Hi and welcome back to Free Science Lessons. By the end of this video you should be able to describe how we can use the chemical 2,4-dinitrophenylhydrazine to identify aldehydes in ketones. Over the last few videos we've been looking at the reactions of aldehydes in ketones.
We've seen that both aldehydes and ketones contain the carbonyl group and this determines how they react. Now we can test for the presence of the carbonyl group using the chemical 2,4-dinitrophenylhydrazine, which is also called 2,4-DNP. And I'm showing you the structure of 2,4-dinitrophenylhydrazine here. Now this molecule looks complicated, but it's actually fairly straightforward.
The molecule is based on hydrazine, which I'm showing you here. In 2,4-DNP, one of the hydrogen atoms in hydrazine has been replaced with a benzene ring. containing two nitro groups. The hydrazine is on carbon 1 of the benzene ring.
The two nitro groups are on positions 2 and 4. Normally 2,4-DNP is used as a solution in methanol and sulfuric acid, and together this is called Brady's reagent. Brady's reagent has a pale orange colour. Now we can use 2,4-DNP to test for the presence of an aldehyde or ketone. 2,4-DNP reacts with the carbonyl group in aldehydes and ketones to form a yellow or orange precipitate.
So if we add Brady's reagent to a chemical and we get a yellow or orange precipitate, this tells us that the chemical must be either an aldehyde or a ketone. The reaction produces a chemical called a 2,4-dinitrophenylhydrazone derivative, and this derivative forms a precipitate. Now the exact name of the derivative depends on the aldehyde or ketone used in the reaction.
And in the exam you won't be expected to name or draw the structure of a derivative. So the formation of a yellow or orange precipitate with 2,4-DNP tells us that we have an aldehyde or a ketone present. Now we can also use 2,4-DNP to identify which specific aldehyde or ketone is present. To do that, we need to determine the melting point of the precipitate produced in the reaction. First, we react our carbonyl compound with 2,4-DNP to produce the yellow-orange solid.
We then separate the solid by filtration. Now this solid will be a relatively impure chemical, so next we re-dissolve the solid in a small amount of ethanol in a warm water bath. We then cool the solution down, causing the yellow-orange solid to recrystallize. These will be relatively pure crystals of the precipitate. We then allow the crystals to dry.
And finally we determine the melting point of these crystals. Now the melting point of the solid will depend on which aldehyde or ketone was used in the reaction. So by looking up the melting point in a database, we can determine which aldehyde or ketone was present in the reaction.
Now sometimes several 2,4-DNP derivatives can have similar melting points. So in these cases, We might also need to consider the boiling point of the carbonyl compound. By combining boiling point data with the melting point of the 2,4-DNP derivative, we can accurately identify our carbonyl compound. In the next video, we look at how to distinguish between aldehydes and ketones.