[owk9688]

Hello,

I'm trying to run a reaction but I'm having difficulty running TLC plates to determine when the reaction goes to completion. Its a monohydrolysis of diethyl malonate in aqueous KOH. I have about 2g (just under 2ml) of DEM dissolved in 200ml water and 20ml acetonitrile, and since the KOH added is aqueous the volume of water really goes up to 250ml after I've added that. Is the reason my TLC plate is just showing a water spot because my starting material is too dilute from the get-go? Should I try to take small amounts of the reaction mixture at certain intervals and try rotovaping off the water or putting it under a high vacuum pump to concentrate my starting material?

Thanks for any help

[Dan]

In KOH solution your product is the potassium carboxylate salt, which you will struggle to mobilise on silica gel.

You could do a mini workup: partition a drop or two of you reaction mixture between 0.1 mL dilute HCl and 0.1 mL ethyl acetate, then TLC the ethyl acetate layer and you should see the carboxylic acid product.

However, if a direct TLC of your reaction mixture shows no starting material, then you have complete conversion and can quench the reaction. This is normally how I judge the endpoint of saponification reactions.

Placing the TLC plate under high vacuum to get rid of most of the water is a generally a good idea when analysing aqueous solutions, but leave it too long and in this case you risk evaporating any remaining DEM (i.e. the conversion may appear higher than it really is).

[owk9688]

That makes much more sense. Thanks so much for the tips!

[Doc Oc]

Quick question: Is your compound UV active? If it's really just diethyl malonate I'd be surprised if it was, but I don't have experience with it myself. If it's not UV active, are you staining it?