Hi everyone!

First sorry but I don't speak english very well.
My question is: when DMT salting with fumaric acid, it can do it in 2 different salts: hydrogen dmt fumarate and dmt fumarate. But in witch proportion? I tried to make calculations, but I'm not sure they are correct, something does not convince me. Someone would be able to correct them or give me clarification? Thank you!

pKa=-log Ka KaDMT = [H3O+][DMT] / [DMT H+] 8,68=pKaDMT KaDMT=2,08 * 10-9
Fumaric acid salts: DMTH+HF- hydrogen dmt fumarate 2(DMTH+)F2- dmt fumarate
Fumaric acid pka: pKa1=3,03 e pKa2=4,44

DMT pKa is greater than fmaric acid pka, so equilibrium is moved to the right.
Reactions are:

DMTH+ + H2O --> DMT + H3O+ H2F + H2O --> HF- + H3O+ HF- + H2O --> F2- + H3O+

KaDMT = [H3O+][DMT] / [DMTH+]

Ka1= [H3O+][HF-] / [H2F] Ka2=[H3O+][F2-] / [HF-]

DMT + H2F --> DMTH+HF- DMT + DMTH+ + HF- --> 2(DMTH+)F2-

keq1=[HF-][DMTH+] / [DMT][H2F]

keq2=[F2-][DMTH+]2 / [DMT][MDTH+][HF-] = [F2-][DMTH+] / [DMT][HF-]

multiplicand Keq1 and keq2 to [H3O+]/[H3O+] we obtain

keq1= Ka1 / KDMT pkeq1= pKa1 – pKDMTH

pkeq1= 3,03 – 8,68 = -5,65 keq1= 4,47 * 10^5

keq2= Ka2 / KDMT pkeq2= pKa2 – pKDMTH

pkeq2= 4,44 – 8,68 = -4,24 keq1= 1,73 * 10^4

This means that if we react DMT and fumaric acid in equal quantities (molar concentration) the formation of the first product (hydrogen fumarate of DMT) is favored for a ratio of 447000 to 1; the second product (DMT fumarate) for a ratio of 17300 to 1. But we don't work with equal molar concetration of DMT and fumaric acid, so how we have to change calculations?

If this calculations are correct, It means that when we are DMT fumarates salts they are 447000/17300= 25.83 hydrogen DMT fumarate each 1 DMT fumarate?

And what's happen with citric acid, that's triprotic acid?

Or maybe all this calculations are useless.....

You forget that fumaric acid is also added dropwise, and this is why that is important.

so DMT should be in molar excess at all times until its concentration has become very small.

Indeed the reaction to form the hydrogen fumarate is faster, that is why dropwise addition is important to allow complete reaction. I`ve never witnessed a problem with this using fumaric acid, but ive seen it happen using concentrated sulfuric acid. However, the hydrogen fumarate salt is much more benign than the hydrogen sulfate.

I was wondering the same question myself, but I`ve run samples of DMT fumarate precipitated by the FASA method in proton NMR, and I got nearly a perfect 2:1 ratio of DMT to the fumaric acid. Whatever the ratio is between the fumarate and the hydrogen fumarate, its negligible as long as the precipitation is done properly.

I am sure if you did a reverse addition, added DMT dropwise to fumaric acid in a 1:1 molar ratio, you could isolate the hydrogen fumarate.

Would the hydrogen dmt fumerate crash out?

I believe it should, if not it could be crystallized.

Fumaric acid is added dropwise, so DMT is always in excess, so DMT fumarate is formed almost exclusively! Yeah, thank you!

"it's negligible as long as the precipitation is done properly"

This is true, but I needed to know that to others reasons.

Do you think the calculations are right?