[batty2279]

This is going to be long winded, and I'm fairly certain the answer is incredibly simple, but I want to make sure I'm as thorough as possible with my question...

Protocol: Synthesis of dimethylacetylphosphonate. (DAP)

In a dryround bottom flask with a magnetic stir bar under positive N2 pressure, acetyl chloride (3.5 mL, 50 mmol, 1 eq) was cooled to 0 C in an ice-water bath. Then, trimethyl phosphite (5.9 mL, 50 mmol, 1 eq) was slowly added over a period of 1 h, which resulted in evolution of MeCl. After full addition of trimethyl phosphite, the colorless solution was allowed to warm to room temperature and stirred until evolution of gas ceased (about 1 h). Unreacted materials were removed under vacuum to afford (14) (7.3 g, 48 mmol, 96%) as a colorless oil. 1 H NMR (500 MHz, CDCl3) δ 2.47 (d, J = 5.28 Hz, 3H, CH3), 3.85 (d, J = 10.71, 6H, OCH3); calcd m/z (C4H9O4P) 152.0328, found 152.0326





Seems simple enough.  After the reaction, I checked the MS looking for the product+1 ion (153).  However, I kept seeing m/z 111






I'm assuming my product is being deacylated somehow, like this:




Dimethyl phosphonate has a mass of 110, therefore the that's probably what I'm seeing at 111, especially because this is reaction is supposed to have 96% efficiency.  My H NMR also has the doublet at 3.8, but none at 2.2 which is what you should see for the DAP.

So my question is, could there be a time-dependent deacylation somehow?  I ran the protocol verbatim.  Or could my acetyl chloride be bad?  It is a fairly old bottle (several years).  If it's "damp" could water be driving the deacylation?

Any insight would be greatly appreciated everyone!  Thanks!

[kriggy]

I woulc check the acetyl chloride, it could decompose into acetic acid over the years and that could be ruining the reaction. Try it again with different batch of acetylchloride if its possible

[Babcock_Hall]

H phosphonates have a very characteristic H-1 NMR and also a very characteristic P-31 NMR spectrum, owing to the roughly 700 Hz coupling constant between the phosphorus and the hydrogen atom.  If you do not have this in the downfield portion of your spectrum, then you don't have the H phosphonate.  Can you obtain a P-31 spectrum?  That would be very helpful.