A friend of mine (called SWIA for simplicity) told me yesterday that he verified the usability of sodium dithionity for reducing P2NP to the oxime - and was very confused about the yield he got!
The poor boy (
) placed a 500ml erlenmayer in a 50°C water bath, in which he suspended 100mmol P2NP in 30ml EtOH, added 50g sodium dithionite (86% purity) and finally 200ml water and started stirring while monitoring the rxn. temp.
First the temperature slowly rose to ~45°C and the dithionite dissolved (yellow flakes appeared). After a couple of minutes, the temp. suddenly shot up to over 70°C while the rxn homogenized to a milky off-white suspension.
Stirring was continued for five minutes, then the water bath was exchanged with a cooling water bath (10°C) and rxn allowed to cool down to ~20°C.
Then, solid Na2CO3 was added until gas evolution ceased, followed by addition of 300ml water and trituration to pH10 with NaOH. The whole lot was then placed in the fridge overnight and suction filtered to give a lot of white, transparent, crunchy needles.
So far, so good.
After the crystals had been air dried for one day, SWIA weighed them - 29 grams!! Even with his poor chemistry knowledge he was able to judge that this was far too much - compared to the amount of P2NP he had put in!
Well 100mmol (16.3g) P2NP should give 14.9g oxime - in the best case possible.
So: what went wrong? are the crystals some carbonate? Hydroxide (probably not)? Is the dithionite involved? Or (my favourite exp.) are they just hydrated, meaning SWIA shoud've dried them with a drying agent/dessicator?
Help is greatly appreciated!