Sciencemadness Discussion Board

Nitration of aniline

adianadiadi1 - 7-6-2018 at 12:07

When aniline is nitrated, para to ortho to meta ratio is 57:2:41. My question is why ortho nitroaniline is just 2%. Ortho position is also activated as that of para.....? I dont think steric hindrance is the reason.

Outer - 8-6-2018 at 01:51

When you use unprotected aniline (and not acetanilide for example), in the presence of acids it is protonated, and this protonated cation (Ph-NH3+) is actually nitrated. Active nitrating agent is NO2+, and because of electrostatic interactions, ortho-position is the least preferable.

Sigmatropic - 8-6-2018 at 09:54

Does anyone know why diazotizations work on anilines? The first step on paper involves the interaction of the nitrogen lone pair with nitrosonium. In my mind I can't picture there being any nitrogen lone pair in conc. HCl (or even diluted HCl for that matter) because the aniline exists as the ammonium salt in such conditions. But I can very much verify that these reactions occur. This discrepancy between practice and theory bothers me.

Tsjerk - 8-6-2018 at 10:16

Apperantly the NO2+ is such a good electrophile that, with amine/NO2+ reaction being irreversible, the reaction proceeds.

Sigmatropic - 8-6-2018 at 10:50

So it is such a good electrophile that it reacts with a N-H bond of an ammonium ion? Hmmm, seems feasible but unlikely although very similar to protonated methane.

Tsjerk - 8-6-2018 at 11:38

Well, nitronium salts have to have a fully oxidized anion to be stable. I have a feeling that extreme electrophiles can do more stuff like this.

If azidozition works, I don't see why for example carboxylic acids couldn't be nucleophiles...

[Edited on 8-6-2018 by Tsjerk]

Outer - 8-6-2018 at 12:05

Quote: Originally posted by Sigmatropic  
Does anyone know why diazotizations work on anilines? The first step on paper involves the interaction of the nitrogen lone pair with nitrosonium. In my mind I can't picture there being any nitrogen lone pair in conc. HCl (or even diluted HCl for that matter) because the aniline exists as the ammonium salt in such conditions. But I can very much verify that these reactions occur. This discrepancy between practice and theory bothers me.

In this reaction PhNH3+ ion interacts with a neutral molecule of HNO2, giving an equilibrium of PhNH2 (aniline) and H2NO2+ (protonated HNO2). The latter one after elimination of water gives NO+ (nitrosonium), which reacts with aniline.

Tsjerk - 8-6-2018 at 12:33

Do I understand you right when I understand you suggesting a weak nucleophile (NH3+) attacking a weak electrophile (HNO2) in a reaction requiring dehydration of a molecule in a aqueous solution while there is an easier and more reasonable explanation for the reaction?

Outer - 9-6-2018 at 01:10

Quote: Originally posted by Tsjerk  
a weak nucleophile (NH3+) attacking a weak electrophile (HNO2) in a reaction requiring dehydration of a molecule

PhNH3+ is not a nucleophile, but after proton transfer (to HNO2) is gives PhNH2 (nucleophile) and H2NO2+ (good electrophile). They can react each with other after dehydration of H2NO2+ (or maybe even without dehydration).

Sigmatropic - 9-6-2018 at 07:56

Wouldn't the proton for dehydration of nitrous come from the acid environment, instead of the ammonium ion? I very much doubt the abysmally small amount of free base amine present in acidic solutions is able to give such a fast reaction (15 minutes at 0c). There must be something else at play here.