Why is the decarboxylation of 2-hydroxybenzoic acid (and related compounds) done 'under nitrogen'? I was reading Cannabinoid Chemistry - 6.2 Extraction &amp; pondered over the following:


  Cannabis must be dried be(fore) it is extracted, because it is not possible to remove more than 50% of the cannabinoids from fresh material. THC-Acid is difficult to extract. If you plant(sic) to convert the THCA to THC, the plant material should be thoroughly decarboxylated by heating it under nitrogen at 105&#8451; for 1 hour before performing a solvent extraction.


Why is decarboxylation done 'under nitrogen'?  My first guess would be that at 105&#8451; the plant begins to oxidize (but that is just my naive guess).  

I've also sourced this graph:



Effect of heating time and temperature on the THC content of an n-hexane
marihuana extract after heating on the glass surface in an open reactor.  Journal of Chromatography 520 (1990).

Which suggests that 122&#8451; for 27-30 minutes will be even more effective for decarboxylation, but if the sample begins to oxidize at 105&#8451; it seems even more important to get the right mixture of gases around it at 122℃.
[&hellip;] it seems even more important to get the right mixture of gases around it at 122℃


No, it's only important to keep the oxygen away while you are heating the sample.

Have a look at the structures: the thermal decarboxylation of the THCA isn't different from the decarboxylation of salicylic acid.

But with the carboxyl group or without, (upon heating,) phenols are easily oxidized by air.

In principle, the sample could be heated under vacuum too. In this case, you might want to use a cooling trap to collect volatile material. But usually, one does that anyway to protect the vacuum pump.