Anthraquinones, isomer separations

Hie above three isolation procedures can usually be conducted in such a way as to effect simultaneous purification from undesired by-product isomers. Thus, with the first method (dilution with water) the pure para isomer of tohienesulfonie acid is obtained since the ortho isomer is soluble likewise, the 1,5- and l, isulfonates (d anthraquinone are separated by taking advantage of different solubilities in sulfuric acid. When the second procedure is applied to the isolation of anthraquinone-l-sulfonate as the potassium salt, filtration at 85°C ensures removal of the 1,5-disulfonate in the aqueous filtrate. Metallic sulfonates are frequently purified further by recrystallization from water, often with the addition of brine to ensure a good yield by salting-out. 

It is relevant to recall that the Japanese in the 1970s imposed a total ban on the discharge of mercury-containing effluent. This has led to the devising of alternatives to the Hg-catalysed sulphonation of anthraquinone leading by amination to the valuable 1-aminoanthraquinone, e.g. the nitration of A/Q, separation of the 1-nitro compound and reduction to the amine or the amination of 1-chloro-A/Q separated from the 2-isomer. In both cases isomer separation is difficult and further investigation is needed. 

Mononitration of anthraquinone at about 50°C gives mainly 1-nitroanthraquinone. At 80 to 95°C, dinitration occurs to give a mixture of the 1,5- and 1,8-isomers. These isomers are important as starting materials for the preparation of other intermediates. In some cases, the mixture is used in others, isomer separation is necessary. 

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. 

Additional noteworthy applications of CEC include natural products such as the plant flavonoids hesperetin and hesperidin [160], anthraquinones extracted from rhubarb and from Chinese medicine [161], and heterocyclic compounds present in oils of bergamot, mandarin, and sweet orange [162], The CEC analysis of retinyl esters has been investigated by Roed et al. in nonaqueous mode for the separation of liver extracts of arctic seal [163]. Carotenoid isomers were also separated on C30 stationary phases by nonaqueous CEC [164]. It was found that CEC offered increased resolution compared to HPLC, and in CEC... 

The mother liquor from the 1,8-disulfonic acid contains about 30 per cent of the original anthraquinone in the form of a complicated mixture of disulfonic acids (mainly the 1,7 acid along with a small amount of the 1,6 isomer, still smaller amounts of the 1,5 and 1,8 acids, and traces of the 2,6 and 2,7 compounds). This mixture cannot be separated in any simple way. In the industry, the residue is worked up to produce "silver salt by prolonged heating of the diluted sulfuric acid solution at 180-200°. This treatment splits out sulfo groups in the 1 position, and the resulting mixture of anthraquinone and anthraquinone-2-sulfonic acid is treated in the manner described under the preparation of sodium anthraquinone-2-sulfonate (page 228). 

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