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In analytical chemistry, Brady's reagent is an aqueous solution of methanol, sulphuric acid, and the active ingredient 2,4-dinitrophenylhydrazine. Brady's is used to test for aldehydes and ketones, which contain a carbon-oxygen double bond, C=O.

Info on 2,4-dinitrophenylhydrazine:

Its mechanism for the detection of carbonyl compounds is fairly simple. 2,4-dinitrophenylhydrazine can be imagined as hydrazine (NH2-NH2) in which one H atom has been replaced by a phenyl radical with some nitro groups attached. Ignore the dinitrophenyl group, which does not take part in the reaction, and consider the hydrazine group - the reaction concerns the outer N atom on this group and the carbonyl C atom in the carbonyl compound.

First of all, the lone pair of electrons on the outer nitrogen atom (in the NH2 part) bonds to the carbonyl carbon, while one of the H atoms in the hydrazine is transferred to the carbonyl oxygen atom.

This addition reaction results in an intermediate compound which is not very stable. The -OH group which has been formed is eliminated along with the other H on the outer nitrogen, releasing water, and the nitrogen and carbon atoms double bond to each other. The compound formed consists of bright yellow-orange crystals, so if you add an organic compound to Brady's and get these crystals, the compound is carbonyl. Compare Brady's reagent, which indicates the presence of an aldehyde or a ketone, with Tollens' reagent, which only reacts with aldehydes.

As an example, imagine adding ethanal/acetaldehyde (CH3-COH) to Brady's (NH2-NH-dinitrophenyl). First, the outer N would bond to the carbonyl C while one H would join the carbonyl O, forming (CH3-CHOH-NH-NH-dinitrophenyl). Water would then be eliminated to leave you with (CH3-CH=N-NH-dinitrophenyl, a yellow crystalline solid).

2,4-dinitrophenylhydrazine has this displayed formula:

      
                  H   H
                  |   |
      H           C - C      
       \         /     \    
        N - N - C   O   C - N - O 
       /    |    \     /     \ /
      H     H     C - C       O 
                  |    \
                  N     H
                 / \ 
                O - O
This is ethanal (acetaldehyde):
    H
    |
H - C - C = O
    |   |
    H   H
And this is created on reaction with Brady's reagent:
                      H   H
                      |   |
    H                 C - C
    |                /     \
H - C - C = N - N - C   O   C - N - O
    |   |       |    \     /     \ /
    H   H       H     C - C       O
                      |    \
                      N     H
                     / \
                    O - O