[Fischer1991]

Hello everyone, first time writing 

I have to perform a reduction of an aromatic nitro group with ammonium formate and Pd/C in refluxing methanol (30 mmol scale). My colleague run that reaction before (1 mmol scale) and noticed a large amount of a white solid crystallizing on the walls of the condenser and obstructing it. Could it be Ammonium Formate subliming (it should not, it should melt at 116 °C) or Ammonium carbonate forming from NH3, CO2 and water (formed by the reduction of the NO2 group)?? Someone had the same problem and knows how to fix it?
Thanks in advance 

[TheUnassuming]

Never had that problem before, but only run that reaction once.  Aromatic nitro groups don't typically require heating to reduce in under an hour or two.  If its an issue you can always go the Fe/HCl route, which has worked wonders for me as long as your compound isn't acid sensitive. 

[kriggy]

I can add another option for you:
Lots of my friends are using sodium dithionite in water/DCM mixture with K₂CO₃+ TBAHS reducing nitro group of nosyl. Might help you since they are doing it on realy small scale since they are doing solid phase synthesis

[Fischer1991]

Thank you for your kind advices. Problem is, the reaction worked really well, it was just impractical because they had to break this strange "plug" of crystals every few minutes (it seems 6 hours at reflux were needed). Substrate is acid sensitive (Boc group around). I'm going to try it on a smaller scale (10 mmol) and maybe at room temperature for a longer time. I'll see what happens.