Does anyone here have any suggestions for making p-dibromobenzene of reasonable purity? The only synthesis I could find was on Rhodium, which requires
pyridine, which for all the effort to get that, I could just try and buy the original.Chemosynthesis - 16-4-2014 at 19:01
You should references if you're not posting in "beginnings."
I assume you mean here:http://www.erowid.org/archive/rhodium/chemistry/bromobenzene.html
I can't find my notes on exactly this reaction, but iron wire or powder, aluminum, triethylamine, etc. can all scavenge free bromine. For the
dibromobenzene, you can try a higher temperature than bromobenzene, which eludes me at the moment. Post back when you try something.
Edit- now this bothers me. I wish I had my notes. Anyway, check out Advances in Synthetic Organic Chemistry and Methods Reported in US Patents by
Thomas F. DeRos, page 81 section 4.3:
To 5 mL benzene in a 100 mL Erlenmeyer flask is added one small piece of iron filings. Cool the flask in ice water and add cautiously 6.0 mL of
bromine. Swirl the flask and allow it to stay at room temperature until the next day. Workup: Add 50 mL of water to the flask and a small magnetic
stirring bar. Heat the contents to boiling with vigorous magnetic stirring. Cool the mixture until the dibromobenzene solidifies, and decant the
water. Repeat the water treatment once, followed by a third washing of 50 mL 2% NaOH. Place the flask on a steambath and add ca. 80 mL of ethyl
alcohol (95%). When the crude product dissolves, filter the hot solution through a fluted filter. Heat the filtered solution to boiling and
carefully add hot water until the solution becomes slightly turbid. Place the hot flask in a larger beaker with cotton wool, cover the mouth of the
flask with a small watchglass, and let the solution cool slowly. Collect the precipitated crystals by filtration, dry in air on a large filter paper.
[Edited on 17-4-2014 by Chemosynthesis]Metacelsus - 17-4-2014 at 04:55
First, get p-bromoaniline (by brominating aniline). Then, do the Sandmeyer reaction.
I think sequential bromination of benzene would form m-dibromobenzene preferentially.
I'm not sure I agree. Halogens tend to be o/p directors due to their resonance.
US 3062899 A and 4762955A also appears to disagree, as does US 3345423A and Influence of Temperature on the Substitution Type in the Bromination and
Chlorination of Aromatic Compounds in the Gas Phase by Sixma and Wibaut.
AlCl<sub>3</sub> use decreases p-dibromobenzene formation with higher temperature kinetics, but I am under the impression yields with
FeCl<sub>3</sub> are adequate without necessitating the additional reactions step and reagents of a Sandmeyer.
Edit- additional patent
You are probably thinking of the aromatic halogenation of toluene, which does require a Sandmeyer to be practical. Verified from Org. Synth. 1925, 5,
21 and the Fundamental Processes of Dye Chemistry section on halogenations of benzene and derivatives (p65-68). The latter citation notes that
dichlorination of benzene results in o/p products, as expected, and that everything pertinent to chlorine is generally applicable to bromine in these
reaction. I have more references on toluene with catalytic iodine to shift products towards p-halogenation at home, but I'll refrain.
