[uvcyclotron]

Here I have this 3-part question which requires me to convert nitrobenzene to
I)1,2-dinitrobenzene (o-isomer)
II)1,3-dinitrobenzene (m-isomer)
III)1,4-dinitrobenzene (p-isomer)

I figured out that I could use Sn/HCl to convert the existing -NO₂ to -NH₂, then perhaps I could use N₂O₅, or Nitrating Mixture (conc HNO₃ with conc H₂SO₄) to attach nitro at ortho, but how to get the amino group back to nitro now..

And what to do for the other parts of the question..?

[AnkurGel]

II can be formed easily by just nitrating mixture reaction giving meta product in majority.
As for I and III, you figured it out much yourself. Convert NO₂ to NH2 by Sn/HCl (Stephens reaction). Then introduce nitro group by nitrating mixture or N₂O₅ in slightly acidic medium. You will get ortha and para product in almost equal proportion.
Now, the main part: To convert that NH2 to NO₂. This can be done easily in two ways

Way 1: By reacting with NaNO₂ + HBF₄ to give diazonium salt and then, by reacting the salt further with HNO₂ in Cu₂O.
Way 2: By oxidation through trifluoroperbenzoic acid i.e, CF₃COOH or Caro's acid (H₂SO₅)

[uvcyclotron]

II can be formed easily by just nitrating mixture reaction giving meta product in majority.

I doubt that, because -NO₂ being a very strong -M group, deactivates the benzene ring to such a large extent that it does not take part in Friedel-Crafts reactions on its own, in fact nitrobenzene is used as a solvent in some FC reactions AFAIK. So I guess, we need some other method for this.

And regarding the ways you provided for the -NO₂ to -NH₂ conversion, I sadly haven't heard about these. Are you really sure about them? Can you give me some more details/ link for the part where you suggest using HNO₂ with Cu₂O  on diazonium salt?
and also for the 2nd method you suggested?

UPDATE:
I looked up in my old notes and discovered that the professor had used AcCl followed by NM on the aniline obtained from Stephen's reduction, while I know that this leads to Ph-NH₂ being converted to Ph-NH-C(O)-Me, which is also a +M group but a weaker one as compared to -NH₂. So How does this help with the reaction..?
Any Ideas?

[SVXX]

If you could expand the following abbreviations -:
AcCl(acetyl chloride?)
NM

I could possibly be of more help to you then. I'm quite bad at most abbreviations.

[AnkurGel]

I can be wrong about direct nitration as I told for (II). To solve this trouble, you can simply use Schotten-Baumann reaction.
You haven't heard about conversion method of -NO₂ to -NH₂ ? Well. Obviously, that's why you put up this question. Anyway, the way I mentioned about is diazonium method. Initially I converted -NH₂ to diazonium salt. Then in the presence of HNO₂ and mild oxidizing agent to facilitate the reaction (Cu₂), diazonium leaves N₂ and HCl and gives you nitro group in lieu of it.

For way 2: This is direct way of obtaining NO₂ from NH₂.  Alas! I don't know about the mechanism and perhaps, any other high qualifier in organic may supply with it. This method is based on vigorous oxidation.
How actually you think you can achieve this by oxidation.. Think and post about it. There is another easy way to do this too.
I suggest you to read your old notes first and make out things initially yourself.

@SVXX: Right! @uvcyclotron: We don't understand chat lingo.

[orgopete]

A common strategy you might find in different textbooks is to make meta-dinitrobenzene and selectively reduce one nitro to the aniline with Na2S. Acylation and nitration will give a mixture of 2,3 and 2,5-dinitroacetanilide. Hydrolysis, diazotizatio, and hypophosphite reduction of the diazonium salt replaces it with a hydrogen.

I don't know if any of the more modern routes is superior to this classic route.

[uvcyclotron]

@SVXX: Right! @uvcyclotron: We don't understand chat lingo.

First of all, thats not chat lingo, Ac is widely used for Acetyl group in chemistry, and for NM that's Nitrating Mixture, I mentioned that in full earlier here, so I thought people would get that. Sorry if you didn't.

As for the Schotten–Baumann reaction you are mentioning, I already said that in my update..

this leads to Ph-NH₂ being converted to Ph-NH-C(O)-Me, which is also a +M group

though I don't understand how this helps in forming the meta product as suggested by you.

And yes, the 1st method explained by you (using the diazonium salt) seems plausible now.

" easy way to do this.." yeah I am looking for the same thing, I found some exotic methods online, but they are beyond me..
http://www.organic-chemistry.org/synthesis/N2O/nitrocompounds.shtm

This one is not for aryl amines, but might be of interest to some.
http://pubs.acs.org/doi/abs/10.1021/jo00084a047

Also, I found out the purpose (or so I think) of using AcCl (Acetyl Chloride !! ), it is used to get the para product in greater yield, because the bigger group would reduce EAS on the ortho positions. The amine group can be recovered later by hydrolysis of the amide.

So finally, I guess the only problem left now is how to get the meta product, and to find an easier way to oxidise NH₂ to NO₂

[SVXX]

http://www.freepatentsonline.com/5998661.html

This might be of some use. The reagent used was a monopersulfate in conjunction with ozone, and the solvent was a mixture of sodium bicarbonate and acetone. It caused direct oxidation of amino to nitro group.

[uvcyclotron]

@SVXX thanks for the help,this one is good, but again this is a bit complicated, I think the diazonium method pointed out by AnkurGel is the simplest so far, hope that it works though.

@orgopete, SVXX :You have any ideas on how to get the meta product?

[AnkurGel]

@uvcyclotron: Yeah! By schotten-baumann reaction the EAS product obtained will be major in ortha & para.
In my class notes, it is specified that direct nitration of nitrobenzene do works. But as you said, it can be seemingly difficult. I thought of something else to solve this; but this can be a bit lengthy.

From nitro-benzene to m.nitro nitrobenzene
Nitro-benzene + Cl₂/FeCl₃ --> m-chloro nitrobenzene (A)
(A) + NaNH₂ / NH₃ ---> benzyne intermediate of nitrobenzne ---> m-nitro aniline (cine product)(B) + o-nitro aniline (direct substitution) (C)
considering deactivating nature of NO2 present on benzene ring, cine product will be major.
Now (B) can be converted to required final product by the methods initially mentioned (either diazonium method or oxidation).
Do correct me where I'm wrong.
-AnkurGel

[SVXX]

Direct preparation of m-dinitrobenzene is possible as given here : http://pubs.acs.org/doi/abs/10.1021/ed016p35
Ankur, the benzyne pathway will not be followed in your case. It requires drastic conditions, and the presence of a highly deactivating group relaxes reaction conditions. The S_NAr product will be major.

[AnkurGel]

Thanks for correcting SVXX. It was just an attempt.