What is 2,4-Dinitrophenylhydrazine? - Structure & Hazards

Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The primary focus of this lesson is an organic molecule that's called 2,4-dinitrophenylhydrazine. Main points of discussion will be its general structure, common uses, and the specific hazards associated with its handling.

A Case of Unknown Identity

It's organic chemistry lab day at the university, and Bill has just walked into the lab ready to go for the day's experiment. His excitement turns into terror when he learns what his professor has in store for the class. Today, each member of the class will be given an unknown compound and be asked to determine its identity. Although it seems like an impossible task given that there are millions of possibilities, Bill feels better when he learns that the only functional group possibilities for the unknowns are ketones, aldehydes, esters, and carboxylic acids.

Why would this make him feel better? Well, because Bill just learned in a lecture about a special reagent known as 2,4-dinitrophenylhydrazine. In this lesson, our focus is going to be on this specific compound. We will soon see how Bill can use it to his advantage during the process of trying to figure out the identity of his unknown molecule. Let's get started!

What Is 2,4-Dinitrophenylhydrazine?

Let's get started by getting up to speed like Bill on what 2,4-dinitrophenylhydrazine is and what its structure looks like. 2,4-Dinitrophenylhydrazine (abbreviated 2,4-DNP for short) is a compound with the chemical formula C6 H3 (NO2) 2 NHNH2. The compound contains a benzene ring, two nitro groups, and a hydrazine (two nitrogen atoms bonded directly to each other) functional group.


Structure of 2,4-DNP
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Notice in the structure that there are two nitro groups, thus the prefix di- in the name. The benzene ring is colored in salmon with the hydrazine portion labeled in purple. The hydrazine part of 2,4-DNP is very important, as we will see shortly.

Why 2,4-DNP Will Help Bill

Now that we know what 2,4-DNP looks like in terms of its structure, let's talk briefly about how Bill can use it to his advantage for his experiment. Since the early beginnings of organic chemistry, 2,4-DNP has been a tried and true test for the presence of aldehyde and ketone functional groups in unknown organic compounds. Since the early chemists didn't have the fancy instrumentation we have at our disposal today, they relied heavily on qualitative tests to help them track down the identity of an unknown. A positive 2,4-DNP test means that your unknown contains either an aldehyde or ketone, while a negative test tells you that it doesn't. It's actually the hydrazine group that reacts with the aldehyde or ketone, producing a positive test.


An example of a positive 2,4-DNP test with an aldehyde or ketone
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When an aldehyde or a ketone is placed in a 2,4-DNP solution, a bright yellow-orange or red solid results, and this is what constitutes a positive test. If no aldehyde or ketone is present, no bright colored solid is observed and the solution remains colorless. This is why Bill is so relieved! He knows that the first thing he can do to help identify his unknown is run a 2,4-DNP test, and if it's positive he knows he has either an aldehyde or ketone. If the test is negative however, his unknown has to be one of the other possibilities.

The nice thing about a 2,4-DNP test is that it is highly selective and will only produce a positive result if an aldehyde or ketone is present. No positive test will be observed for esters and carboxylic acids. This is very helpful for Bill because he can very quickly rule out half of the functional group possibilities of his unknown simply by determining the results of this one test.

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