[Susan]

I am working on the synthesis of a diester.  The starting materials are a long (C9) chain acid and a C5 diol.  I need to have at least 95% purity, and the catalyst needs to be pretty nontoxic.

I have tried using tin oxide at over 200 C but I can't get it to go to completion.  I also don't have a good way of really monitoring the reaction.  I am using xs acid, so watching the acid peak on FTIR only gets me so far.  I am having a tough time visualizing it on TLC (using fluorscent plates) or HPLC (RI and PDA detectors).  We don't have a GC or GC-MS.

I am thinking that I need to use a better catalyst but I don't have a ton of experience making esters - is p-TsOH the best or will I get side products?  How about an organotin compound?  I just an not sure I will be able to get one of those by our safety officer.

I have put this project on hold for a bit but I really need to get back to it next week so any advice would be greatly appreciated!

[sjb]

Is there any reason you can't convert the acid to a acid halide or similar?

Tin compounds are probably going to be fairly toxic - what is the actual diol - could you use something like catalytic YbCl₃ or similar?

[Susan]

I hadn't considered converting to an acid chloride, but I don't think thionyl chloride would fly here.  I barely got my carboxylic acid in.

The diol is 3-methyl-1,5-pentanediol.  I don't know anything about using YbCl₃ - would it work?  What are the conditions?

[sjb]

Typically you just stir the carboxylic acid, Lewis acid and alcohol together at room temperature or perhaps heat slightly, in something like THF. Is your carboxylic acid precious (by which I guess I mean did it take several steps and / or is expensive)?

[Custos]

I'd try an acidic resin like Amberlyst (http://www.amberlyst.com/sac.htm) because you can just filter it off at the end. The acid+alcohol to ester reaction is obviously a dehydration so another option is to use whatever acid catalyst you want and to drive the reaction to completion by azeotropic removal of the water (refluxing benzene in a Dean-Stark trap for example, or by using A4 molecular sieves if it's on a small scale).

A further option is you want to move away from "toxic chemicals" is to use an enzymatic process. Yeast lipase (Candida cylindracea) quantitatively converts carboxylic acids and alcohols to esters in organic solvent. See JACS 1985, 7072.

[Captain Sci]

I'd try an acidic resin like Amberlyst (http://www.amberlyst.com/sac.htm) because you can just filter it off at the end. The acid+alcohol to ester reaction is obviously a dehydration so another option is to use whatever acid catalyst you want and to drive the reaction to completion by azeotropic removal of the water (refluxing benzene in a Dean-Stark trap for example, or by using A4 molecular sieves if it's on a small scale).

I wonder if a silica-bound acid catalyst like "SiliaBond Tosic Acid" would be useful in this context. It is a versatile catalyst, fully end-capped (which means it is completely insoluble) but also, being a silica-based material, it is a moisture scavenger and may adsorb the water that is eliminated during the coupling reaction.

Athan

[g-bones]

would a carbodiimide like DCC be out of context?