[G-Head]

Hi guys,
In some experiments i tried to react fumaric acid, or maleic acid wirh different alcohols to form a corresponding diester. As catalyst sulphuric acid was used.

The reactions with maleic acid worked well without major side products.

But the reactions with fumaric acid often got cloudy/turbid.
I guess, this is because of polymerisation via the double bond. Likley via radical mechanism..

My questions are:
Why fumaric acid ester polymerize, but maleic acid not ??
How can i suppress this polymerisation??

With some alcohols, fumaric acid reacts well, but with some not.

How can the alcohol affect the polymerization??


Thanks for your help

[Orcio_87]

@G-Head I heard that lowering concentration of the reagent lowers degree of polymerization.

[Babcock_Hall]

Did you obtain a specific protocol from the literature for fumaric acid?  If so what was the reported yield?

[wildfyr]

So I work for a company that manufactures monomers, and just had a bit of additional extra training on this precise topic  of stabilizing double bond containing reactions .

Two things: you should consider adding phenol inhibitors like BHT, MeHQ, etc, and also possibly phosphite inhibitors to consume peroxides. Additionally, consider actively sparging air... that what is done industrially when doing something like esterifying acrylic acid. The bit of water that gets in doesn't matter because you're azeotroping it off anyways.

Fumaric acid is just plain more radically reactive than maleic due to the electronics of the cis vs trans double bond... though it should be said that at raised temperatures they will interconvert. I forget what ratio you end up with and how long that takes to happen.

The alcohol really shouldn't affect things too much, unless you're comparing primary, secondary, tertiary, or you have a volatile alcohol that likes to boil out. Phenols also won't react well under Fischer esterification conditions.

[G-Head]

@Orcio_Dojek: i don't use a solvent, just the alcohol in excess.

@Babcock_Hall: No, I more like just mixing the alcohols with the acid, becaus i thought it a simple reaction.

@wildfyr: I already thought aboit inhibitors. Which concentration of them is neccessary? Are they than left in the endproduct?

Sparging air?? I used N2 for inert conditions, because i thought oxygen forms radicals which start the polymerisation??

Thanks for you answers.

[Orcio_87]

@Orcio_Dojek: i don't use a solvent, just the alcohol in excess.

Adding other solvent wont help in any way. If (for example) solution contains fumaric acid / ethanol 1:10, and (after dilution) fumaric acid / ethanol / acetone in ratio 1:10:100, then - there is still the same chance (10 %) that fumaric acid will meet other fumaric acid molecule than the ethanol one - it's just that reaction rate will be lower.

The problem is that sulfuric acid catalysts both estrification and cationic polymerization.

[Babcock_Hall]

@OP, Do you have access to SciFinder?  As a biochemist who has increasingly had to turn to organic synthesis in research, I have found it to be extraordinarily valuable because it alerts me to problems that I would not have known existed otherwise and directs me to useful protocols that would have been difficult to find.  You can see examples in some of the threads that I have started.

[wildfyr]

Oxygen will create peroxides, but it also quenches spontaneous free radical reactions that occur under raised temperatures. On balance under air is better. The standard is to do these things under air, and then add inhibitors (100-1000 ppm is typical) to counteract the peroxide formation. In some cases not doing things under air will cause runaway exotherms or make a reactor gel.

Inhibitor classes are phenols like BHT, phosphites, thioethers, and a few other specialized ones.

Yes, they are often left in the endproduct but depending what the product is they can and sometimes are removed.