395) has shown that sterilised linseed oil exposed t o sterilised air also absorbs oxygen, thus proving that the change is not induced by the presence of micro-organisms. Exposing the oil to bright sunlight increases the rapidity of tlhe oxygen absorption, and i t has been shown by Genthe (Zeitsch. angew. Chem., 1906, 19, 2087) that linseed oil exposed to ultra-violet light is capable of absorbing as much as 34 per cent. of its weight of oxygen from the atmosphere.

With regard to the chemical changes which are involved in the oxidation of linseed oil, i t has been observed frequently that volatile products are formed and liberated during the process

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FATTY UILS AND UNSATURATED FATTY ACIDS. PART I. 139

(Kissling, loc. cit.). Noerdlinger (D.R.-P. 167137 of 1906) has shown that the volatile products contain fatty aldehydes which begin to boil a t 150°, and fatty acids beginning t o distil a t 190°, whilst small quantities of volatile alcohols and esters are said to be present. Recent investigations by Baly ( J . SOC. Chem. Ind., 1912, 31, 515), Gardner ( J . Ind. Eszg. Chern., 1914, 6, 91), and King ( J . Iiid. B n g . Cheni., 1915, 7, 502), who have each examined the volatile products from oxidised linseed oil with a view to ascertain why the inhalation of vapours from linseed oil paints f requently induce sympt,oms of poisoning, have yielded some interesting results. Gardrier and King show that carbon monoxide and carbon dioxide are evolved during the drying of linseed oil, whilst Bzly states that an unsaturated aldehyde of a poisonous character is formed. The volatile products emanating from oxidised linseed oil are, however, comparatively small in amount, and by far the greater portion of the oil is converted into a non- voslatile, varnish-like solid. Efforts have, been made t o ascertain the character of this substance, but without much success. It is evidently a mixture, and no compound of undoubted homogeneity has been isolated from it. Orloff ( J . Russ. Phys. Chem. SOC., 1910, 42, 658) has separated i t by water into a soluble and insoluble1 portion, which he suggests are oxidation products of linolic and linolenic glycerides respectively, possessing the follow- ing constitutions :

CH,Me*CH.(~H*CH2.FH*~H*[CH,1,,.C(),R \/ 0---0 0

CH2Me*CH*CH*CH,*yH*f! H-CH,*CEI*CH*[CH2~7.C0,R 0--0 \/

V \/

0

There is, however, no evidence that these substances are homo- geneous.

Various conjectures have been made regarding the mechanism of the ‘‘ drying ” process. Fahrion (Zeitsch. angew. Chem., 1910, 23, 723) believes that the unsaturated compounds of linseed oil first form peroxides, which then undergo intramolecular changes with the formation of one or more ketoxy-groups according t o the number of unsaturated linkings involved in the oxidation, thus :

R*CH:CH*R’ + R*YH*YH*R’ + R*CE€(OH)*CO*R’ 0---0

The ketoxy-compounds are then supposed by Fahrion to undergo condensation with elimination of water, thus forming the substance known as linoxin. Fokin ( J . Rztss. Phys. Chem. SOC., 1908, 40, 276)

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140 SALWAY : STUDIES ON THE OXIDATION OF UNSATURATED

considers, on the other hand, that the primary product in the oxidation is an oxide represented by the formula R0CR.CH.R’.

\/’ 0

Kitt (Chem. Rev. Ir’ett-Ham-lnd., 1901, 8, 40) has endeavoured to ascertain, by acetylation of the oxidised oil, whether hydroxy- compounds are formed, but the results were inconclusive.

From the foregoing brief survey of the literature it will be seen that comparatively little is known regarding the chemical changes which linseed oil undergoes on oxidation. It is, moreover, evident that a satisfactory explanation of the process cannot be reasonably expected until the oxidation of each of the constituents of that oil is fully understood. For that reason it would be better to attempt the solution of the problem by invest.igating the action of air o r oxygen on the glycerides of linolenic, linoleic, and oleic acids suc- cessively, but as these substances cannot readily be obtained in a pure state recourse must be1 had to the corresponding free fatty acids, which are more readily obtained pure (compare Erdmann, Zeitsch. physiol. Chem., 1911, 74, 179). This method of investiga- tion of the “drying” process seems quite permissible, since it is known that the glycerol of fatty oils plays no active part in the oxidation. Accordingly, the author has undertaken an investiga- tion of the products formed when unsaturated acids are exposed to air or oxygen. The present communication, however, deals entirely with the new observation that acrolein is one of the volatile products of oxidation of linolenic acid and of oils containing glycerides of this acid. It has previously been noticeid by Dunlop and Shenk ( J . Amer. Chem. SOC., 1903, 25, 826) that a sample of linseed oil gave, on drying, an odour of acrolein, and the forma- tion of this substance was attributed t o the action of oxygen on the glycerol in the oil. This view of its formation is, however, erroneous, f o r in the light of thel results recorded below there is no doubt that the acrolein is formed directly from the fatty acids. Its is here shown that when linolenic acid, or the fatty acids from linseed oil, are caused to absorb oxygen, acrolein is one of the volatile substances evolved, but no trace of this substance is produced when oleic acid is oxidised. This observation is of con- siderable interest in view of the light it may throw on the constitu- tion of linolenic acid. Erdmann and his pupils (Ber., 1909, 42, 1324; Zoc. c i t . ) have concluded from their investigations that there are two stereoisomeric linolenic acids (compare, however, Rollett, Zeitsch. Physiol. Chem., 1909, 62, 422), each of which is repre- sented by the constitutional formula :

CH,Me*CH: CH*CH,*CH: CH*CH,*CH: CH*[CH,],*CO,H.

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FATTY OILS AND UNSATUR\TED FATTY ACIDS. PART I. 141

It is difficult, hawever, to explain satisfactorily the formation of aerolein from such a compound. On the other hand, supposing linolenic acid contained the grouping R*CH:CH*CH:CH*CH:CHR’ 110 such difficulty would be encountered. Thus, in the first place, the linolenic acid (I) absorbs oxygen with the formation of an oxygenide (compare Salway a.nd Kipping, T., 1909, 95, 166), and since the central point of unsaturation should be relatively stable there would be a tendency to form the dioxygenide (11). This would then decompose, either spontaneously or in the presence of moisture, with the formation of aldehydes, including fumaraldehyde as shown below. This unknown aldehyde is unstable, and immedi- ately suffers direct disruption with the formation of acrolein and carbon monoxide, or, 011 the other hand, is partly oxidised, with subsequent. elimination of carbon dioxide and acroledn :

R*CH R.CIZ-() R-CHO I I CII,:CH*CHO and CO

CH--U CHO I ‘ C [I

I I /

CO,H I

I1

I

CH ~

(:€I c 110

c IT,: c H c HO and CO,

This explanation of the1 change not only accounts f o r the forma- tion of acrolein, but also furnishes a reason for the production of carbor, monoxide and carbon dioxide, as observed by Gardner and King (Zoc. cit.).

As already indicated, this view of the constitution of linolenic acid is not in agreement with the conclusions of Erdmann. The discrepancy possibly arises from the presence in linseed oil of two structurally isomeric linolenic acids, the one of which is the linolenic acid of Erdmann, whilst the1 other contains the unsaturated grouping suggested above. There is some support for this supposition, since i t has been observed that different specimens of linolenic acid gave, under comparable conditions, varying quanti- ties of acrolein on oxidation. It is, however, hoped that further investigation will reveal the cause of these differences.

It will be observed that linolenic acid, according to the view here given, is a derivative of hexatriene. It is known that hexatriene readily absorbs oxygen from the atmosphere, but as far as the author is aware the products of oxidation have not been closely inveshigated. For that reason i t would be desirable to examine

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142 SALWAP : STUDIES ON THE OXIDATION OF UNSATURATED

hex2 triene, and compounds of this type, in order t o ascertain whether ccrolein is forlned from them by oxidation.

The formation of acrolein by the1 oxidation of highly unsaturated oils suggests a theory which may be advanced, with due reserve, for the explanation of the " drying " of oils. In the first place, the glyceride of linolenic acid (I) present in the oil absorbs oxygen, and the oxygenide (11) thus obtained is wholly or partly decomposed with the formation of aldehydes, including acrolein. The greater portion of the aldehydes theln polymerise, and form, with the unchanged oil, the well known elastic varnish-linoxyn. From this point of view the '' drying " of linolenic glyceride would be repre- sent'ed by the following scheme, and the dry oil would then be expected t o contain polymerised acrolein and glyoxal :

CH,Me CH,-Rle C H,Me

AH CH-0 bHO

I I I CUH2I2 P 3 2 1 2 I p * 1 2

I 1

d f € UH UH,:CH*CHO),, CHO

1 1 CH-0 CH I

& 4 8! + AH CH-0 I ClHO \* (CHO*CHO),,

CHO I 1

CH-0 I I CH

(1- 1 (11.) TiVhilst there is considerable evidence in favour of this theory of

the " drying " of oils, the8 author recognises that further research is necessary before its validity can be considwed established.

E ~ P E R I M E N T A L .

Specimens of linseed oil which have been exposed to the atrno- sphere for some time always possess a strong odour, and occasionally the characteristic sharp acrolein sinell can be detected. The amount of acrolein formed under theee conditions is not, however, sufficient to enable i t to be definitely identified. On the other hand, by con- ducting the oxidation a t looo in an atmosphere of oxygen, as described below, sufficient acrolein was obtained to prove its identity beyond doubt.

It For convenience, the glycerol residue is represented by R. It should also be understood that the author does not wish to commit himself with regard to the position of the hexatriene group in the chain of 18 carbon atoms.

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FATTY OlLS A N D UNS9'1'URA'I'ED FATTY ACIDS. PART I. 143

The Formation of Acro1pi.n by the Oxidation of Linseed Oil.

The vessel in which the Oxidation was effected consisted of a large pipette bent iii the shape of a U-tube, the bulbed limb of which was connected with a wash-bottle containing water, whilst the other limb was in direct communication with a manometer and an oxygen cylinder. For each experiment the pipette was about half filled with the oil and oxygen Lien introduced under two atmospheres' pressure, the exit t8ube leading t o the wash-bottle being closed. The pipette was then immersed in water a t looo and vigorously shaken by a mechanical agitator with vertical motion, by means of which intimate contact of ths oil and oxygen was effectad. When the pressure of gas in the pipette had been reduced t o one atmo- sphere by absorption of oxygen, the exit tube was opened and oxygen passed through the pipette. I n this way the gaseous products of oxidation were made to bubble through the water contained in the wash-bottle. The pipette was then closed again and the process of oxidation and collection of volatile products repeated until the absorption of oxygen had almost ceased.

I n a typical experiment with linseed oil the absorption of oxygen a t the commencement of operations was very slow, but after a period of about twenty minutes the oxidation was rapid and continued so until the1 oil had absorbed 5 litres of oxygen per 100 grams of linseed oil. The wash water, through which the gaseous products of oxidation had been passed, had the strong and characteristic odour of acrolein; it also readily decolorised bromine water and reduced ammoniacal silver nitrate solution; it was free from acidic sub- stances, 1 drop of N/lO-alkali being sufficient t o render the liquid alkaline. I n order t o identify the acrolein definitely, a quantity of the aqueous solution, obtained as described above, was shaken with freshly precipitated silver oxide until the odour of acrolein had entirely disappeared. The mixture was then filtered and t h e filtrate, concentrated to a small bulk under diminished pressure. The concentrated solution was again filtered, to remove a small amount of silver which liatl separated, and then set aside to cool, when a crystIalline silver salt was deposited. A further crop of crystals was obtained by the addition of alcohol t o the mother liquors. The two fractions of silver salt were analysed separately with the following results :

(a) 0.0130 silver salt gave 0.0079 Ag. Ag=60*8. (b) 0.0110 ,, ,, ,, 0.0066 Ag. Ag= 60.0.

C,H,O,Ag requires Ag = 60.3 per cent. C"3H502Ag 9 , Ag=59*7 9 , 3,

It is evident from these figures that the substance is either silver

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144 STUDIES ON THE OXIDATlON OF UNSATURATED FATTY OILS.

acrylate or silvelr propionate. From other evidence, namely, the odour and the unsaturatioii of the original aldehyde, i t is clear that the latter is acroleiii, and not propaldehyde.

The Formation of Acrolein by the Oxidation of the Fatty Acids from Linseed Oil.

A quantity of linseed oil was saponified and the soap solution treated with dilute mineral acid. The liberated fatty acids were then well washed with water, dried with anhydrous sodium sulphate, and filtered. The product had an iodine value of 197.2 and was free from glycerides o r glycerol. The oxidation of this substance was conducted in the manner already described in connexion with the oxidation of linseed oil. The water used for washing the gases issuing from the oxidation had the penetrating odour of acrolein, and silver acrylate was formed by agitating the solution with silver oxide. The silver acrylate was separated into two fractions by crystallisation, and then analysed. (Found, Ag = 60.1, 59.6. C,H,O,Ag requires Ag = 60.3 per cent.)

The amount of silver acrylate obtained from the oxidation of the fatty acids of linseed oil was greater than the amount in a corre- sponding experiment with linseed oil. Thus 100 grams of the fatty acids yielded 0.510 gram of silver acrylate, whilst linseed oil under the sanie conditions yielded 0.370 gram. There is therefore no doubt that the acrolein obtained in these oxidations is derived from the fatty acids, and not from the glycerides.*

Formatioia of Acrolein b y the Oxidation of Limlenic Acid.

The linolenic acid employed for this oxidation had an iodine value of 269.3, and a molecular weight, as determined by titration, of 283. Fourteen grams of the acid were oxidised in the usual way a t looo, about 1 litre of oxygen being absorbed. The aqueous liquid, through which the gaseous products of oxidation were passed, had a strong odour of acrolein, and the latter was identified by conversion into silver acrylab. The amount of silver acrylate par 100 grams of linolenic acid was 0.34 gram.

Oxidntzon of Oleic ,4cid.-Thirty grams of pure oleic acid were treated with oxygen a t looo in the apparatus described above. The absorption of oxygen was relatively slow and hdd almost ceased when 300 C.C. had been taken up. There was no odour of acrolein in the water used for washing the volatile products, and no silver

* Pure glycerol did not yield acrolein when treated at 100" with oxygen in the manner here described.

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THE COLOURING MATTER OF COTTON FLOWERS. PART 111. 145

acrylate could be isolated from it. It is thus evident that acrolein is not formed by the oxidation of oleic acid.

It would be of considerable interest to ascertain whether pure linoleic acid yields acrolein on oxidation, but there, appears to be no trustworthy method of obtaining this acid in a state of purity. A specimen of Kahlbaum’s linoleic acid was found to yield acrolein, but the acid had an iodine value of 195, and evidently contained linolenic acid, so that the result was inconclusive. From theo- retical considerations one would not expect pure linoleic acid t o yield acrolein on oxidation. There appears to be little doubt that the acrolein obtained in all these experiments is derived from linolenic acid, especially as other fatty oils known t o contain this acid havs been found to yield considerable quantities of acrolein on oxidation.

The author desires, in conclusion, to express his indebtedness t o Lever Bros., Ltd., and Mr. J. L. Bnchanan, Managing Director of the Research Department, for permissioii t o publish the results of this investigation.

RESEARCH DEPARTMENT, LEVER BROTHERS, Lt,tl.,

PORT SUNLIGHT. [Received, Decembev 23rd, 1915 3

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