[OccamsAftershave]

This was a question in an exam, and honestly it put me off for a bit. Here's what I did, please tell me if I was right or wrong ?

1) To get a meta arrangement, I need a meta director, so I start with Nitration with HNO3 and H2SO4.

2) I then chlorinate the Nitrobenzene with Chlorine and Ferric chloride, and then hydrogenate with hydrogen and platinum as a catalyst.

3) I add sodium nitrite and lower the pH, then add CuBr and HBr to carry out bromination via diazo mechanism.

There were other options and they were :

a) 1) Sulphonation by conc. H2SO4, 2) Chlorination as above, 3) Bromination via PBr3.

b) 1) Chlorination as above, 2) Nitration as above, 3) Hydrogenation as above, 4) NaNO2 in low pH as above, 5) Addition of CuBr and HBr as above.

-> I rejected this due to the chlorine directing NO2 towards Ortho and para spots.

c) 1) Nitration as above, 2) Hydrogenation as above, 3) Adding CH3CO-O-COCH3 and pyridine, 4) Chlorination as above, 5) NaNO2 in low pH as above, 6) CuBr and HBr.

Which option was right ? And why ?

[Hunter2]

I think only the first 3 steps are ok. Some you already x out. The last c you forgot to hydrolyse the amide.

[Raphael]

The way you did it was my first thought too.

[OccamsAftershave]

The hydrolysis was implied in the options ( by a h3o+ ) so I take it that I did it right.

Why are a and c incorrect, exactly ? Bromine can't displace SO3H ?

[Raphael]

The hydrolysis was implied in the options ( by a h3o+ ) so I take it that I did it right.

Why are a and c incorrect, exactly ? Bromine can't displace SO3H ?

I am far from an expect on these things, but I have never heard of PBr3 being used that way. It is usually used to convert primary and secondary OH groups into alkyl bromides. I see no reason why it would displace the SO3H group. Considering that in theory you can use the SO3H group as a blocking/directing group and then remove it, it would make me believe that there would be no change to the group by adding PBr3.
Someone correct me if I'm wrong.

[Pranav]

c is incorrect because if you chlorinate after 3), you get para (major) and ortho isomer. What's formed after step 3) ?

[OrganicSyn1.0]

I agree with your three steps. Although I can also synthesize m-chlorobromobenzene by displacing the nitro group on m-chloronitrobenzene(from benzene nitration and then EAS with FeCl3/Cl2)  with a Bromine nucleophile via a SNAR mechanism.

[neobenzene]

Here's an alternative method.
First do nitration, so you will get nitrobenzene.

Then carry out bromination, so you'll get m-bromonitrobenzene.

Then, react this with Fe and HCl, which will convert the -NO2 into -NH2...so you'll get,
m-bromoaniline.

Then do diazotization, which will give m-bromo benzene diazonium chloride.

Finally, react this with CuCl and HCl (Sandmeyer's reaction), which will give you the m-chlorobromobenzene

Hope it helped!