The amount of the DMF in the Sonogashira coupling - why it is of importance?

I was looking for some protocols for Sonogashira coupling and have found that in many papers the amount of the DMF used is exactly 2 ml per mole of the substrate.


Is there any reason for that? Usually, for other reactions, we can see, that articles report more-less random amount of solvents, like 10, 50, 100 ml - some "simple" numbers. But in this case I see numbers like 68 ml (why not 70?). Why must it be exactly 2 ml/mole?


some refs:


<http://pubs.acs.org/doi/suppl/10.1021/ol049474j/suppl_file/ol049474jsi20040410_035107.pdf>


<http://www.sciencedirect.com/science/article/pii/S0040402000008772>


<http://pubs.acs.org/doi/suppl/10.1021/jo8020264/suppl_file/jo8020264_si_001.pdf>


