A Level Chemistry: Going 2,4DNP(H)-free
08 March 2024
Andy Brunning, Chemistry A Level Subject Advisor
Chemistry practicals involving 2,4-dinitrophenylhydrazine pose increasing challenges in terms of storing the compound safely and even getting hold of it in the first place. Here, I summarise the issues surrounding 2,4-dinotrophenylhydrazine and look at what alternatives are available.
What is 2,4-DNPH?
2,4-dinitrophenylhydrazine is commonly abbreviated to 2,4-DNPH (or sometimes 2,4-DNP, though this risks confusion with 2,4-dinitrophenol). It reacts with aldehydes and ketones to give red and orange derivatives, the melting points of which can be used to determine the identity of the parent aldehyde or ketone. To carry out the initial reaction, 2,4-DNPH is mixed with phosphoric acid (a mixture known as Brady’s reagent), then added to the aldehyde or ketone.
What’s the problem with 2,4-DNPH?
2,4-DNPH hit the news a few years ago due to the risk of explosion if it is inappropriately stored. It’s friction and shock-sensitive, so CLEAPSS advise that technicians store its container inside a larger container that contains a small amount of water. A reminder of this requirement in 2017 led to number of schools realising it had been stored incorrectly, with “explosive destruction” (in short, calling in the bomb squad) the prescribed remedy.
Additionally, 2,4-DNPH is an undocumented casualty of Brexit. The lone supplier is Germany-based Merck and import challenges have led to lengthy delays of over a year to deliveries. Consequently, schools that want to use it as part of their A Level Chemistry practical work are finding it difficult to get hold of.
How does this impact A Level Chemistry practical work?
Though knowledge of 2,4-DNPH derivatives is required by our A Level Chemistry A specification (6.1.2(d)), carrying out the reaction in practical work is not required and it doesn’t feature in any of our suggested practical activities. Consequently, we don’t expect students to have used it during their work towards the practical endorsement.
We know that teachers regularly continue to carry out this reaction with students, though, and it’s safe to continue doing so. In fact, if your school is in the lucky position of having ample supplies of 2,4-DNPH, it’s probably a good idea to continue to deplete them, to avoid storage issues further down the line.
However, if you don’t currently have 2,4-DNPH in your school, the current recommendation from CLEAPSS is that you should not attempt to restock. This is due to the potential lengthy delays in procuring it and the afore-detailed lack of requirement to use it from exam boards.
What are the alternatives?
If you’re all out of 2,4-DNPH, you might be wondering if there are any alternatives you can turn to. Obvious candidates are the other reagents to identify carbonyls featured in A Level Chemistry specifications, Tollens’ reagent and Fehling’s reagent. However, both give a reaction with aldehydes and not ketones and don’t provide the opportunity for melting point determination afterwards.
Another qualitative alternative, detailed in this Journal of Chemical Research paper, is phenylhydrazine chloride in water. When added to aldehydes, this reacts to give a precipitate, while no precipitate is seen with ketones. A mixture of phenylhydrazine hydrochloride with sodium acetate trihydrate does, however, produce a precipitate when reacted with ketones.
Again, phenylhydrazine chloride doesn’t provide the opportunity for quantitative melting point analysis. So, it does seem that the options for that for this practical are limited without 2,4-DNPH. With that said, some suppliers do provide Brady’s reagent (2,.4-DNPH dissolved in phosphoric acid), and this does not have the same storage issues as solid 2,4-DNPH – though any unused reagent must be disposed of and cannot be stored.
Remember, if you have concerns about 2,4-DNPH or Brady’s reagent, we recommend you visit the CLEAPSS website or call their hotline for further guidance.
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About the author
Andy joined OCR in September 2017 as the subject advisor for A Level Chemistry. He has a Chemistry BSc and a Secondary Science PGCE from the University of Bath. Before joining OCR, he worked as a chemistry teacher in Bournemouth and Cambridge. He also sidelines as a science communicator and has produced infographic projects for the Royal Society of Chemistry and the American Chemical Society.
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