What is the Mechanism of Trinitration of Phenol?

[proton007007]

What is the mechanism of trinitration of phenol ?

Is it one by one substitution of nitro groups ?
For which wouldn't the nitro groups stop the reaction as they are deactivating groups

Or is it the simulataneous activation of ortho and para positions of phenol and the nitro groups add on together ?
For this mechanism , deactivating nature won't be a concern . but the probability of such a collision would be less . And this can be reason of the low yield .

Please clarify the correct concept of the above .

 

[Raghav Gupta]

Note you have to use conc. HNO₃ for forming trinitrophenol otherwise using dilute will form in majority para nitro phenol.
You know that OH is a para and ortho activating group and nitro is a ortho and para deactivating group meaning a meta activating group.We have excess of NO₂⁺ in conc hno3.
So trinitration product is formed.

 

[proton007007]

What about the mechanism ? Is it one by one ?

 

[Raghav Gupta]

What about the mechanism ? Is it one by one ?

Yes it should be.
Theoretically first a para nitro is formed, then it acts as meta directing and obviously phenol is ortho directing.
So then ortho nitro also bonds and it also being meta directing further helps in making a trinitrophenol .

 

[proton007007]

but wouldn't the first nitration deactivate the ring?

 

[Raghav Gupta]

but wouldn't the first nitration deactivate the ring?

Yeah it is deactivating ortho positions and para positions with respect to nitro group. But for OH group that positions are meta and where OH group is attached and if you look there are no nitro groups there.

 

[proton007007]

But in that case trinitration of toluene shoulalso happen which is not correct

 

[proton007007]

even a weak o-p director should give trinitration but all don't give

 

[Raghav Gupta]

Now in toluene I effect takes place and in phenolic group mesomeric effect or resonance effect which is because of lone pair of electrons of O. In toluene carbon does not have that lone pair.

 

[proton007007]

the methyl group activates o-p by HC , if nitration indirectly activates meta position , why shouldn't toluene give trinitration . Since your argument is not based on the activating group , even a weakly activating group should give the rxn .

 

[proton007007]

And you have got the reason of toluene wrong . Its not +I effect but the hyperconjugation .

 

[Raghav Gupta]

And you have got the reason of toluene wrong . Its not +I effect but the hyperconjugation .

Yeah I saw it is hyperconjugation. Do you know pi bonds are more stable than hyperconjugated bonds?
That may be the reason between phenol and toluene as one is donating lone pair and other showing hyperconjugation. Do you agree?

even a weakly activating group should give the rxn .

Can you give some examples? Then we'll verify.

 

[proton007007]

Do you know pi bonds are more stable than hyperconjugated bonds?
That may be the reason between phenol and toluene as one is donating lone pair and other showing hyperconjugation. Do you agree?

Of course the lone pair resonance is stronger . Thats why toluene will take harsher conditions than phenol .

And I meant that by your theory , "even weakly acrivating groups will give the reaction" .

 

[proton007007]

Anyway this was what I posted out there ( same question ) and got the answer . Thanks !

 

[Raghav Gupta]

So it's all temperature dependent?
So in toluene case also we get TNT, but temperature is more required?
Sorry that I am in your thread asking for more.
But chloro benzene when nitrated gives paranitrochlorobenzene, why that is not showing trinitration?
If we raise temp. Does it also shows trinitration?

Also why toluene gives on normal conditions on nitration orthonitrotoluene but phenol with dil.HNO3 gives paranitrophenol and chloro benzene also paranitrochlorobenzene.
Why the steric hindrance factor not comes here?

 

[proton007007]

Almost all reactions are .

Yes . TNT is produced by step by step nitration with increasing temperature and purity of acids .

No problem .

Chloro group is weakly deactivating itself .

In chlorobenzene case , polynitration would be dependent on temperature to an extent but stating anything more without referring recent literature would not be a good idea .

Toluene gives ortho as major . Its steric effect is very less and its +I effect is more significant on ortho position (hence more ease for EAS).
Phenol gives a mixture , though ortho is slightly more due to H-bonding . -OH group has negligible steric effect . But significant para is formed (though ortho is slightly more , para is not a minor )due to -I effect of -OH which is less at para .

Chlorobenzene gives para as both -I effect and steric reasons are in favour of para .

The line breaks will distinguish your multi-question-filled answer .

 

[Raghav Gupta]

Thanks a lot.
Can you do a favour as I need desperately the answer for this since you have started the first reply in this thread?

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