Pumukli
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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.
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Tsjerk
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5-Chlorosalicylic-acid? MP 171, soluble in acetic acid (pH<7). The phenol being formed by oxidation of the amide arm, with the phenol becoming a
para director for the chlorination.
In acidic solutions hypochlorite partially behaves like straight chlorine. I can perfectly imaging the 5-position of salicylic acid being very prone
to chlorination by chlorine.
https://patents.google.com/patent/CN106831396A/en
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Pumukli
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Tsjerk, you are the man! Your hint made my day! ![:)](./images/smilies/smile.gif)
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! )
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Tsjerk
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Good luck! Keep us posted!
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Pumukli
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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!
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Tsjerk
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I did check the chlorsalicylic acid and its reaction with Fe(III) and it indeed forms an intense violet complex... Too bad. I will have another look
tomorrow and see if I can think of something.
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Pumukli
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Well, there's a bit of progress!
I tried the sublimation of the "mystery compound" and it sort of worked!
The compound separated into two fractions. The first one (about 10-15% of the whole mass) sublimated easily and produced long, thin, white needles.
The melting point of this fraction is around 120.5 - 121.5 C. Based on the melting point data and my experience with sublimated benzoic acid I tend to
think this fraction is actually benzoic acid. (Yes, the mp. is off a bit, but there was something what sublimated together with this benzoic acid and
made the needles a bit "oily". I mean they tend to stick together. Also, the glass surface of the sublimator is covered with this unknown substance,
the glass is a bit sticky The ammount of this contamination is not big, say 10% of the whole benzoic acid, or around 10 mg, mostly condensed on the
glass surfaces.)
Then the remaining fraction refused to sublimate.
I took a sample of this remaining white powder and found its melting point being around 182-185 C. Above this temperature, around 187 C it starts
bubbling in the melting point capillary (release some sort of gas or water vapour?) but does not discolorate otherwise.
Btw. a second crop of white crystalls appeared in the mother liquor of the mystery compound on the second day. Filtered, washed with distilled water,
dried, the melting point of this second crop is between 179-181 C. Above this temperature it also shows bubbling and starts climbing up in the melting
point capillary. I assume that this second crop and the second (non-sublimated) fraction of the first one is actually the same compound. Their
appearance is also similar. They are surely a bit contaminated though.
Maybe a recrystallisation would help?
Anyway, the "new" melting point range of the mystery compound is between say 180-184 C.
To do: checking (qualitatively) whether -COOH group is present in the mystery compound.
Molecular mass determination, either with acid/base titration (-COOH is present) or the melting point depression method (-COOH is absent).
Maybe a qualitative check of halogen (chlorine in this case) would also be helpful.
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Boffis
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Are you sure that your compound isn't just impure phthalic acid (+ some benzoic acid) and that it eventually starts to decompose to the anhydride and
water, hence the water seen in the mp tube?
Try recrystallizing it several times from a large amount of hot water
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Pumukli
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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!) ![:D](./images/smilies/biggrin.gif)
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. ![:)](./images/smilies/smile.gif)
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!
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Tsjerk
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Congrats! Sounds like a likely candidate. Even more so if you can measure the anhydride.
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Pumukli
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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! ![:)](./images/smilies/smile.gif)
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