If you are reading this in America, Finland, Norway, Russia, Australia and China or wherever, do let me know. As always a “no” is a result jut as much as a “yes”.
Schools in the UK have had quite an amazing two weeks during late October 2016 with this chemical. (However, CLEAPSS (www.science.cleapss.org.uk), we have been wrestling with it for many years.)
24-DNP is used to make Brady’s Reagent which appears to have been discovered in the 1920s. There are two basic methods of making Brady’s reagent, the original method used 24-DNP dissolved in concentrated sulfuric acid and methanol and another with phosphoric(V) acid with ethanol. The latter appears much safer to me as it would less prone to evaporation when stored. When Brady’s reagent is added to an aldehyde or a ketone, a beautiful yellow or orange precipitate appears. On careful crystallization small glistening crystals appear (see picture on right). It is claimed that the crystals have a sharp melting point from which the carbonyl compound can be identified.
Its use, in organic qualitative analysis and melting point determination, was published in Arthur Vogel’s Text Book of Organic Chemistry. It is certainly in the 3rd edition published in 1948. I still have a copy of this marvellous book. It must have been used by every university chemistry student in the UK during the next 50 years. My suspicion is that this experiment, was taken into classroom laboratory work by qualified teachers as the reaction is visibly quite stunning and subsequently appeared in our A-level syllabuses. For more on Vogel see microchemuk.weebly.com/2-blog-is-this-supposed-to-happen/archives/10-2015.
The CLEAPSS Hazcards, which are now in every English, Welsh and Northern Ireland School, have been in publication since the 1970s. In that first edition, we stated that the substance should be kept damp. Suppliers added (and still do) 30% by weight of water and the bottles then carried an explosive hazard label. The information was that the substance was explosive when dry and in dust form. This is one of those claims that one just cannot pin down but the latest ECHA information classifies it as a flammable solid (a flammable solid is a solid that is readily combustible, or may cause or contribute to fire through friction).
If any reader has experience of this material exploding or catching fire on storage and on opening, it would be useful to know as CLEAPSS has no direct evidence of this actually happening. Yet the warning is there! What is interesting that Vogel makes no mention of the explosive nature of 24-DNP in the book. In the 3rd edition he even has a method of synthesising 2,4-DNP.
(He does have warnings for picric acid though. He writes. “It is advisable to keep the picric acid in the moist condition (containing about10 per cent, of water) in a bottle with a cork stopper. Small quantities may be safely stored whilst dry, but this is not recommended in the interest of safety. Under no circumstances should glass stoppers be employed for potentially explosive substances, since on replacing the stopper some of the material may be ground between the stopper and the neck of the bottle and an explosion may result.”
In calls to the CLEAPSS Helpline technicians and teachers asked how they should maintain 2,4-DNP damp during storage. In the subsequent editions of Hazcards, we suggested it was stored in another container containing water.
Our A-level chemistry syllabuses (for 16-18 year old students) continued to use it in from the 70s to 90s. The teacher would present a series of “unlabelled” carbonyl compounds to the students, ask them to make the hydrazone adduct of one of them using Brady’s reagent, determine the melting point and identify the unknown from the melting points provided in a table. That is what teachers thought real chemists did, though I, even in 1964 at Sheffield University, was putting compounds into an IR spectrophotometer, obtaining a mass spectrum or a NMR spectrum to identify organic chemicals. Of course, in the school environment spectroscopy is too expensive.
In the 90s the use of 24-DNP gradually dropped out of some syllabuses. Schools still kept the chemical though. During the “noughties”, the UK A-level practical exam was rather moribund with contrived investigation activities which were much abused by over-rehearsing and possibly cheating. Now in the new regime, we are testing skills and one of them is to measure melting points using melting point equipment. Many experienced chemistry teachers and those in our exam boards revived the Brady’s reagent test for carbonyls. Technicians went to the stores to find the chemical. It was not in a puddle water; it had not been used for within the working memory of these technicians and teachers; bottle of Brady’s reagent had gone “dry”. Dates on bottles suggested they might have been bought in the 70s (and you could get a penny back for returning the bottle). Although sold in 25g pots by specialist educational suppliers even then, some teachers had “saved money” by buying in bulk from more general suppliers so there were 500g bottles on the shelves, dry as a bone.
Another issue then appeared. If schools wanted to remove old material, the waste contractors and the waste holding stations would not take it if were or might be dry. A school could be charged several thousand pounds to remove 1 bottle by highly specialist waste contractors. In the past picric acid, now a class 1.1 explosive had been used in schools. Occasionally an old cupboard would be opened in a school and there, lurking in the back, would be an old bottle of picric acid. To remove it, our army “bomb squad” would be mobilise to deal with it. With that experience in mind, CLEAPSS senior staff did sterling work over the 2016 Autumn half term in a setting up procedures with the police and the bomb squad in dealing with the removal of dry 2,4-DNP from hundreds of schools. In the space of 2 weeks leading up to our November 5th fireworks night (good camouflage), there we many little explosions over the country as the dry 24-DNP was exploded with a small explosive charge, often on the school playing fields. This news even appeared in the press, the radio and television. There were some interesting stories how these events were received by non-science trained Head teachers, school governors, other teachers and the general public (“the noise was "immense" and "worrying", especially when families were out celebrating Halloween” “The explosion was no louder than a loud firework” ““The bang was a lot louder than they told me it was going to be.”).
To me, the use of 2,4-DNP in schools is “redundant chemistry”.
- 2,4-DNP is not a diagnostic test for carbonyls compounds as amines and esters can produce precipitates.
- I found it took several recrystallizations to obtain a melting point close to the published value.
- Recrystallization is an important but a time-consuming practical activity as damp solids need to be dried. Can teachers afford the time?
- The melting points of hydrazones are often over 200°C. The use of expensive electrically-heated melting-point apparatus is slow to these temperatures and have to be very carefully carried out. The equipment takes time to cool and in the context of the school lesson time, so it is not very efficient teaching. (It has taken me a a good 20 to 50minuts to take a meting point. It is mind-numbingly boring.
- Melting points can be carried out in hot liquids in Thiele tubes or boiling tubes but the liquid needs to cope with high temperatures. The use of organic phthalates is not encouraged as they have environmental issues. Liquid paraffin can be used and sometimes fresh glycerol. This involves heating glass equipment full of organic liquids to quite high temperatures with a Bunsen burner. Hmmmm! Not good really.
- Mercury thermometers can no longer be bought. Schools do not have lots of thermometers available reading over 200°C. Spirit thermometers at high temperatures are not very accurate. Digital thermometers can be used. (“This is cheating as students are not reading a scale!”)
- There are other melting points students can do in their A-level work which are lower and more useful, eg methyl 3-nitrobenzoate and benzoic acid (and substituted benzoic acids) from saponifications the esters. In both cases they are checking purity and can be used alongside thin layer chromatography.