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(2,4-Dinitrophenyl)hydrazine, a popular laboratory reagent is used to identify carbonyl compounds by forming 2,4-dinitrophenylhydrazones, which are often red or yellow colored precipitates.

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Why is the specific 2 and 4 positions that cause precipitate formation with carbonyls, and make it a useful reagent? Can (2,6-dinitrophenyl)hydrazine work this way?

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Probably any DNP could work if you could get it. But, because of the directing character of substitutents in electrophilic substitution you inherently favor the 2,4 isomer:

  • the hydrazine group (or more accurately the chlorine atom which is converted to a hydrazine group) is ortho/para directing.

  • the nitro groups are deactivating and meta directing.

So you get the nitro groups meta to each other and ortho/para to the hydrazine group. Therefore 2,4 DNP.

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  • $\begingroup$ So in this way, a 2,4,6-TNP should also work right? $\endgroup$
    – Pritt says Reinstate Monica
    Commented May 17, 2017 at 2:02
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    $\begingroup$ If you can get it, try it. The nitro groups are deactivating and getting three of them on the ring is tough. Might having that much nitro in the molecule present other hazards? (TNT.) $\endgroup$
    – Oscar Lanzi
    Commented May 17, 2017 at 2:07
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    $\begingroup$ Also, sterics probably come into play with 2,6-DNP. With nitro groups on both sides, I think that hydrazine has a hard time getting to the electrophile with the right orientation. $\endgroup$
    – Andrew
    Commented Feb 2, 2020 at 13:08

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