Anthraquinone preparations

Here is report on my second experiment with fused aluminium nitrate;

Experiment 2; preparation scale procedure.
10.006g of technical grade anthracene and 28.360g of laboratory grade aluminium nitrate hydrate were ground together in a pestle and mortar until a fine, even dispersion was achieved. The mixture was then placed in a tall-form, stainless steel beaker (a repurposed SS drinking vessel off Ebay) and suspended in an oil bath at 120°C for 20 minutes. The mixture was stirred with a piece of iron wire once it began to melt. After about 10 minutes the brown melt began to foam and brown nitrogen oxide fumes were released. After 20 minutes the melt was allowed to cool a little and then 50ml of 2M hydrochloric acid was added. The vessel was allowed to cool to room temperature, about 7°C, and then a spatula as used to dislodge most of the organic melt adhering to the inside of the vessel and to break up the larger pieces of foam. The contents of the beaker were poured into a small Buchner funnel and the solid washed with copious water to remove aluminium salts and acid. The dark brown solid was dried at 45-50°C and cooled. Even at this temperature the solid becomes plastic and tends to fuse together. The yield was 12.619g which is >100% (theory assuming 100% pure anthracene is 11.69g).


Crude acid washed anthraquinone

The crude anthraquinone was added to 100ml of warm methanol, circa 50°C, stirred at this temperature for about 10 minute and filtered hot using a preheated Buchner funnel. The light brown cake was washed with about 20ml of fresh methanol and sucked dry. The crude anthraquinone was then dried at 45-50°C for 5 hours to give 9.512g of soft granular material that no longer become plastic at low temperature but which is till much too dark in colour.

The crude product a recrystallised from 100ml of glacial acetic acid (10ml per gram), treated at boiling point with 1ml of kieselguhr and filtered hot through a preheated Buchner funnel. The filter solution began to crystallise so was reheated almost to boiling and allowed to cool slowly. The crystals were filtered off, washed with a little glacial acetic acid, then 80% acetic acid, water and finally methanol. The dried massive consisted of mainly of tiny, golden brown, lustrous needles but a small amount of darker red-brown plates were also present in the product. The former is almost certainly impure anthraquinone but the latter is unknown but is presumably derived from the 10% of impurities such as carbazole and phenanthrene. The yield was 4.979 which is about 45%.


Crude anthraquinone, methanol leached


Anthraquinone recrystallised from acetic acid

Conclusions
While this process may work on an industrial scale where the anthraquinone product can be sublimed out of the reaction mixture with superheated steam or air as per the patent, it is not a practical laboratory scale procedure. The reaction appears to work but the product is very impure and recrystallisation from glacial acetic acid results in large loses (about half) and the resulting product is still visibly impure. Reheating the initial filtrate and adding a little water until it becomes cloudy and cooling results in the recovery of further product but it is contaminated with an orange tarry material. The dark crystals that contaminate the recrystallised product are not a hydroxyanthraquinone and are not soluble in hot alcohols as phenanthraquinone would be.

The chlorate or chromic acid oxidation routes are better options in the laboratory.

Preparation of Anthraquinone by Chlorate oxidation of Anthracene

This procedure is a scaled down version of that published on the Orgsynth.com web site (Coll. Vol. 1, p482) and is also that suggested by Niklas on the Science Madness forum: http://www.sciencemadness.org/talk/viewthread.php?tid=160105

70ml of roughly 2% sulphuric acid was prepared by adding 0.9ml of conc. sulphuric acid to 69ml of water. 30.011g of technical grade (~90%) anthracene was placed in a 1 l conical flask equipped with a large stir-bar. 23.507g of finely ground sodium chlorate, 0.188g of vanadium pentoxide and 335ml of glacial acetic acid were then added to the flask and the mixture stirred to produce an even grey green dispersion. The dilute sulphuric acid was then added slowly over 5 minutes. Once the latter had been added, a thermometer was suspended in the mixture and the hotplate turned on. The mixture was warmed until the suspension began to turn golden yellow at about 70-75°C when the hotplate was temporarily removed. The temperature continued to rise very slowly and eventually reached 79-80°C and stabilised there for nearly 20 minutes before starting to fall. At this point the hotplate was reinstated, the thermometer removed and the mixture refluxed very gently for an hour.

The deep yellow reaction mixture slowly turned slightly brownish green towards the end. The hot reaction mixture was poured into a beaker and allowed to cool slowly. The flask was rinsed with a little cold water and this was added to the beaker with stirring. When cold, the suspension was filtered using a 9cm Buchner funnel and sucked as dry as possible. The cake was washed three times with 150ml batches of warm water on the filter and drained between each wash. After the final wash the cake stilled smelled strongly of acetic acid so 500ml of cold water was drawn through the cake and then it was sucked as dry as possible again. After drying overnight at 50°C the cake was greenish brown and weighed 37.542g (i.e. >100% of theory!).

The methanol leach used to purify the product of other experiments into anthraquinone was tried on the product of this procedure. The cake was dispersed into 200ml of nearly boiling methanol, stirred for 10 minutes and filtered hot, the cake was washed repeatedly with a further 100ml of methanol and drained hard. The cake was dried again at 50°C to give a much paler greenish yellow product and a dark brown filtrate that deposited much dark brown tar on cooling. The product now weighted 29.860g which is about 85% yield, however, given that the starting material is claimed to be only about 90% anthracene the yield is closer to 95% based on anthracene which is very close to the figure given for the procedure when using purer anthracene. The final dried cake had slight brownish stains on the surface but this is due to incomplete removal of the methanol leach and a further methanol wash will resolve this.


The final, methanol-washed anthraquinone product

I am going to try this material in my future experiments without further purification.

Quote: Originally posted by unionised

Have you seen this? https://pubs.acs.org/doi/pdf/10.1021/ja01856a504