2C-H Bromination using KBr/H2O2 and 2C-B Yield

[sponsan]

Hello again everyone!  It's been a while.  I have a question for the professionals regarding 2C-H bromination using bromine generated in-situ using KBr + H2O2.  The actual method used was posted by Chromic a few times.  8.5g of 2C-H freebase went into the bromination procedure, but only 5.6g of 2C-B HCl was recovered.

During the 2C-B freebase distillation, some crystallisation was noticed on the sides of the vacuum adapter.  It was assumed that this was unreacted carbonate salt of 2C-H.

Would that assumption be correct?  If this was unreacted 2C-H, could anything be done to increase the bromination yield using KBr/H2O2?  Perhaps using a double excess of KBr, instead of 120mmol KBr/60mmol H2O2, use 220mmol KBr/60mmol H2O2?  Has anyone else had the same problems?

I must admit, that the bromination was only allowed to stand for about 30 minutes after the second load of H2O2 was dripped in.  Perhaps the low conversion is due to that?  Maybe a 24 hour reaction time is nesessary to obtain a higher conversion result?

Thank you,
Sponsan.

[Rhodium]

The actual method used was posted by Chromic a few times.

Could you please link to those writeups?

During the 2C-B freebase distillation, some crystallisation was noticed on the sides of the vacuum adapter.  It was assumed that this was unreacted carbonate salt of 2C-H. Would that assumption be correct?

In the vacuum adapter? As in between the condenser and the recieving flask? Strange that you got crystal formation there...


Yes, 30 min sounds a little short to me, but 24h sounds like overkill. Try 2-3h.

Also, try to isolate the HCl salt of 2C-B before distilling it and evaluate the yield at that point rather than after the distillation, as that step might be where you lose yield.