Keras
International Hazard
Posts: 1013
Registered: 20-8-2018
Location: (48, 2)
Member Is Offline
|
|
Why is the nitrating power of nitrites and nitrates so different?
Folks,
when I need a couple of grams of p-aminophenol, I usually make it in two steps starting from phenol (~ 1 g in 15 mL water), 37% sulphuric acid (~ 2
mL) and a small quantity of sodium nitrite in 10 mL water. Of course, the erlenmeyer containing the phenol solution is cooled in an ice bath until it
is well under 0 °C and the reaction works well under these conditions, producing p-nitrosophenol that I reduce immediately with sodium hydrosulphite.
Today, I was planning to make o-aminophenol because I’m interested in catechol (after diazotisation). So instead of sodium nitrite I used potassium
nitrate, but to my surprise, nothing happened at all at 0 °C. I waited for the solution of phenol, acid and nitrate to warm up to room temperature,
but even then, all I got was a sort of small dark red blotch of something floating on the surface. So I decided to try and steam distill (one never
knows) and… the reaction picked up at some point (can’t say exactly when, I had the flask wrapped in aluminium foil, but certainly over 50 °C).
Finally, I got a fair amount of dark orange solid stuff that I immediately proceeded to reduce using sodium hydrosulphite and zinc metal (not finished
yet, have to filter).
I’m quite puzzled, because I was expecting phenol, which has an activated aromatic cycle, to react much more easily with nitrate, as it does with
nitrite. Is nitrite such a powerful oxydant, or nitrate such a poor one?
[Edited on 3-3-2025 by Keras]
|
|
|
SplendidAcylation
Hazard to Others
Posts: 209
Registered: 28-10-2018
Location: Starving in some deep mystery
Member Is Offline
Mood: No one I think is in my tree.
|
|
Unfortunately I don't really have any idea about your question, but I was wondering, have you considered making p-aminophenol by hydrolysis of
paracetamol?
Certainly sounds easier, but I know amide hydrolysis can be a bit slow and stubborn.
Anyway, sorry I can't contribute anything more useful.
|
|
|
Keras
International Hazard
Posts: 1013
Registered: 20-8-2018
Location: (48, 2)
Member Is Offline
|
|
Lol, I precisely do the opposite, ie synthesise paracetamol from phenol 
Over here in Europe, you can’t really buy Tylenol pills by the kilo as you can in North America…
|
|
|
MrDoctor
Hazard to Self
Posts: 60
Registered: 5-7-2022
Member Is Offline
|
|
nitrite, and, nitrous- acid, is not the same as nitrate- and nitric acid. my understanding is that nitrite/nitrous acid can behave as a radical and in
some circumstances will dissociate to facilitate a reaction. nitrite readily converts back to NO/NO2 especially if there is something to take the
place of that ion.
you can probably think about it being analogous to chlorites vs chlorates
|
|
|
Keras
International Hazard
Posts: 1013
Registered: 20-8-2018
Location: (48, 2)
Member Is Offline
|
|
Well, in both cases, after the formation of the respective acid, the acid loses a OH group because of protonation by sulphuric acid.
KNO₃ + H₂SO₄ → HNO₃ + KHSO₄ (1)
HNO₃ + H₂SO₄ → NO₂⁺ + H₂O + HSO₄⁻ (2)
And NO₂⁺ proceeds to attack the phenol.
With nitrite:
NaNO₂ + H₂SO₄ → HNO₂ + NaHSO₄ (1')
HNO₂ + H₂SO₄ → NO⁺ + H₂O + HSO₄⁻ (2')
And NO⁺ attacks the phenol.
So I must somehow conclude that NO⁺ is a way better electrophile than NO₂⁺. It is somehow suggested that nitrites are more powerful oxidants,
but I didn't suspect the gap was that big, especially given that most 'standard' nitrations I watched being made used an ice bath to keep the reaction
under control. In my case, except for a precipitate of potassium bisulphate, nothing much happened until 70 °C circa.
|
|
|
Sulaiman
International Hazard
Posts: 3779
Registered: 8-2-2015
Member Is Offline
|
|
I came across the activity of the nitrite ion when I researched cleaning mercury.
There is a reference a few posts down
https://www.sciencemadness.org/talk/viewthread.php?tid=16034...
CAUTION : Hobby Chemist, not Professional or even Amateur
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
