Mystery compound, a challenge!

Dear All,

I'd like to compile a prepublication article on phthalimide and its offsprings: anthranilic acid, isatoic anhydride and related compounds. I achieved a thing or two but now I'm stuck.

I prepared phthalamic acid, melting point seems right, but failed to convert it into N-chloro-phthalamic acid. For the conversion I used sodium hypochlorite in faintly acidic solution. (The same method worked well in the N-chlorination of phthalimide.)

I did produce something though: it is a fairly light, snow white powder with a fair melting point between 169-172 C. The substance does not decompose around its melting point, I could heat it to 190 C without blackening. (N-chloro amides tend to decompose on melting.)
The substance contains only a trace of N-chloro-amide, 1% max, estimated by its weak reaction with KI in acid. (N-chloro-amides readily set free iodine under these circumstances, actually it can be used to determine their purity.)

The mystery substance does not appreciably dissolve in water but dissolves readily around pH=6 in a citric acid / citrate buffer.
Phthalamic acid also dissolves in this buffer, but its melting point is different, around 143-144 C.
The compound can't be phthalimide, phthalic acid, phthalic acid anhydride, or N-chloro-phthalimide based on the big differences in melting points.

Help would be really appreciated in the identification!
I'm also looking for the melting point of the N-chloro-phthalamic acid, could not find it on the net.

Tsjerk, you are the man! Your hint made my day!

I run out of hobby time, so I'm just planning methods to identify 5-chloro-salicylic acid with what I have at hand.

First, a purification step seems necessary, sublimation should be fine. I could get a more accurate melting point on the sublimated compound.

Fe³⁺ - for the phenol group, qualitative identification.

And this unknown substance is a good driving force for me to finally do what I should have done before: calibrate my pipette, burette, measuring flask, etc. With the calibrated equipment I can - hopefully - do a titration with enough precision to be able to make a reasonable estimation of the molar mass of this compound! (And many to come! )

The challenge continues!

What I have is not a phenol!
Did not react with Fe³⁺ solution, while pure salicylic acid produced a deep purple coloration. I assume 5-chloro-salicylic would be the same, it would produce the same (maybe deeper) purple!

So the challenge goes on, I'm open to suggestions! Organic chemists, wake up!

Btw. the decarboxylation/chlorination route has a flaw: even if it produced salicylic acid in the first step, this compound is sensitive to halogenation, likes to decarboxylate further and give trichloro-phenol in the end. I did not smelled any phenol stench though.

Anyway, its 171.5 C melting point was spot on! Nice find Tsjerk!

Even closer to the solution!

I've just finished the molecular weight determinations by titration. As it turned out the compound in question does contain carboxylic group(s), because it bubbled when mixed in Na2CO3 solution!

Guess what the molecular weight is?
(Now I should put here all the relevant data: concentration of NaOH solution, HCl solution, exact weights of mystery compound dissolved in said NaOH solution, volume of NaOH solution, consumed HCl standard solution during back-titration, etc. to give you the opportunity to work out the maths!)

Not being such a sadist, I'll spare you the tiresome details and give you what I came up with: the equivalent weight is somewhere between 80.6 and 83.3 g/mol! Taking into account that the titration tended to give low values (atmospheric CO2 interferred), 83 seems to be acceptable.
It automagically brings us a few consequences: the lowest molecular weight benzene-aromatic carboxylic acid is benzoic acid, with M=122 g/mol. Obviously the 83 g/mol value is acceptable only if there are more than one COOH groups in the molecule! Say two... Which means that the molecular weight of the compound is around 166 g/mol... Which is "accidentally" very close (spot on?) the molecular weight of phthalic acid! Which also means that we can safely forget any chlorinated compounds, the heavy chlorine just does not fit in these values. Also, it is very unlikely to say the least, that more than two COOH groups would be squeezed into the molecule, so it seems case closed! Whoa!

Yes, Boffis, you are right. The bubbling is caused by the conversion into phthalic anhydride. To test this -just to be sure- I'll take a melting point again after the bubbling finished. I expect this time a much lower mp, around 130 C, because phthalic anhydride melts there.

The morale of the story: don't trust your melting point data. The original, roughly 9-10 : 1 phthalic acid : benzoic acid mixture melt nicely within 3 degrees C and was very convincing as a pure entity. Some sort of Occam's razor also seems to be working here: don't expect cool nuclear halogenations and such when actually there's only good, old, simple hydrolysis and decarboxylation is only in the background.

Anyway, I'm glad that I could finally solve the riddle, it required a fair ammount of work on my side. Thanks for the contributions of the contributors, it really helped me going and finishing what I started!

At the weekend I did the "melt it twice" experiment.
The second melting point was not really a "point", it was a range between roughly 125 and 136 C. So it seems phthalic anhydride was produced, as we all knew in the end it should!