Bean - 7-4-2016 at 09:23
Can an aqueous acid such as hydrochloric acid solution be used as a catalyst for acid catalysed ketalization ? (In particular I am trying to make
fructone from ethyl acetoacetate and ethylene glycol) Or can p-toluenesulfonic acid be used as this is available to me. I know reducing the amount of
water present in the reaction would shift the reaction to the right, increasing yield, but to what extent ? Could 30% HCl be used for example ? Many
thanks and sorry if the question is silly.
Darkstar - 7-4-2016 at 15:52
You really want as little water around as possible, especially if you have no real means of removing it. Keep in mind that the ketalization reaction
itself is also going to be producing water, continuously adding more and more water to your reaction mixture on top of all the water from your aqueous
acid catalyst. Not only will this water hydrolyze the fructone back into a hemiketal, but it will also hydrolyze the ethyl acetoacetate and fructone
esters into carboxylic acids. Not only will this reduce your yield, but if it's the smell of fructone that you're after, the presence of even a small
amount of the carboxylic acid of fructone will significantly alter its characteristic fruity smell.
Since it works well and is already available to you, just use a small amount of dry TsOH (monohydrate) as your catalyst. Also, if you have one, a
Dean-Stark apparatus to remove the excess water as it forms would also be preferable.
UC235 - 7-4-2016 at 16:38
pTsOH is more or less the ideal catalyst for ketalization. Nonvolatile, non-nucleophilic, nonoxidizing, soluble in organics. You will want a dean
stark trap (or distill slowly, though this will require much more solvent) and either toluene or preferably benzene as the azeotroping agent.
Using benzene and a Dean Stark, I collected the theoretical quantity of water in an hour and a half and after vacuum distillation got a 63% yield of
fructone.
Attachment: ed050p216.pdf (1.3MB)
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Attachment: Jchemed-ketalization-toluene.pdf (85kB)
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[Edited on 8-4-2016 by UC235]
Bean - 8-4-2016 at 04:33
Thank you very much for responding.