[PopeUrban]

In my advanced chemistry class (it's mainly a precursor to college organic chemistry classes) we got a work sheet.  On it, there is a problem that goes as follows "Using benzene or toluene as the only aromatic organic starting material, devise a synthesis for each of the following."  My problem is I can't figure out how to devise a synthesis and it's the weekend so I can't take the problem to my teacher.  The first problem under that is m-bromonitrobenzene and if anyone could help me understand how to do this I would greatly appreciate it.  (I should probably mention that I don't actually need help solving all the problems, because once I understand the process I catch on quickly.  What I need to know is the process for solving them.)

-Pope urban

[Yggdrasil]

The first thing you need to look at is how to get each of your substituents onto the benzene ring.  Almost always, you will do this through electrophilic aromatic substitution.  So for m-bromonitrobenzene, you need to introduce a bromine onto the ring and a nitro group onto the ring.  So, the first step would be to look up the appropriate reactions for introducing bromine and nitro groups via electrophilic aromatic substitution.

Next, you figure out the order you put these thing onto the ring.  One (minor) consideration is whether one substituent will interfere with the reaction conditions necessary to put the other substituent on (this mainly applies to amine groups).  However, the major consideration is getting the substituents in the right place on the ring.  Here you must consider whether your substituents are ortho/para- dirrecting or meta-dirrecting.  In the case of m-bromonitrobenzene, you want to add the meta-dirrecting substituent first so that the second group will be in the meta position.

Feel free to post the solution you come up with and we'll be happy to check it.

[PopeUrban]

Here's what I came up with (and thusly feel rather idiotic because of how obvious it seems after the fact).  Start with Benzene, add Bromine to it and you get Bromobenzene.  To Bromobenzene you add Nitric Acid (concentrated I think, but I'm not totally sure if it matters) and thus receice m-bromonitrobenzene.

[Yggdrasil]

Incorrect.  For a few reasons:

Bromine is not good enough as an electrophile to react with an aromatic system.  You need some sort of catalyst like AlBr₃.

Nitric acid alone will not react with an aromatic system.  Again, you need additional reagents, this time conc. sulfuric acid.  A mixture of conc. sulfuric acid and conc. nitric acid will generate nitronium ion, which is a good electrophile for EAS.

Bromine is an ortho/para-dirrecting group because the halide's lone pair can be donated to the aromatic ring.  Bromination followed by nitration results in o-bromonitrobenzene and p-bromonitrobenzene.  In order to obtain the meta product, you must nitrate first.  The nitro group is a strongly electron withdrawing group, so it is meta-dirrecting.  Bromination of nitrobenzene will result in m-bromonitrobenzene.

I would suggest reading over the section on electrophilic aromatic substitution in your book to learn the reagents you need to perform all of the basic EAS reactions (halogenation, sulfonation, alkylation, acylation, nitration, and amination) as well as reviewing your ortho/para dirrecting and meta dirrecting groups.

[plu]

Bromine is not good enough as an electrophile to react with an aromatic system.  You need some sort of catalyst like AlBr₃.

Br₂,FeBr₃ would do the trick for this bromination.  We save our aluminum halides for our friedel-crafts alkylations and acylations

[GSR]

hi,

for the synthesis of m-bromonitrobenzene here is the pathway


          HNO3/H2SO4                    Br2/Febr3
C6H6----------------->  C6H5NO2 ------------------> m-Bromonitrobenzene

if you first brominate and then nitrate you get  ortho and para isomers.

for designing and organic synthesis  the chemists usually follow the retrosynthesis( working backwords)/disconnection approach or FGI ( functional group interconversion) methods.

here are some links (from google)  to now how to design a synthesis

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch14howsynthesis.html

http://www.uea.ac.uk/~c286/Interactive/start.htm

http://www.uea.ac.uk/~c286/notes/retrosynthesis.htm

and a book by stuar warren on designing organic synthesis the synthon approach and workbook for organic synthesis, the disconnection approach are the excellent ones.

I hope this will help you.

regards

GSR
 

[PopeUrban]

Thanks everyone for that, it took me a bit but I finally realised it was m-bromonitrobenzene and not o-bromonitrobenzene.  That misunderstanding explains why I wanted to brominate first.  

But anyways, thank you to everyone who helped me, I appreciate it very much.