The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918
(Tekstiin lisätty kappaleita lukemisen helpottamiseksi. // Some paragraphs added to the original text for making reading easier.)
Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.
Madder is the ground root of the Rubia tinctorium (Linn.) which has been cultivated for dyeing purposes from a remote antiquity, so remote indeed that one is unable to say with certainty in which countries it originated. It is known to have been employed by the ancient Egyptians, Persians, and Indians, probably by the last in the first instance, and more recently by the ancient Greeks and Romans. About the time of the Crusades the cultivation of madder was introduced into Italy and probably also into France. The Moors cultivated it in Spain, and during the sixteenth century it was brought to Holland. Colbert introduced it into Avignon in 1666, Frantzeninto Alsace in 1729, but only towards 1760-1790 did it become important. During the wars of the Republic, its cultivation was largely abandoned, and only after 1815 did this again become regular.
Owing to the beauty and fastness of the tints it yields, and the range in colour that can be produced from it by a variation in the mordant, it was considered until recently as perhaps the most important of all dyestuffs. Although its commercial value has been greatly reduced through the introduction of artificial alizarin, it has still considerable scientific interest.
The plant is an herbaceous perennial belonging to the natural family of the Rubiaceæ, and its valuable portion is entirely the root, which is usually of considerable length but does not exceed an ordinary slate pencil in thickness. Old roots are richer in colour than young ones, and the plant is consequently left in the soil for at least eighteen and sometimes for twenty-eight months. When removed it is usually washed with water, allowed to dry in the sun or artificially by means of kilns, then finely ground and packed in casks. In certain districts it was stored in pits for several months before grinding, whereby its tinctorial power was said to be greatly enhanced; but these and other refinements of its preparation are now of so little importance as to be hardly worthy of mention. The root in many countries bears the name "alizari" or "lizari," whence we have the name "alizarin". Madder was principally cultivated in Holland, France, and Turkey, and to a less extent in Belgium, Italy, and Germany, and North and South America, but the small quantity which enters this country is principally obtained from Holland. Perhaps no substance was submitted to so much examination by the older chemists as madder, and in many of the earlier works on dyeing much space is occupied by a description of these researches.
The isolation of the most important colouring matters of madder, alizarin, and purpurin, occurred as far back as 1826 and 1828, and was due to the chemists Robiquet and Colin; but it is doubtful whether they were successful in obtaining these substances in a state of chemical purity. By many of the earlier workers it was considered that these colouring matters did not exist as such in madder, but were present in combination with sugar or some other substance. About 1823 Kuhlmann extracted a bitter-sweet yellow amorphous compound from the root and named it xanthin, and a similar yellow substance was also isolated by Runge and Watt. In 1848 Higgin observed that if a cold aqueous solution of madder, which has a deep yellow colour and an intensely bitter taste, was allowed to stand for some time or was heated to 50°, it lost these characteristics, and a gelatinous flocculent precipitate was formed in which all the tinctorial power of the original infusion resided. Higgin considered, therefore, that the xanthin of madder must, during this process, have been converted into alizarin, and that the change was probably brought about by the action of some ferment contained in the madder, and extracted along with the xanthin by cold water. Somewhat latter (1851) Schunck isolated from madder a substance which he called rubian, as a dark, brownish-yellow transparent, amorphous hard mass, which by hydrolysis with acids or by the action of the special madder ferment, which he termed "erythrozym," was converted into glucose, alizarin, and other substances.
The next important step was due to Rochleder, who prepared the alizarin glucoside in a crystalline condition and named it ruberythric add. It appeared to possess the formula C20H22O11, and its hydrolysis could be represented according to the following equation: C20H22O11 = H2O + C14H8O4 (alizarin) + C6H12O6 (sugar.)
Subsequently, Schunck prepared a crystalline compound, rubianic acid, which he regarded as an oxidation product of rubian, and which proved to be identical with the ruberythric acid of Rochleder.
Finally, this portion of the subject was exhaustively examined by Liebermann and Bergami, who assigned the formula C26H28O14 to ruberythric acid, and proved that its hydrolysis with acid proceeds as follows: C26H28O14 + 2H2O = C14H8O4 + 2C6H12O6
For the preparation of the glucoside, madder (i kilo.) is extracted with boiling absolute alcohol (8-9 litres) for two to four hours, and the mixture filtered hot. The alcoholic extract is evaporated to about one-quarter its bulk, and on cooling a yellowish-brown crystalline precipitate of the impure glucoside separates. After filtering and evaporating the filtrate still further, crystals of cane sugar separate, and by adding water to the remaining alcoholic solution impure alizarin is precipitated. In this way, according to Liebermann and Bergami, i kilo, of madder gave
Impure glucoside … 50-60 grams (5-6 per cent.)
sugar … 15-30 grams (1,5-3 per cent.)
colouring matter … 30-40 grams (3-4 per cent.)
The impure glucoside, which becomes resinous on drying, is dissolved in water, the solution is precipitated with lead acetate, filtered, and the filtrate treated with basic lead acetate. The pink coloured precipitate is well washed, suspended in water, decomposed with sulphuretted hydrogen, and the lead sulphide, which contains also the liberated ruberythric acid, is collected and washed with cold water. The ruberythric acid is removed from the lead sulphide by means of boiling alcohol, the yellow extract is partially evaporated, water and some quantity of barium hydroxide solution are added, and, after filtering off a white precipitate, an excess of barium hydroxide is added to the filtrate. The dark cherry-red precipitate of barium ruberythrate is dissolved in acetic acid, the solution is filtered, barely neutralised with ammonia, and then treated with basic lead acetate. The resulting red precipitate is washed with alcohol, suspended in alcohol, and decomposed by hydrogen sulphide, and the liquid and precipitate together are heated to boiling and filtered hot. On cooling, the amber- coloured solution yields pale yellow needles of ruberythric acid which are recrystallised from hot water. The latter portion of this process, employed by Liebermann and Bergami, is due to Rochleder.
Ruberythric acid crystallises in silky needles of a pure yellow colour, melts at 258-260°, and when strongly heated yields a sublimate of alizarin. It dissolves in caustic alkali solutions with a cherry-red colour, which on boiling changes to violet, and on acidification yields a precipitate of alizarin. With potassium carbonate solutions, dark red needles of potassium ruberythrate are produced. Ruberythric acid is not precipitated with lead acetate, but basic lead acetate gives a red flocculent precipitate. It possesses no dyeing power.
By the action of sodium acetate and acetic anhydride, Liebermann and Bergami obtained an octoacetyl derivative C26H20O6(C2H3O2)8, which crystallises in yellow needles, melting at 230°.
Schunck and Marchlewski, by means of the method of Schotten and Baumann, obtained a hexabenzoyl and a heptabenzoyl compound. The fact that ruberythric acid gives an octoacetyl derivative renders two constitutions possible for this substance, viz.: [KUVA PUUTTUU] and of these the second is more probably correct, as an explanation is thus afforded of the well-characterised red-coloured salts which can be obtained from it.
Erythrozym, the madder enzyme, was obtained by Schunck by extracting madder with water at a low temperature (38°) and precipitating the solution with alcohol. When dried, it consisted of a brown amorphous mass. Under its influence, ruberythric acid is hydrolysed with formation of alizarin and glucose. This reaction no doubt takes place in the incompletely dried root on storing, and it is evidently due to this fact that madder was said to dye more readily after this treatment. In dyeing with madder, moreover, the presence of this enzyme will no doubt exercise a beneficial effect, because as it is frequently the practice to employ at first a cold dye-bath and then to gradually raise the temperature, hydrolysis of the glucoside, which is itself devoid of tinctorial property, will thereby occur with formation of the colouring matter.
Though purpurin is considered to exist in madder in the form of glucoside, such a compound has not yet been isolated, and some uncertainty exists on this point. During some experiments carried out by Perkin, it was found that an alcoholic extract of madder, on standing in cold weather, deposited a considerable quantity of cane sugar contaminated with a red precipitate. This latter was soluble in water, and on treating the solution with cold dilute acid, gave a precipitate of impure purpurin and appeared to consist of an acid calcium salt of this substance. On the other hand, it was not ascertained whether all varieties of madder behave similarly in this respect, and the matter requires further investigation. Presuming, however, that a purpurin glucoside is present in madder, it is evident that this compound is far less stable than ruberythric acid, and is hydrolysed by dilute acids at a temperature at which the latter is unaffected.
Kopp's Process for the Extraction of Madder.
Based on this assumption, the commercial process of Kopp was devised, and this is specially interesting as it affords a fairly complete method for the isolation of the phenolic constituents of this dyestuff.
Ground madder is extracted with a cold aqueous solution of sulphurous acid, and the solution, after addition of 2-3 per cent, of hydrochloric acid (33 per cent.), is heated to 60°. A red flocculent precipitate of purpurin is thus thrown down which was collected, washed, dried, and sold under the name of "commercial purpurine" or "Kopp's purpurine". This product was until recently prepared to a very small extent in France for the manufacture of a rose-red lake, and for this purpose gives results differing in some respects from those produced by the artificial dyestuff.
Kopp's purpurine, in fact, is not pure purpurin, but consists mainly of a mixture of this colouring matter with three other substances : pseudo- purpurine, purpuroxanthin, and purpuroxanthin carboxylic acid or munjistin.
Pseudo-purpurin was first isolated from Kopp's commercial product by Schützenberger and Schiffert, but the fact that it consists of a purpurin carboxylic acid is due to the investigation of Rosenstiehl. It consists of small red prismatic needles, and differs from purpurin in that it is more readily soluble in benzene. It melts at 218-220° with evolution of carbon dioxide and formation of purpurin, and this decomposition is said to occur gradually at from 180-195°. Purpurin is also produced by boiling pseudo-purpurin with dilute caustic alkali, or by long boiling with water or alcohol. The constitution of pseudo-purpurin is represented by the following formula: [KUVA PUUTTUU]
It may be prepared synthetically (D.R.P. 260765) by dissolving 1.2 dihydroxyanthraquinone 3 carboxylic acid in 20 parts of sulphuric acid and slowly treating the solution at 15-20° with 0.3-0.4 parts of manganese dioxide. In place of the 1.2 dihydroxy the 1.4.3 dihydroxycarboxylic acid may be employed (D.R.P. 272301), in which case the 2 hydroxyanthradiquinone carboxylic acid is the first product of the reaction [KUVA PUUTTUU]. This by means of sodium hydrogen sulphite solution is reduced to pseudo-purpurin.
It is interesting to observe that an isomeric compound which is obtained by the oxidation of alizarin carboxylic acid, and for which also the two formulæ are possible, differs markedly from pseudo-purpurin, and is an exceedingly stable compound (Perkin and Cope).
Purpuroxanthin or xanthopurpurin, which forms glistening yellow needles, melting at 262-263°, is a dihydroxyanthraquinone isomeric with alizarin, and was isolated from Kopp's purpurin by Schützenberger and Schiffert. These authors also found that purpuroxanthin can be produced by digesting purpurin with phosphorus iodide and water, or by the action on it of a boiling alkaline stannous chloride solution. The reverse action occurs, according to Rosenstiehl, when an alkaline solution of purpuroxanthin is boiled with excess of air, purpurin being thus produced. Purpuroxanthin was synthesised by Noah by heating 3: 5-dihydroxybenzoic acid with benzoic acid in the presence of sulphuric acid, and possesses the following constitution: [KUVA PUUTTUU]
According to Plath, the dimethyl ether melts at 178-180°, and the diacetyl derivative (Liebermann) at 183-184°.
Purpuroxan,thin dyes aluminium mordanted fabrics a yellow colour (Schützenberger and Schiffert).
Purpuroxanthin carboxylic acid (munjistin) was discovered by Schunck and Römer in the crude purpurin. It crystallises from acetic acid in golden yellow leaflets, melts at 231°, and dissolves in alkaline solutions and ammonia with a red coloration. By heating above its melting-point or by boiling with alkalis, it is converted into purpuroxanthin. It has not been prepared synthetically, but probably contains its carboxyl group in a similar position to that present in pseudo-purpurin. It is said to dye aluminium mordanted fabrics an orange-red colour, which is, however, not fast to the action either of soap or light.
GREEN ALIZARIN.
The sulphurous acid liquid from which the purpurin precipitate has been removed is boiled for two hours, when the ruberythric acid and certain other glucosides present are hydrolysed and a deep green precipitate separates. This at one time was a commercial article, and was known under the name of "green alizarin".
CHLORORUBIN.
The green tinge of this product arises from the presence in madder of a considerable quantity of a peculiar substance, possibly a glucoside, termed chlorogenin or rubichloric acid, but little or nothing is known of its chemical nature. It is also present in chay root, morinda root, in certain species of galium and in the Gardenia grandiflora. This compound, which has been obtained in the form of a colourless syrup and to which the formula C14H8O9 has been assigned, on digestion with boiling dilute mineral acid, is converted into chlororubin and formic acid. Chlororubin consists of a dark green amorphous powder which is insoluble in all the usual solvents, but dissolves in alkaline solutions with a blood-red colour.
YELLOW ALIZARIN.
In order to obtain this product, the dried and finely powdered "green alizarin" was extracted at 150° with petroleum (toluene or coal-tar solvent naphtha is more suitable for laboratory purposes), by which means the alizarin and other phenolic constituents pass into solution, whereas the chlororubin remains undissolved. The petroleum extract after cooling is agitated with 10 per cent, caustic soda solution, and the dark violet-coloured alkaline liquid thus produced is removed and neutralised with acid. The bright yellow precipitate was collected, washed and dried, and sold under the name of "yellow alizarin".
The alizarin prepared in this manner is not completely pure, as it contains a small quantity of a mixture of non-tinctorial substances, which are derivatives of anthraquinone. To remove these, an alkaline solution of the "yellow alizarin" is treated with milk of lime, which precipitates the alizarin in the form of its calcium compound. This when collected, well washed, and decomposed with acid, gives a very pure alizarin which is best crystallised from solvent naphtha.
If the reddish-brown filtrate from the calcium alizarate is neutralised with acid, a small quantity of a dull yellow precipitate separates which is approximately equal to 0.02 per cent, of the madder employed. A preliminary examination of this product indicated that it consisted of at least four yellow crystalline substances, with no special properties that would permit of their ready separation.
Schunck during his examination of madder obtained various yellow crystalline and amorphous products by the action of acids and alkalis on his rubian. The individuality of most of these substances, to which the names rubiretin, verantin, rubiadin, rubianin, rubiafin, rubiagin, rubiadipin, rubidehydran, rubihydran, and rubiacic acid were assigned, has been doubted by later writers, and but one of these namely, rubiadin has been characterised. On the other hand, it is quite possible that certain of these may exist in the mixture of yellow non-tinctorial substances previously referred to. For a description of his compounds, the original papers of Schunck should be referred to.
Rubiadin glucoside (Schunck and Marchlewski). Madder is extracted with boiling water, the solution precipitated with lead acetate, and the filtrate treated with ammonia, by which means a second lead precipitate is formed. The latter is decomposed with sulphuric acid, the lead sulphate removed, and the clear liquid boiled with addition of hydrochloric acid. A dark green precipitate separates, only a portion of which dissolves in boiling alcohol. On treating the alcoholic extract with lead acetate, the alizarin present can be removed, and addition of baryta water now precipitates the barium salt of the rubiadin glucoside, which is decomposed by dilute hydrochloric acid. It crystallises from alcohol in citron yellow needles, melts at 270 with decomposition, and when hydrolysed by acid gives rubiadin and glucose
C12H20O9 + H2O = C15H10O4 + C6H12O6
Penta-acetylrubiadin glucoside, yellow needles, melts at 237°.
According to Marchlewski, the constitution of this glucoside is best expressed as follows: [KUVA PUUTTUU]
Rubiadin prepared by the hydrolysis of the glucoside, forms yellow needles, melting about 290°, soluble in alkalis with -a red coloration. By oxidation with chromic acid it gives phthalic acid. Rubiadin, according to Schunck and Marchlewski, is a methyl purpuroxanthin and possesses the following constitution: [KUVA PUUTTUU]
Rubiacin (Runge's madder orange) is a yellow crystalline substance obtained directly from the madder root, and is formed, according to Schunck, by the decomposition of a glucoside. It separates in small quantity from an infusion of madder made with only a little cold water, after it has become sour by twelve hours' standing. It crystallises in the form of plates and needles, having a strong reddishgreen lustre. Alkalis dissolve it with a purple colour.
The following table illustrates the analysis of madder by the sulphurous acid extraction method:
Madder is extracted with dilute sulphurous acid solution and the extract heated to 60°.
Precipitate consists of Purpurin, Pseudo-purpurin, Purpuro-xanthin carboxylic acid, Purpuroxanthin.
Filtrate is digested with boiling dilute H2SO4, and the resulting precipitate of green alizarin extracted with boiling toluene or petroleum.
Chlororubin remains undissolved.
The toluene extract is agitated with caustic soda solution, and the alkaline liquid is treated with baryta water.
Precipitate consists of calcium prealizarate, which when decomposed with acid gives alizarin.
Filtrate on acidification gives a precipitate of yellow non-tinctorial derivatives of anthraquinone.
Commercial Preparations of Madder.
The principal of these were: Garancine, Garanceux, Flowers of Madder, Commercial Alizarin or Pincoffin, and Madder extract.
Garancine. The preparation of this product results from the observation in 1827 of Robiquet and Colin, that by treating ground madder with an equal weight of concentrated sulphuric acid, the various principles of the madder were destroyed with the exception of the colouring matter alizarin. We now know further that the glucoside of the root is decomposed by the action of the acid. This first product was termed charbon sulphurique, but soon the method of its preparation was slightly altered, and it then received the name garancine.
Garancine is made by mixing, in a wooden tank with false bottom, 100 kilos, ground madder, 1000 litres water, and 2 kilos, sulphuric acid, 168° Tw. (sp. gr. 1.84), stirring up and allowing the whole to macerate for about twelve hours. The liquid is then drawn off, the residue mixed with a little water and 30 kilos, strong sulphuric acid, and the whole boiled for 2-3 hours. After running off the acid liquor, the garancine remaining is washed with water till free from acid, drained, pressed, dried, and ground.
The colouring power of garancine is three to four times that of good madder, it dyes more readily, giving yellower toned reds and pinks, and greyer lilacs. They are not quite so fast to soap as the madder colours, but since, in the case of printed calicoes, the unmordanted white parts are not so much soiled in the dye-bath, the operation of soaping can be omitted.
Garanceux or Spent Garancine was introduced in 1843 by L. Schwarz of Mulhouse. It was simply a low quality of garancine prepared in the above manner from the spent madder of the dyebaths, and made by each calico-printer for himself, by way of economy. Its colouring power is about one-fourth that of good garancine.
Flowers of Madder was first made in 1851 by Julian and Rogner of Sorgues. It can be prepared by macerating ground madder for several hours with cold water very slightly acidulated with sulphuric acid (1-2 per cent, on the weight of madder), then washing, draining, pressing, drying, and grinding. In this manner all soluble, mucilaginous, and sugary matter, etc., is removed, decomposition of the glucoside by fermentation occurs, and the residue has nearly double the colouring power of the original madder. The waste liquors were neutralised, allowed to ferment with the addition of yeast, and then distilled to gain the alcohol. 100 kilos, madder yielded 45-60 kilos, flowers of madder and 10 litres alcohol, suitable for making varnish, etc.
Commercial Alizarin or Pincoffin was introduced in 1852 by Schunck and Pincoff, who prepared it by submitting ordinary garancine to the action of high pressure and superheated (150°C.) steam. By this treatment the verantin and rubiretin present in the garancine were said to be destroyed, while the alizarin remained intact, and the product yielded in consequence more brilliant purples, and less soaping was required to clear the whites or unmordanted portions of printed calicoes.
Madder Extracts. Already in 1826 attempts were made by Gaudin to apply mordants along with the colouring matter of madder directly to calico, in the form of an extract, i.e. as a steam-colour, instead of by dyeing, and in 1837 Gastard succeeded in doing this successfully on a large scale by means of a product named colorine. The expense, however, of this and other early madder extracts retarded their application, but their utility having been clearly demonstrated, the endeavours of numerous chemists were directed to their production in a reasonably cheap manner. Madder extracts consisted of variable mixtures of the two colouring matters of madder, alizarin and purpurin, or of each separately, in a more or less pure condition. By the introduction of artificial alizarin, just when their manufacture had been perfected, they lost all their importance. The following were the chief methods of production employed.
Leitenberger's process consisted in first extracting all the purpurin from ground madder by water heated to 55°C., and afterwards dissolving out the less soluble alizarin from the dried residual madder by means of wood-spirit. The aqueous solution was precipitated by lime, the washed calcium-purpurin lake was then decomposed with hydrochloric acid, the liberated purpurin collected and washed, when it was ready for use. The alcoholic solution of alizarin was merely precipitated by water, collected and washed. Alizarin and purpurin extracts were thus obtained.
Parafs method (1868) consisted in extracting madder with superheated water, with or without the addition of a small quantity of alum or sulphuric acid, then collecting and washing the flocculent alizarin precipitate which separated out on cooling.
The modes of preparing Kopp's "purpurine," "green alizarine," and "yellow alizarine" have already been given.
Pernod's madder-extract, once largely used, was prepared by extracting garancine with boiling water very slightly acidified with sulphuric acid, collecting and washing the precipitate thrown down on cooling, and extracting the dried precipitate with boiling alcohol. After recovering the major portion of the alcohol by distillation, the remaining solution was mixed with water, and the precipitated alizarin was collected and washed.
Use of Madder in Dyeing.
Previous to 1870 madder and its derivative garancine were the dyestuffs par excellence of the calico-printer and Turkey-red dyer.
By the former, it was used because of its characteristic property of yielding a variety of colours with the aluminium, tin, and iron mordants, viz. red and pink, orange, lilac, and black; also brown or chocolate, by employing a mixture of aluminium and iron mordants. Further, all these colours are fast to soap and light. To the calico-printer both the alizarin and the purpurin of the madder were of use, though undoubtedly the alizarin would, in most styles of work, be the essential colouring matter. The Turkey-red dyer employed madder, and afterwards garancine, because they yielded, by his peculiar process, the most brilliant and most permanent red on cotton which was known. In this case the alizarin was the allimportant colouring matter, since the purpurin, although fixed on the fibre at first, was more or less removed during the operations of clearing. Alizarin, in conjunction with aluminium and iron mordants, gives a bluish-red and a comparatively bright like; purpurin, a yellowish-red and a greyish-lilac, respectively.
The method of applying madder in Turkey-red dyeing was similar to that now employed in the case of alizarin.
Another interesting feature in connection with the application of this dyestuff is that, if the madder was deficient in lime, it was necessary to add a certain proportion of chalk to the dye-bath; it now appears that calcium is a normal constituent of the madder colours, especially those obtained with aluminium and iron mordants.
Madder has also been used in the past, and is even now employed to a small extent, by the indigo dyer and the woollen dyer.
Literature
Robiquet and Colin, Ann. Chim. Phys., [ii.], 34, 225
Robiquet, ibid., 63, 311;
Annalen, 20, 196;
Kuhlmann, J. Pharm. Chim., 14, 354;
Zenneck, Pogg. Ann., 13, 261;
Decaisne, J. Pharm. Chim., 24, 424;
Gaulthier de Claubry and Persoz, Annalen, 2, 31;
Runge, J. pr. Chem., [i.], 5, 362, 374;
Higgin, Phil. Mag., [iii.], 33, 282;
J. pr. Chem., [i.], 46, i;
Schützenberger, Bull. Soc. Chim., [ii.], 4, 12;
Schunck, Phil. Trans., 141, 433; 142, 67; 145, 389;
Annalen, 66, 174; 81, 151,344; 87, 345, 351; Schunck and Romer, Ber., 10, 172, 551, 790;
Debus, Annalen, 66, 351;
Wolff and Strecker, ibid., 75, 3;
K. Moy, ibid., 54, 346; H. Koechlin, ibid., 59, 344;
Schiel, ibid., 60, 79;
De Lalande, J., 1874, 486;
E. Kopp, Bull. Soc. Ind. Mulh., 1861, 31, 9; 1867, 37. 437; Rochleder, Annalen, 80, 323; 82, 207;
Schwarz, ibid., 80, 333; Willigk, ibid., 82, 339;
Stenhouse, ibid., 130, 341, 343;
Bolley and Rosa, Dingl. poly. Jahr., 171, 446;
Strecker, J., 1868, 479; Rosenstiehl, Ber., 7, 1546, 10, 1178;
Liebermann and Bergami, Ber., 25, 2241;
Liebermann and Plath, Ber., 10, 1618;
Liebermann and Friedlander, Ber., 29, 2851;
Schunck and Marchlewski, Chem. Soc. Trans., 63, 969, 1137; 65, 182;
Gmelin, Handb., 16,32; 14, 129;
Perkin and Cope, Chem. Soc. Trans., 65, 848.
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