Diethyl oxalate

Diethyl oxalate, or diethyl ethanedioate, is a interesting ester used as an organic building block. It has an ethereal smell similar to grape juice. Diethyl oxalate is also used as a specialty solvent for various resins, and as a pigment carrier. In the pharmaceutical industry, diethyl oxalate is used to produce barbiturate steroids. I plan to use it to prepare ethyl acetopyruvate and oxalamide. Diethyl oxalate can be made from dry ethanol and anhydrous oxalic acid. I tried this out.

To get a good yield, the hydrated oxalic acid used must be made anhydrous.

To a large crystallizing dish, I added 100g of oxalic acid dihydrate. I then placed the crystallizing dish
on a hotplate and lowered in an overhead thermometer. I heated the oxalic acid at around 140 C until no more water boiled off. During this process, the oxalic acid melted and a large amount of vapor
(mostly steam) was boiled off. This is not a very effective way to produce anhydrous oxalic acid, but I got there in the end.

To a 500ml round-bottom flask, I added the anhydrous oxalic acid prepared above. I then added 200ml of ethanol and a drop of 98% sulphuric acid, then set up for reflux. I allowed the mixture to reflux for 2 hours. The anhydrous oxalic acid dissolved but apart from this, there was not much visible change in the mixture. After 2 hours of reflux, I removed the condenser and set up for simple distillation. The first fraction came over at 71-81 C,  consisting of ethanol/water azeotrope. About 175ml of this was collected. The temperature of the distillate then rose to 100 C, and I changed the receiving vessel. I kept collecting distillate until nothing was left in the distilling flask. The temperature had climbed to 220 C at the end of the distillation. I had collected 50ml of clear liquid, predominantly diethyl oxalate.

To this liquid, I added 60ml of saturated sodium chloride solution. I shook the mixture up then let it settle. The diethyl oxalate separated, forming a layer above the surrounding liquid. I collected the diethyl oxalate product with a syringe, dried it over anhydrous calcium chloride and finally transferred it to an amber glass bottle for storage. I got 42ml of diethyl oxalate which is a 34% yield.


Adding a drop of sulphuric acid isn't absolutely required as oxalic acid is a strong enough acid to catalyse the reaction.

H2C2O4 + 2 C2H5OH ==(- H2O)==> (C2H5)2C2O4 + 2 H2O