2,4 dinitrochlorobenzene and things to do with it

Just recently I have acquired from eBay a considerable amount of this compound and more crops up fairly regularly. This compound is a useful intermediate in the preparation of numerous compounds due to its reactive chloro moiety adjacent to one of the nitro groups. Although not as reactive as the chlorogroup in picryl chloride it is significantly more reactive than that in 2 or 4 mono nitrochlorobenzene. With hydrazine hydrate in triethylene glycol it give 2,4 dinitrophenylhydrazine in good yield (A. Vogel; A Textbook of Organic Chemistry: Longman; 3rd ed). With Ammonia or urea it gives 2,4 dinitroaniline (Vogel & US patent 1752998) and with aqueous alkalis it gives 2,4 dinitrophenol (Vogel & several anonymous or un-cited internet references). It also reacts with organic substituted analogues of the above compounds, so for instance it reacts with aniline in the presence of calcium carbonate to give 2,4 dinitrodiphenylamine, the starting material for the preparation of the hexanitrodiphenylamine (COPEA).

My interest lies in the derivatives of the dinitrophenol and dinitroanilines. When I received one of the batches I purchased one of the 100g jars had been crushed so for my first experiment I used the whole contents. The heavy polythene packaging was carefully removed will wearing nitrile gloves and the entire contents glass and all rinsed through a seive with water. 2,4 dinitrochlorobenzene is a fine dust and its dust too toxic to screen dry. The aqueous slurry was filter and used whilst still damp.

Experimental

2,4 Dinitrophenol via Sodium Carbonate Hydrolysis
Based on Vogel 3rd Ed (quantities double for reason above)
125g of anhydrous sodium carbonate was dissolved in 500mL of water and poured into a 3L RB flask. 100g of commercial 2,4 dinitrochlorobenzene was rinsed into the flask using a powder funnel and 500ml of water. The flask was swirled until the nitro compound had dispersed. The mixture wasthen heated to boiling and then refluxed for 24 hours. The nitro compound melts and forms a heavy oil at the bottom that is disturbed by the bouts of sudden boiling but otherwise does not appear to mix with the aqueous phase.


Figure 1 The 3L flask with sodium carbonate solution and the2,4 dinitrochlorobenzene at commencement.

The solution soon turned yellow and after 24 hours was deep orange yellow. After 24 hours the mixture was allowed to cool a little and then poured into a 2L beaker. There was a large amount of un-reacted oil at the bottom and as the mixture cooled this solidified and then a volumous yellow-orange precipitate of sodium 2,4 dinitrophenate formed. When completely cold the aqueous phase was stirred up and decanted through a Buchner funnel to collect the sodium salt. The filtrate was returned to the 3L flask and the sodium salt removed from the funnel and dried at room temperature (18°C) the yield was 10.16g of sodium dinitrophenate (note 1), a weekly energetic deep yellow compound that stains the skin. The unreacted dinitrochlorobenzene was warmed until it melted and poured back into the 3L flask and refluxed again but this time a multi-neck adapter was fitted to the flask and to allow an overhead stirrer to be used in an attempt to get a better mixing and therefor reaction rate.


Figure 2 Sodium carbonate/ dinitro-chlorobenzene mixture after 24 hours refluxing.


Figure 3 Sodium carbonate/ dinitro-chlorobenzene mixture after 48 hours refluxing

After a further 24 hours refluxing the now dark orange brown aqueous phase was poured into the 2L beaker and allowed to cool a little. Before crystallisation commenced the aqueous phase was decanted into a 3L beaker and the heavy oil stirred with 100ml of fresh warm water and left to cool. When the residual dark red brown dinitrochlorobenzene had solidified the aqueous phase was pour off through a small stainless steel straining sieve and the aqueous phase combined with the main aqueous sodium carbonate-phenate solution. The cake of unreacted 2,4 dinitrochlorobenzene was placed on a couple of filter papers and left in a cool place to drain and dry.


Figure 4 Crude 2,4 dinitrophenol – 1st dried filter-cake

The combined aqueous phase was heated to re-dissolve the sodium salts that were starting to crystallise and the small crop of sodium dinitrophenate previously recovered added to the solution and stirred until dissolved. The dark orange brown solution was then acidified with 28% hydrochloric acid until strongly acid (about 280ml were required) which was added slowly with vigorous stirring to break up the froth which forms. The product precipitates as pale brown “biscuit-crumb” like solid in a brown solution. The suspension was allowed to cool to room temperature and filtered using an 11cm Buchner funnel, washed with a little cold water until the filtrate was almost colourless and dried. After drying in a desiccator over silica gel the crude brown material weighed 31.6g indicating about 34.7% conversion after 48 hours.

2,4 Dinitrophenol via Sodium Hydroxide Hydrolysis
The unreacted residue weighed 68g but was clearly contaminated with occluded liquid and probably bis(2,4-dinitrophenyl) ether. Searching on the internet I found an anonymous quote on the Chemicalland web site that claimed 2,4 dinitrophenol could be prepared by the hydrolysis of the dinitrochlorobenzene with aqueous sodium hydroxide at 104°C and the experiment below was an attempt at this method.

From Chemicalland web site; Quote;

"This compound can be prepared from 2,4-dinitrochlorobenzene by first heating water in a beaker to 60 °C and adding the molten 2,4-dinitrochlorobenzene, heating is continue to 90 °C. Then sodium hydroxide solution is added (1mol equi). The temperature is controlled to no more than 102-104 °C with vigorous stirring and the insulated and stud for 30 minutes. A little HCl is added to ensure that the final solution is acid and then cool and filter to recover the product."

Another comment found in the internet search suggested that this reaction is conducted commercial using an autoclave with 3-4 fold excess of sodium hydroxide in order to minimise the formation of a diphenyl ether derivative. The experimental details below are my attempt at this process.

Experimental
Based on the above quotations the 68g of residual nitro compound was placed in a large beaker and warmed until it melted. 40g of sodium hydroxide was dissolved in 400ml of water and poured over the heavy oil. The mixture was warmed on a hot plate and stirred with a thermometer. It was heated to between 100 and 105°C and then this temperature maintained for 4 hours but occasionally adding a little cold water, careful adjustment of the hotplate controls or just lifting off the hotplate for a minute or two. After this time the solution had become dark brown and most of the oil had dissolved though a little remained. A porcelain basin with a little cold water in it served to avoid loss of dinitrochlorobenzene vapour which is extremely irritant and toxic.


Figure 5 The residual dinitro-chlorobenzene in sodium hydroxide solution after only 60 minutes

The beaker was then cooled and most of the aqueous solution decanted from the small amount of residual dinitrochlorobenzene which was washed with cold water causing it to solidify allowing the remaining liquid to be strained off.

The aqueous phase was acidified with 28% hydrochloric acid and cooled to 4-5°C overnight before being filtered at the pump. The cake was washed with cold water and dried in a warm place. The yield was 43.91g of a crude medium brown “biscuit crumb” like solid as before, suggesting about 48,3% conversion of the original compound to dinitrophenol. The unreacted, low melting point residue weighed 17.16g.


Figure 6 The 2nd filter-cake of crude but dried dinitrophenol

The three recovered fractions, two of crude dinitrophenol and one of residual dinitrochlorbenzene therefore account for almost exactly 100g of starting material. Some dinitrophenol, which is slight soluble in water, must be lost in the filtrate too though the solubility of the phenol in the strong brine solution that results from neutralisation may be very low.

Purification
Time did not permit the full purification of all of the crude material but a small orientation study was carried out as follows: 1 g of the crude filter cake was treated with 10ml of reagent grade methanol, boiled for a few seconds and filtered hot through a 23mm Hirsh funnel. Pale crystals formed slowly from the dark brown solution, presumably the fairly pure dinitrophenol. The methanol was evaporated on a water bath to a very small volume and cooled to leave a second crop of dark brown crystals. The later were completely and rapidly soluble in dilute sodium hydroxide and deposit a buff coloured precipitate when acidified, they appear to be impure dinitrophenol. Charcoal treatment may be require or to re-crystallise it from hydrochloric acid. The light grey granular residue left on the filter is partly water soluble and the remainder soluble in strong (40%) hot caustic soda solution; the former is probably salt and the latter tar.

Discussion
Vogel’s sodium carbonate hydrolysis seems optimistic without at least a fast stirrer and possibly a PTC or an emulsifying agent to improve mixing of the two phases. The sodium hydroxide method looks like a better technique particularly if a rapid stirrer was used. The use of a stronger solution of sodium hydroxide may also speed up the reaction.

Surprisingly very little evidence for the formation of a tetranitrodiphenyl ether (Mp 195 C) was seen the unreacted residue remained a low melting point material to the end.

Note 1; Sodium 2,4-dinitrophenate is fairly soluble so that the recovered salt probably represents less than half that present.

When time permits I will attempt to purify the bulk of the crude material and conduct experiment to convert it to 4-nitro-2-aminophenol, picric acid and picramic acid.