[cayax]

Hello everyone,

My partner and I are doing undergraduate research. Currently, we are trying to double brominate 1,12-dodecanediol to simply have Br substitute both -OH groups. We keep on running the experiment, but based on TLC analysis, there's always a mixture of the monobrominated and dibrominated products. Of course, we tweak some parameters, and our current protocols are as follows.

A quick summary of our procedure:
1. 0.5 g of 1,12-dodecanediol.
2. 3 mL of 8.9M HBr
3. Set it up in a reflux apparatus.
4. High heat.
5 Leave it there for two - three days.

Based on stochiometric ratios, we always put an excess of HBr solution to have both -OH groups substituted. Even with excess HBr, we are still getting a mixture of monobrominated and dibrominated products. Some theories, on to why there a mixture, are that water (solvent) is acting a nucleophile replacing the Br or the temperature is too high. At low temperatures, there is no bromination observed. If you have any suggestions or pointers, it will be much appreciated. Thank you for your time!

[rolnor]

I think you can add som sulphuric acid. This is described in Fieser&Fiesers reagents for organic synthesis Vol1
First I would use more hydrobromic acid. There could be a problem with solubility, there will be a two-phase system and its possible that the acid cant react with the monobromoalcohol when its in the organic, product-phase. Good stirring can be important.

[Borek]

Getting 100% yield would be surprising, wouldn't it?

[rolnor]

100% yield would be suprising, yes, but 100% conversion of startingmaterial should be possible, this is not an equilibrium.

[kriggy]

What about using different reagents such as PBr₃ or PPh3 + CBr₄ ?

[cayax]

I think you can add som sulphuric acid. This is described in Fieser&Fiesers reagents for organic synthesis Vol1
First I would use more hydrobromic acid. There could be a problem with solubility, there will be a two-phase system and its possible that the acid cant react with the monobromoalcohol when its in the organic, product-phase. Good stirring can be important.

Thank you for your advice! We saw what the Fieser & Fieser books said about hydrobromic acid and sulfuric acid. This is actually a great practical book to use for experiment. It also gave the exact concentration of HBr that we use. Not sure if 48% HBr is common for synthesis. I also forgot to mention that we had magnetic stirrers in the round bottoms.

We set up this run with more HBr and H₂SO₄. Hopefully, it should yield better results. We thought of re-running the product mixture we had before with xs HBr and H₂SO₄ to run it to completion. We found out today that methanol can easily dissolve the monobrominated product while not being a good solvent for the dibrominated product, so we may use that method to purify dibromo product. One thing we notice is the solution turns a yellow/brown color whenever we don't add a little water. Not sure what that is. We will let you know what happens.

Again, thank you for your advice and the resource!

[cayax]

What about using different reagents such as PBr₃ or PPh3 + CBr₄ ?

Thank you for suggestion! I forgot to mention that we are limited with our reagents. If we want other chemicals, we have to go through a long process of getting them. We will check if we can get those though.

[rolnor]

For purification silica-gel filtering would be good, the dibromo is very lipophilic, even pure hexane can be used as eluent.