Anthraquinone separation

Experiment.—Dissolve 1 g. of purest anthracene in just sufficient good glacial acetic acid at the boiling point without further heating add 3 c.c. of concentrated sulphuric acid and, drop by drop, 4 g. of sodium dichromate dissolved in a quite small amount of water. (Neglect any turbidity or precipitate which appears after the addition of the sulphuric acid.) A very vigorous reaction occurs and the chromic acid is used up almost immediately after all the dichromate has been added boil for five minutes longer. Dilute the solution. The anthraquinone separates in a flocculent condition. After... 

Anthraquinone is obtained by oxidation of anthracene using sodium bichromate plus sulfuric add, and is purified by dissolving in concentrated sulfuric acid at 130 Cl and pouring into boiling water, whereupon anthraquinone separates as pure solid, and is recovered by filtration, Further... 

Treatment of 2-benzoylbenzoic acid with concentrated sulfuric acid effects cyclodehydration to anthraquinone, a pale-yellow, high-melting compound of great stability. Because anthraquinone can be sulfonated only under forcing conditions, a high temperature can be used to shorten the reaction time without loss in yield of product the conditions are so adjusted that anthraquinone separates from the hot solution in crystalline form favoring rapid drying. 

For the manufacture of alizarin on a large scale a very pure anthraquinone is required, and this is generally prepared by oxidation of anthracene with sodium bichromate and dilute sulphuric acid. The anthracene is generally a 50-per-cent, product which has oeen converted into a soft powder by subliming with superheated steam. The oxidation takes place in lead-lined vessels in which the mixture is heated by direct steam. By employing a pure anthracene and a not too concentrated oxidation-mixture, the anthraquinone separates as soft grey powder, which is freed from acid by washing wdth water. The crude product is then dried, dissolved in concentrated sulphuric acid, and precipitated with water. A further purification is effected by sublimation with superheated steam. 

The sublimed anthracene is dissolved by heating in a test-tube with a little glacial acetic cid it is treated with about double its weight of chromic anhydride, and heated a short time to boiling. The solution is then diluted with several times its volume of water, the anthraquinone separating out is filtered off, washed with some dilute sulphuric acid, then with water, and is finally crystallised in a test-tube from a little glacial acetic acid. Long colourless needles of anthraquinone, which melt at 2770, are thus obtained. 

Allow the reaction mixture to cool and attain the room temperature and pour the contents directly into a 600 ml beaker containing 300 g of crushed ice with vigorous stirring with a glass rod when light-yellow crystals of anthraquinone separates out. 

Sodium anthraquinone-p-sulphonate ( silver salt ). Place 60 g. of fuming sulphuric acid (40-50 per cent. SO3) in a 250 or 500 ml. round-bottomed flask and add 50 g. of dry, finely-powdered anthra-quinone (Section IV,145). Fit an air condenser to the flask and heat the mixture slowly in an oil bath, with occasional shaking, so that at the end of 1 hour the temperature has reached 160°. Allow to cool and pour the warm mixture carefully into a 2 litre beaker containing 500 g. of crushed ice. Boil for about 15 minutes and filter off the unchanged anthraquinone at the pump. Neutralise the hot filtrate with sodium hydroxide and allow to cool, when the greater part of the sodium anthra-quinone-p-sulphonate separates as silvery glistening plates ( silver salt ). Filter these with suction and dry upon filter paper or upon a porous plate. A second crop of crystals may be isolated by concentration of the trate to half the original volume. The yield is 40-45 g. 

Use of mercuric catalysts has created a serious pollution problem thereby limiting the manufacture of such acids. Other catalysts such as palladium or mthenium have been proposed (17). Nitration of anthraquinone has been studied intensively in an effort to obtain 1-nitroanthraquinone [82-34-8] suitable for the manufacture of 1-aminoanthraquinone [82-45-1]. However, the nitration proceeds so rapidly that a mixture of mono- and dinitroanthraquinone is produced. It has not been possible, economically, to separate from this mixture 1-nitroanthraquinone in a yield and purity suitable for the manufacture of 1-aminoanthraquinone. Chlorination of anthraquinone cannot be used to manufacture 1-chloroanthraquinone [82-44-0] since polychlorinated products are formed readily. Consequentiy, 1-chloroanthraquinone is manufactured by reaction of anthraquinone-l-sulfonic acid [82-49-5] with sodium chlorate and hydrochloric acid (18). 

Efforts have also been made to overcome compHcated processes. Methods to reduce the number of steps or to use new starting materials have been studied extensively. l-Amino-2-chloro-4-hydroxyanthraquinone (the intermediate for disperse red dyes) conventionally requires four steps from anthraquinone and four separation (filtration and drying) operations. In recent years an improved process has been proposed that involves three reactions and only two separation operations starting from chloroben2ene (Fig. 2). 

Anthraquinone-l,5-disulfonic acid [117-14-6] (44), and anthraquinone-1, 8-disulfonic acid [82-48-4] (45) are produced from anthraquinone by disulfonation in oleum a higher concentration of SO than that used for 1-sulfonic acid is employed in the presence of mercury catalyst (64,65). After completion of sulfonation, 1,5-disulfonic acid is precipitated by addition of dilute sulfuric acid and separated. After clarification with charcoal, 1,5-disulfonic acid is precipitated as the sodium salt by addition of sodium chloride. The 1,8-disulfonic acid is isolated as the potassium salt from the sulfuric acid mother hquor by addition of potassium chloride solution. 

Sodium anthraquinone-l,5-disulfonate (HjO) [853-35-0] M 412.3. Separated from insoluble impurities by continuous extraction with water. Crystd twice from hot water and dried under vacuum. 

Water-soluble polymeric dyes have been prepared from water-insoluble chromophores, viz., anthraquinone derivatives. Unreacted chromophore and its simple derivatives, which are all water-insoluble, remain in solution due to solubilization by the polymeric dye. A method has been developed to separate and quantitate the polymeric dye and these hydrophobic impurities using Sephadex column packing. The solvent developed has the property of debinding the impiirities from the polymer, and further allows a separation of the imp irities into discrete species. This latter separation is based on the functional groups on the impurity molecules, having a different interaction with the Sephadex surface in the presence of this solvent. The polymer elutes at the void volume... 

Anthraquinone glycosides and aglycones can be readily separated on silica layers rising moderately polar developing solvents [41 3]. The best such solvents eonsist of ethyl acetate modified to increase polarity by the addition of alcohols or water for the glycosides or changed to decrease polarity by inclusion of hydrocarbon components. 

Glycosidic anthraquinones may be developed using ethyl acetate-methanol-water systems (100 10 10) with suitable adjustments made for polarity. Similarly, aglycones can be separated using a somewhat less polar solvent such as petroleum ether (40 to 60°C)-ethyl acetate-formic acid (75 25 1). Some chosen retention data may be found in a recent monograph [24]. Pigments may be recovered by extraction of the absorbant with acetone or methanol after removal of the individual zones. 

The effect of substituents on colour in substituted anthraquinones may be rationalised using the valence-bond (resonance) approach, in the same way as has been presented previously for a series of azo dyes (see Chapter 2 for details). For the purpose of explaining the colour of the dyes, it is assumed that the ground electronic state of the dye most closely resembles the most stable resonance forms, the normal Kekule-type structures, and that the first excited state of the dye more closely resembles the less stable, charge-separated forms. Some relevant resonance forms for anthraquinones 52, 52c, 52d and 52f are illustrated in Figure 4.3. The ground state of the parent compound 52 is assumed to resemble closely structures such as I, while charge-separated forms, such as structure II, are assumed to make a major contribution to the first excited state. Structure II is clearly unstable due to the carbocationic centre. In the case of aminoanthraquinones 52c and 52d, donation of the lone pair from the... 

In the world of red colourants, anthraquinones, which are obtained from plants or animals, are the largest group. They can be separated by RPLC due to diverse polarity caused by the presence of various polar groups in their structure. However, forms of identified compounds depend mostly on extraction and hydrolysis conditions. 

RPLC separation with spectrophotometric detection is often applied to the identification of the anthraquinone colour components of cochineal, lac dye and madder. [28,40,41,50 53] In particular the latter, containing many colourants, is the object of many research studies. Due to the large number of anthraquinones isolated from plants of the Rubiaceae family, their unambiguous identification solely by UV-Vis detection is not always possible,... 

Electrophilic substitution at the anthraquinone ring system is difficult due to deactivation (electron withdrawal) by the carbonyl groups. Although the 1-position in anthraquinone is rather more susceptible to electrophilic attack than is the 2-position, as indicated by jt-electron localisation energies [4], direct sulphonation with oleum produces the 2-sulphonic acid (6.3). The severity of the reaction conditions ensures that the thermodynamically favoured 2-isomer, which is not subject to steric hindrance from an adjacent carbonyl group, is formed. However, the more synthetically useful 1-isomer (6.7) can be obtained by sulphonation of anthraquinone in the presence of a mercury(II) salt (Scheme 6.4). It appears that mercuration first takes place at the 1-position followed by displacement. Some disulphonation occurs, leading to the formation of the 2,6- and 2,7- or the 1,5- and 1,8-disulphonic acids, respectively. Separation of the various compounds can be achieved without too much difficulty. Sulphonation of anthraquinone derivatives is also of some importance. 

Carry out the reduction and filter the red solution. Anthraquinone soon separates again from the filtrate on exposure to the air. 

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