Maui3 - 18-12-2024 at 06:26
Hi! I am trying to make dibromomethane like this:
CH2Cl2 + HBr = CH2BrCl + HCl
CH2BrCl + HBr = CH2Br2 + HCl
I know water will interfere with the reaction, but I do not know how dry the HBr needs to be. I know I can't use 48% HBr, but will generating some HBr
from the reaction of sodium bisulfate and sodium bromide be dry enough, or do I need to dry it through calcium chloride or similar?
Dibromomethane is ridiculously expensive where I live, but HBr and dichloromethane are both very very cheap.
Sir_Gawain - 18-12-2024 at 06:35
If your sodium bisulfate and bromide are anhydrous, there should be no water produced. Just dry them both in the oven to drive off all moisture.
woelen - 18-12-2024 at 23:45
Keep in mind that sodium bisulfate usually is a hydrated salt (NaHSO4.H2O), and even at low temperatures, like 60 C or so, it already loses water. So,
making HBr from NaBr and sodium bisulfate will give a humid product. You first can heat the NaHSO4.H2O to well over 200 C to get rid of the water and
then mix that with NaBr. I am not sure, how well it produces HBr when the mix is completely dry. From experience with other solid-solid mixes (e.g.
NaCl, NaHSO4 for making HCl, or NH4Cl, NaOH for making NH3) I know that a little water in the mix helps a lot for producing the desired gas. Most
likely the reaction then takes place in a liquid high concentration mix, around the solid crystals.
Just give it a try. Pre-heat your sodium bisulfate and your NaBr as well to 200 C or so (the latter is a little hygroscopic and may contain a little
water as well).
[Edited on 19-12-24 by woelen]
teodor - 19-12-2024 at 00:43
Can bisulfate + Bromide really work? As far as I know it is very hard to replace Br- with Cl- without Br- oxidation to Br2 at high temperatures. I
think dehydration of aqueous HBr with P2O5 is the only practical method if you start with Br- ion. All others preparation ways for anhydrous HBr
require elemental bromine. Well, probably you can get some HBr by methods, similar to HCl generation, but most yield will be Br2, so why not start
with Br2 itself.
[Edited on 19-12-2024 by teodor]
Maui3 - 19-12-2024 at 10:21
Thank you woelen for the tips! I will try it.
Teodor, I am pretty sure it works with bisulfate + bromide..