Indole 2.7 g
NaHSO3 5.4 g
Water 15 mL
MeOH 5 mL
+Some added water and EtOH after 12 hours for stirring.
After standing overnight, the sodium indoline-2-sulfonate reaction mixture was filtered and washed with ether. The ether wash was moderately strongly salmon-coloured, indicating some unreacted indole, but the product retains a significant odour of indole after much washing. Sulfur dioxide is also detectable by scent, indicating the decomposition of sodium bisulfite.
The product is an amorphous mass, nearly chalk-white with a pinkish-tinge. It retains significant wetness of feel, probably from some sort of absorption of water.
Van Urk-magenta seems visible after a while, but strong yellowing is immediately visible, maybe somewhat different from when testing pure sodium bisulfite.
The product is still in the process of drying, but I fully expect a mass of more than 100% of the indole used, given the chemical similarity of sodium alkyl sulfonates and sodium bisulfite. Does anyone have any ideas on how to separate remaining sodium bisulfite?
Still, a van Urk reaction is odd. In the Chem. Ber. article, don't they say that the sulfonate moiety survives conditions of 50% nitrating acid (H2SO4/HNO3), and is only cleaved when neutralising with NaOH?
Addendum to "Speculations on tryptophan...": A number of mistakes appear in my notes on tryptophan, a serious one being that I make the mistake of saying that indole can be nitrated directly in the 5-position, giving a reference to
http://chemistry.sogang.ac.kr/lecture/db/021-429/file/indole-1.doc
. Obviously this is not correct, as I have missed the presence of a full methyl group in the 2-position.
About polyphosphoric acid: I find it curious, this report of "very small conversion" in the Kornfeld/Woodward article. What does that mean, surely one must wonder? Does it mean that most of the precursor stayed intact, with only a small yield of the cyclized product, or did most of the precursor yield biproducts?
Given a case of the former, one might feel intrigued to experiment with PPA using other conditions. For example, water would appear to be a biproduct of acylation with carboxylic acids. In newer articles, I have seen mention of continuous removal of reaction-water using a Dean-Stark trap, but this does not appear to have been in much use in the time of the Kornfeld article.
Edit: Yield, after a bit of microwave-drying to speed things up: 7.2 g. Molecular weight is 221.21, so yield calculated on indole is 141%. Obviously a lot of bisulfite must be present, even allowing considerable moisture-content.
Editedit:
In DMAB/HCl, the particles of indoline-SO3Na immediately turn a strong yellow like ferric chloride hexahydrate. The color gradually turns red, then brown.
This does not seem the standard van Urk reaction, but indole is known to give either a "yellow or magenta coloring, depending on conditions" (Dibbern & Rochelmeyer, Arzneimittel-Forschung 13, 7-16 (1963). Pure tryptophan slowly turns magenta, and then some hours later, deep blue like ink.
In HCl, particles immediately turn red. Color seems to significantly lessen with time.
Experiments to try:
Basify sodium indoline-2-sulfonate. Does this give indole?
Acidify to redness, then basify. Does this give indole?
What is the yield?
Van Urk-test pure indole in the presence of sodium bisulfite. Does this also give the strong yellowing reaction?
Does the tryptophan+bisulfite reaction-mixture give the preliminary yellowing reaction?
Estimate solubilities of the reaction dyes in different solvents.
Borrow an NMR-scanner from the grocery-store.
