[squeak]

Hello,

I need to dry acetonitrile that we have in the lab. Vogel (4th Ed.) suggests the use of molecular sieves (4A)/silica gel (what mesh size, does anyone know?) for preliminary drying followed by treatment with calcium hydride.

How do I pre-treat these molecular sieves? We don't have calcium hydride in the lab so I googled and found an alternative - P₂O₅. But I don't know how to go about using P₂O₅ - do I need a certain amount w.r.t. the amount of solvent I take? Are there some precautions I need to take?

I also read somewhere that I could distill my solvent with benzene whereby I'd get the benzene-water azeotrope. Is that a do-able technique for getting dry acetonitrile? 

Or, well, if there is a different method that would work?

Danke. 

[macman104]

Hello,

I need to dry acetonitrile that we have in the lab. Vogel (4th Ed.) suggests the use of molecular sieves (4A)/silica gel (what mesh size, does anyone know?) for preliminary drying followed by treatment with calcium hydride.

How do I pre-treat these molecular sieves?

Put the molecular sieves in an oven to dry them out.

[Rico]

Hey squeak

I am not sure if it is a good idea to use P₂O₅ as a substitute for CaH₂. When P₂O₅ reacts with water it forms phosphoric acid, which probably to some extent is soluble in acetonitrile. You would then have phosphoric acid as an impurity instead. The reason why CaH₂ is such a good drying agent in this context is that it forms calciumhydroxide upon reaction with water and calciumhydroxide has a very low solubility even in water. Isn't it possible for you to just buy some CaH₂, it is available from most chemical suppliers and its not very expensive.
Now back to the drying procedure. When I need dry acetonitrile I distill it using fractional destillation over CaH₂ (B_p = 79-80^oC) and then I dry it over 4 A mol sieves (the mol sieves have been dried by heating them in a round bottomed flask under oil pump vacuum with a flame). This works well for me.

Rico   

[alphahydroxy]

In full agreement with Rico here - Distilling from calcium hydride onto 4A sieves has worked well for me. Not sure that fractional distillation is necessary though, if you are starting with HPLC grade acetonitrile...

I use this instead of acetonitrile obtained from Grubbs solvent system, as there has been an issue with ehtnaol contamination with the latter...

[insertwittyname]

Just in case someone is also considering using phosphorpentaoxide as a drying agent, one problem that may prove to be a pain in the neck is that quite a few times, upon absorbing water it forms a weird resin like substance which makes it near impossible for a stir bar to stir the mixture.

[Babcock_Hall]

Check Burfield, Lee and Smithers (1977).  IIRC calcium hydride is not that effective but phosphorus pentoxide is, despite having some issues (as noted upthread).  Sieves can be activated by heating on an oil bath under a vacuum (Cornell).  Williams and Lawton used 3 Å sieves very successfully to dry acetonitrile.  Burfield also did a comparison between 3 vs. 4 Å for this solvent (JOC 1984) and found that 3 Å was more efficient.

[kriggy]

I can suggest this piece
J. Org. Chem. 2010,  75, 24, 8351-8354

TLDR:
Drying ACN with 10% m/v 3A sieves for 48hrs give you verry low water content (lower than detection limit of Karl Fisher titration)

[Babcock_Hall]

https://pubs.acs.org/doi/abs/10.1021/jo101589h
"Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants," D. Bradley G. Williams and Michelle Lawton
J. Org. Chem., 2010, 75 (24), pp 8351–8354.  DOI: 10.1021/jo101589h