[Janey]

Hello everyone  ,
we had a homework on benzene and this far I have solved all tasks left one:
finding a way to synthesize 1-bromo-3-chloro-benzene out of chlorobenzene.
But as far as I know chlorine is ortho-para directing  
Is there maybe something I have to substitute to the benzene before substituting the bromine and remove it afterwards?
Unfortunetaly I can't find any such an example how to get it into meta position in one of my books *aarrrgh*
I'd be really happy if someone had an idea ^^

[ksr985]

1 bromo 3 chloro benzene... i am assuming this is NOT asking for the most viable lab synthesis:

1. Nitrate benzene in mild conditions -----> nitrobenzene
2. Brominate the product-----> 1 nitro 3 bromo benzene
3. Prepare the diazonium salt with NaNO2/ HCl
4. Sandmeyer reaction( CuCl 60-100 degree C) will give the desired product.

[Janey]

I'm afraid it is not, here it says chlorobenzene is the educt.
Thx for your reply, but there has to be another way...

[movies]

Ksr's route is certainly practical if benzene where the starting material.  It may be possible to use the same idea from chlorobenzene:

First, make the Grignard reagent from chlorobenzene and quench with an electrophilic NO₂ source like tetranitromethane or N₂O₄.  That ought to give nitrobenzene and then you can go about ksr's route.

[GCT]

Hello everyone  ,
we had a homework on benzene and this far I have solved all tasks left one:
finding a way to synthesize 1-bromo-3-chloro-benzene out of chlorobenzene.
But as far as I know chlorine is ortho-para directing  
Is there maybe something I have to substitute to the benzene before substituting the bromine and remove it afterwards?
Unfortunetaly I can't find any such an example how to get it into meta position in one of my books *aarrrgh*
I'd be really happy if someone had an idea ^^

I imagine it can be accomplished with blocking groups, addition of bromine and subsequent removal.  I wonder, addition of nitro so that all ortho-para positions are filled, subsequent mild electrophilic addition of bromine...removal of nitro groups.  Movies, you think it can be done?

Have anyone of you considered a nucleophilic route, I'll need to review my text, but this might be a better alternative perhaps.

[Donaldson Tan]

the gist is:
1. introduce a 3-directing substituent to benzene
2. add bromo group
3. substitute the 3-directing substituent with chloro

using -NO2 as the 3-directing group faciliates its subsequent substitution via the diazonium route.

[movies]

I imagine it can be accomplished with blocking groups, addition of bromine and subsequent removal.  I wonder, addition of nitro so that all ortho-para positions are filled, subsequent mild electrophilic addition of bromine...removal of nitro groups.  Movies, you think it can be done?

Yeah, that was my first thought, but you would need a blocking group that would be an ortho-director and I don't recall any.  The most common one (sulfate) is a meta-director.

The S_NAr route is interesting.  Might be a bit steppy though.

[dexangeles]

guess what I found....

I believe the Zn/HCl is suppose to be the Clemmensen Amalgam of Zn(Hg), HCl

[movies]

guess what I found....

I think that only the second one would give the right product, don't you?  And that uses the wrong SM.

[GCT]

Yeah, that was my first thought, but you would need a blocking group that would be an ortho-director and I don't recall any.  The most common one (sulfate) is a meta-director.

The S_NAr route is interesting.  Might be a bit steppy though.

Not neccessarily SNAr, but nucleophilic substitution in general, I thought this would be a neat approach, although it may not turn out to be practical.

[dexangeles]

I think that only the second one would give the right product, don't you?  And that uses the wrong SM.

isn't the Clemmensen amalgam a reducing agent?

[ksr985]

looks like only the second one is correct.

[movies]

isn't the Clemmensen amalgam a reducing agent?

Yes, in this case you would be reducing a nitro group to an amino group.  I don't think you actually need to use the amalgam for that reduction, just zinc dust and an acid.

[dexangeles]

a) ok the nitro gourp is going to be para
then reduced to p-chloroaniline


b) the chlorine will be meta to the nitro group
then reduced to m-chloroaniline

is this correct so far?
what exactly does the 3rd synthesis step do?
with aniline in the mix this will control where the bromine is added right which would be ortho to it A and para to it in B, is this correct so far?

[dexangeles]

ok I think I get it....it forms a diazonium salt. so it is B then