6
$\begingroup$

This is a commonly used confirmatory technique in organic functional group analysis. The test is performed by addition of meta dinitrobenzene to the given organic sample followed by addition of a base such as sodium hydroxide. The indication is given by a violet colouration.

Is the probable mechanism based on carbanion formation? Also it would be helpful if exceptions to this reaction and other variants in colour produced by different ketones. My main requirement is to know why aldehydes do not respond to this test.

$\endgroup$
3
$\begingroup$

In the Janovsky reaction, an aldehyde or ketone which contains at least one $\alpha$ methylen group reacts with 1,3-dinitrobenzene and a strong base (like $\ce{NaOH}$) to form a colored Meisenheimer complex (source). The enolate acts as a nucleophile which attacks the aromatic ring that is electron-deficient due to the presence of multiple electron-withdrawing nitro substituents.

enter image description here

image source

As the reaction mechanism relies on the formation of an enolate, non-enolizable aldehydes like, for example, formaldehyde or benzaldehyde, do not resond to this test. Likewise, non-enolizable ketones (like benzophenone) cannot react in this way. The color of the formed complex varies with the number of nitro substituents of the benzene ring, with 1,3-dinitro compounds giving blue complexes, while trinitro compounds yield a red coloration (source).

This paper contains a list of the colors produced by various aromatic nitro compounds when reacted with acetone and $\ce{NaOH}$. It is reported that most mononitro compounds do not produce any color at all, with the exeption of 3-nitro-4-aminotoluene (orange) and 3-nitro-4-aminotoluene benzoate (orange-red). Some examples for the colors obtained from the reaction of dinitro compounds are: m-dinitrobenzene (purplish blue), 2,4-dinitrotoluene (blue), 2,4-dinitrophenol (yellow-orange), 2,4-dinitroresorcinol (brownish-green) and p-dinitrobenzene (green-yellow).

$\endgroup$
  • $\begingroup$ It would be great if you could append a list of the colour of various complexes formed in the Janvosky (and Zimmerman reaction). Also, most of the info I have found regarding Janvoski reaction has been confined to Wikipedia (including one that you posted) and a couple of papers (most behind a paywall), so incase you do happen to find more, could you please append to this? Anyways, I have accepted this answer. Thanks a lot. $\endgroup$ – Varun Feb 13 '16 at 15:15
  • $\begingroup$ @Varun I have added some examples for the colors of various complexes. $\endgroup$ – Jannis Andreska Feb 13 '16 at 15:40
  • $\begingroup$ It would be great if you could append a list of the colour of various complexes formed in the Janvosky (and Zimmerman reaction) I used the Janovsky reaction (e.g. Zimmermann too) for a project about the synthesis of acridines in the seventhies. I can send you a list as wished published in Microchimica Acta (Wien) 1968, 739-747 and the paper too. The author is L. Legradi (Hungary). For this purpose, I need your email address. The paper is in German but you can find a summary in English for all the papers of L. Legradi. All these infos are free of charge in Google or Google Scholar. $\endgroup$ – Alan Bic Jan 23 '18 at 18:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.