Sciencemadness Discussion Board

Optimal route to nitrobenzene from benzoic acid?

garphield - 3-8-2022 at 17:54

I just got some benzoic acid, mainly for the purpose of making nitrobenzene. I'm not sure if it would be better to convert it to benzene by decarboxylating it and then nitrating the benzene or if it would be better to make 3-nitrobenzoic acid by nitrating the benzoic acid, then decarboxylating that to make nitrobenzene. The nitro substituent should make 3-nitrobenzoic acid easier to decarboxylate than unsubstituted benzoic acid, but I'm not sure if it would be by enough to let you just heat it in a normal flask until decomposition instead of needing to deal with the weird paint can/pipe still you need for heating it a ton with sodium hydroxide. However, I haven't done either before so I really don't know, which is why I'm asking you guys in case any of you have an idea.

clearly_not_atara - 4-8-2022 at 05:58

Pragmatically, it's probably more efficient to nitrate benzene. Benzoic acid is resistant to nitration and the yield of decarboxylations is low, plus large amounts of nitric acid present handling difficulties. So get a low yield in the first step with cheap reagents and then you can use less HNO3.

But there is a third route you haven't considered: conversion to benzamide by heating with urea (usually high yield), Hofmann degradation with bleach (cheap, ~50% is good yield), and oxidation to nitrobenzene (various methods, usually with peroxide).

For the oxidation of aniline, usually H2O2 is the terminal oxidant, and the choice of catalyst depends on what you have on hand.
One possibility is formic acid / NR4Br: https://pubs.acs.org/doi/full/10.1021/acsomega.9b00543
DCM and phosphotungstic acid also works well: https://www.scielo.br/j/jbchs/a/CcKGMkYP6CRCY6hFRpjkx7P/?for...
cat. RuCl3 + NR4Br:https://pubs.acs.org/doi/pdf/10.1021/jo00275a043
calcined MgO or CaO @TiO2 photocatalysts with 1 atm O2 give nitrosobenzene: https://www.sciencedirect.com/science/article/pii/S092058611...

Main advantage of this route is that you don't need nitric acid.


PirateDocBrown - 4-8-2022 at 07:37

Pretty sure direct oxidation of aniline would be a problem. Normally, you'd go through a diazonium intermediate.

Ph-NH2 + NaNO2 + HCl = Ph-NN-Cl + HBF4 = Ph-NN-BF4 + HNO3 (Cu) = Ph-NO2

digga - 4-8-2022 at 09:05

I would do the decarboxylation to benzene route. Nitrating benzene with nitric acid reliably gives mono nitrobenzene. Nitration of the benzoic acid might well yield a mixture of mono and di nitrobenzoic acids.

When you decarboxylate the benzoic acid with sodium hydroxide, the raw product is contaminated with polycyclic aromatics like tetracene and chrysene. These are carcinogenic. They also fluoresce under UV light. Distillation removes them. When you are done, shine the UV light over your bench and see just how neat you are. I wasn't.

Texium - 4-8-2022 at 09:23

Quote: Originally posted by PirateDocBrown  
Pretty sure direct oxidation of aniline would be a problem.
Perhaps you should read some of the references that clearly_not_atara posted, which provide some quite convincing looking alternative methods. I particularly like the look of the RuCl3 catalyzed one (60% yield of nitrobenzene reported), having recently obtained a couple grams of it for free. Filed that paper away for whenever I finally get back to amateur chemistry.

PirateDocBrown - 4-8-2022 at 13:31

Quote: Originally posted by Texium  
Quote: Originally posted by PirateDocBrown  
Pretty sure direct oxidation of aniline would be a problem.
Perhaps you should read some of the references that clearly_not_atara posted, which provide some quite convincing looking alternative methods. I particularly like the look of the RuCl3 catalyzed one (60% yield of nitrobenzene reported), having recently obtained a couple grams of it for free. Filed that paper away for whenever I finally get back to amateur chemistry.


That's true. Sure, there's lots of ways, but I just showed the most common. You get a decent yield, too.
RuCl3 can be pretty pricey, but if you have it, by all means use it.

Aniline can have runaway oxidations, so caution is recommended.

Σldritch - 5-8-2022 at 12:00

Hunsdiecker? And if you can't go straight for the nitro group due to, say, exotic reagents required: I would suggest using Nitrosyl Chloride as the oxidizer to go to nitrosyl which can be easily oxidized to nitro any number of ways IIRC. I don't have much experience with organic chemistry though.