Basic Chem Questions

[awalmartbag]

Hi

I was reading this Syrian Rue harmine/harmaline extraction guide and I'm confused about how it works from step 13 onward. It says:

Saturate the solution with salt. Have the tea boiling in a pot, and add 100-120g salt per liter of water. Let it cool down to room temperature slowly over the next couple of hours, and then you can stick it in the fridge. After 3-5 hours, the harmalas will have crystallized. If you add more salt (closer to 200g per liter), the harmalas precipitate faster but less pure and more powdery than crystally.

What is happening here is that with excess of salt, the harmalas get the Chloride ions from salt and precipitate in the salt harmala form, harmine and harmaline HCl, which are insoluble in salt saturated water therefore precipitate. In this step one removes the uterotonic and possibly more toxic alkaloids vasicine and vasicinone, dangerous specially for pregnant women because they are abortifacient. Analysis done by Shulgin and others have shown salt-precipitated harmalas to be indeed free of these mentioned alkaloids.

We have a hot alkaloid acetate solution, add NaCl, then cool to crystallize.
Why is alkaloid⁺ Cl⁻ the salt that precipitates, not alkaloid⁺ MeCOO⁻? Wouldn't it be more favorable to have the Cl⁻ ions interacting with water instead of MeCOO⁻? It has a chunky nonpolar Me group and spreads negative charge across 2 O...

Also, what keeps vasicine and vasicinone from precipitating? I would have thought adding salt gives greater yield (fewer total organic ions in solution), but worse purity, not better.

 

[downwardsfromzero]

It's to do with the lattice energy of the harmala hydrochlorides, along with what is known as the common ion effect. This is how we explain the observed precipitation. Some of the alkaloids do stay in solution after the Manske step, however.

In a way, there is little point in asking "Wouldn't it be more favorable to have the Cl⁻ ions interacting with water instead of MeCOO⁻?" without doing the relevant calculations as that is not what is observed. Besides, the two oxygens in the acetate anion can both form hydrogen bonds with two water molecules. Compare the enthalpy of hydration of acetate anions with that of chloride anions for further insights on this.

It may seem that the crystallisation of the harmala hydrochlorides is entropically unfavoured, but this is balanced out by the remaining solution being a mixture of chloride and acetate.

Also, a methyl group is far from "chunky". You might as well want to consider how acetic acid is completely miscible with water. Or you could try dissolving harmine hydrochloride in glacial acetic acid to see if that precipitates crystals of harmine acetate. (Actually that's not so realistic, maybe salting out from a solution of harmala hydrochlorides in dilute acetic acid using sodium acetate would be a fairer comparison.)

what keeps vasicine and vasicinone from precipitating?

Firstly, they are typically present only in trace quantities and secondly they would appear to have chlorides that are more soluble than those of the betacarboline salts.

Basic Chemistry