Question

Propose a reason for why 2,4-DNP or semicarbazone derivatives usually precipitate out of their ethanolic solutions.

Answer 1

2,4-dinitrophenylhydrazine is often abbreviated to 2,4-DNP or 2,4-DNPH. A solution of 2,4-dinitrophenylhydrazine in a mixture of methanol and sulphuric acid is known as Brady's reagent.

Although the name sounds complicated, and the structure looks quite complicated, it is actually very easy to work out.

Start with the formula of hydrazine. That's almost all you need to remember!

Hydrazine is:

In phenylhydrazine, one of the hydrogens is replaced by a phenyl group, C₆H₅. This is based on a benzene ring.

In 2,4-dinitrophenylhydrazine, there are two nitro groups, NO2, attached to the phenyl group in the 2- and 4- positions. The corner with the nitrogen attached is counted as the number 1 position, and you just number clockwise around the ring. Details vary slightly depending on the nature of the aldehyde or ketone, and the solvent that the 2,4-dinitrophenylhydrazine is dissolved in. Assuming you are using Brady's reagent (a solution of the 2,4-dinitrophenylhydrazine in methanol and sulphuric acid): Add either a few drops of the aldehyde or ketone, or possibly a solution of the aldehyde or ketone in methanol, to the Brady's reagent. A bright orange or yellow precipitate shows the presence of the carbon-oxygen double bond in an aldehyde or ketone. This is the simplest test for an aldehyde or ketone. The overall reaction is given by the equation: R and R' can be any combination of hydrogen or hydrocarbon groups (such as alkyl groups). If at least one of them is a hydrogen, then the original compound is an aldehyde. If both are hydrocarbon groups, then it is a ketone. Look carefully at what has happened. Provided you take care to draw the two starting molecules lined up right, working out the structure of the product is easy. The product is known as a "2,4-dinitrophenylhydrazone". Notice that all that has changed is the ending from "-ine" to "-one". That's possibly confusing! The product from the reaction with ethanal would be called ethanal 2,4-dinitrophenylhydrazone; from propanone, you would get propanone 2,4-dinitrophenylhydrazone - and so on

Dinitrophenylhydrazine does not react with other carbonyl-containing functional groups such as carboxylic acids, amides, and esters. Forcarboxylic acids, amides and esters, there is resonance associated stability as a lone-pair of electrons interacts with the p-orbital of the carbonyl carbon resulting in increased delocalization in the molecule. This stability would be lost by addition of a reagent to the carbonyl group. Hence, these compounds are more resistant to addition reactions. Also with carboxylic acids there is the effect of the compound acting as a base, leaving the resulting carboxylate negatively charged hence unable to be attacked by this nucleophile.