In this post, Ron discusses the reasons why carboxylic acid derivatives such as amides, acids and esters do not give a positive result with 2,4-dinitrophenylhydrazine. He mentions that it is largely due to the fact that the carbonyl carbons in these compounds are not sufficiently electron-deficient to react with the hydrazine derivative due to strong conjugation within these compounds. What about acyl chlorides and possibly, acid anhydrides as well? These carboxylic acid derivatives are clearly rather electron-deficient, certainly much more so than aldehydes and ketones, as evident from their great ease of hydrolysis. Why don't they react with hydrazine to give hydrazone?

I think that the reason may be that $\ce {Cl^-}$ and $\ce {RCOO^-}$ may be good leaving groups that leave after the initial formation of the tetrahedral intermediate. In the case of acyl chloride, we may get a hydrazide instead of a hydrazone. This is likely the same for the case of acid anhydrides. Is my thinking correct?

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    $\begingroup$ You get the acyl hydrazide RCONHNHPh. These form easily with acyl chloride and anhydrides but the are not coloured llike hydrazones. $\endgroup$
    – Waylander
    Commented Mar 31, 2019 at 8:06


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