TechTakes

alright i'm gonna start by noting that this synthesis - the real one - is not particularly hard or dangerous, and if you know what are you doing it can be even semi-safely done in shed. that said, it's also not exactly normal compound and in few aspects it's a corner case. importantly for spicy autocomplete, this procedure is well disseminated over internet

whatever chatgpt cooked there is not workable at all, because it's three unrelated procedures mashed together in a way that makes little sense. i think i've been able to pick up where did some weird, disastrous numbers came from. here's some random synthesis https://web.fscj.edu/milczanowski/eleven/luminol.pdf it sounds like everyone copies this procedure from everyone else with small changes if any

overall synthesis of luminol goes like this: phthalic anhydride (or acid) gets nitrated to 3-nitrophthalic acid (+ 4-nitrophthalic acid), this step has been omitted and i don't know why, because this is nice if a little bit dangerous synthesis for first organic lab ever (after basics of purification methods are covered). it requires some care, but more importantly requires understanding of crystallization because that's how these two isomers are separated. it's not particularly hard either. anyway, it's not shown there. that would be step zero. step one is synthesis of hydrazide from acid, at rather high temperature (200C) with glycol-type solvent (triethylene glycol or straight up polyethylene glycol). it's variant of this method https://repository.ukim.mk/bitstream/20.500.12188/11328/1/XIII_0540.pdf it's rarely discussed, because not many compounds of interest survive these conditions. here, however, it works fine because of combination of factors, one of them being unusual nucleophilicity of hydrazine, the other being additional driving force coming from the fact that newly formed ring is aromatic (which is covered in probable training material, but chatgpt can't infer so it doesn't matter)

then second step is reduction of nitro group to amine. there are many ways to do this, but if i was to choose, i'd pick some modern method like catalytic hydrogenation on Pd/C, or transfer hydrogenation with formic acid. what can be found out there uses either tin(II) salts or sodium dithionite, or some method using hydrogen sulfide. then again some purification is necessary, of which good choice would be crystallization. unusually hydrochloride salt of luminol has low solubility in water, and this gets useful during workup.

chatgpt instead cooked something that won't work under any conditions. like i said previously, first step would be synthesis of phthalohydrazide directly from acid, but spicy autocomplete had little info about this type of synthesis in probable training material so it doesn't come up. what was, however, was actual synthesis of luminol and variety of amide synthesis methods. these syntheses use some kind of activated acyl group equivalents, starting with anhydrides, acyl halides, isourea derivatives etc the important bit is anhydrides. anhydrides do form amides, and cyclic anhydrides can be formed on heating. spicy autocomplete conflated heating in synthesis of phthalhydrazide with making 3-nitrophthalic anhydride - step that does not exist in real synthesis

so let's start with good synthesis, that looks for example like this:

Combine 0.300 g of 3-nitrophthalic acid and 0.4 mL of 10% aqueous hydrazine in a side arm test tube. Heat the test tube over a microburner until the solid dissolves. Add 0.8 mL of triethylene glycol to the test tube. Clamp the test tube to a ring stand, insert a thermometer into the test tube (use a two holed rubber stopper, one hole for the thermometer and the other to make certain that the system is not sealed), and connect the side arm to a vacuum source. Heat the solution to 200 °C and keep the solution at 210–220 °C for two minutes. Allow the test tube to cool to 100 °C and add 4 mL of boiling water to the test tube. Cool the suspension to room temperature by running tap water along the outside of the test tube. Collect the brown solid on a Hirsch funnel.

botslop version:

i'm just gonna highlight worst bits because it's ridiculously, irritatingly verbose

Hydrazine hydrate (80% aqueous solution)

this is rocket fuel grade stuff and not suitable for undergrads. real procedure uses 8%, 10% hydrazine or something of similar concentration

Step 1: Formation of 3-Nitrophthalic Anhydride

1. Dehydration of 3-Nitrophthalic Acid:

Place 3-nitrophthalic acid (e.g., 10 g) in a dry round-bottom flask. Heat the flask gradually to 200–220°C under reduced pressure to remove water and form the anhydride. Maintain the temperature until no more water distills off.

zeroth of all, there's no motherfucking "for example" in procedures. it's described for strictly defined amount of whatever was put there, and also i'd like to see it recalculated into mmols and equivs so that both student can show that they know what they are doing and it's easier to spot mistakes

this step is completely made up and unnecessary. the 200C temperature was taken from real synthesis of phthalohydrazide. distillation manifold appears magically out of thin air when it's necessary to notice that no further water is distilled off, and depending on how hard that vacuum would be, this could result in loss of material by evaporation. 3-nitrophthalic acid is a solid, and remains a solid (sigma lists mp as 205 with decomposition) where would you even put thermometer? some solvent is necessary for good heat transfer, but there none is shown. in real procedure it is used. this entire step is footgun number 1

entire substep for cooling? come on

Step 2: Synthesis of 3-Nitrophthalhydrazide

Procedure:

1. Reaction with Hydrazine:

Dissolve the 3-nitrophthalic anhydride in 50 mL of ethanol in a round-bottom flask.

after this point, what you get instead is ethyl and diethyl 3-nitrophthalate. this is wrong and bad form. again, no scale is provided

Add hydrazine hydrate (excess, e.g., 5 mL) to the solution. Attach a reflux condenser to the flask.

in real life hydrazinolysis of esters works in much milder conditions, at room temperature. this probably is workable, but in terms of lab exercise like this it would probably be better to make ester directly from acid through Fischer esterification, maybe with Dean-Stark adapter. this is very typical preparation. anyway, there's easier option there, so it's not used. moving on

Pour the mixture into 200 mL of ice-cold water with stirring. Collect the precipitated 3-nitrophthalhydrazide by filtration. Wash the solid with cold water to remove impurities.

when crude product is used in next step without purification you are excused, but there, after purification, even as rudimentary like this, i'd like to see at minimum expected yield and melting point (after crystallization). how are you even supposed to grade this

footgun number 2 approaching. reduction of nitro group

Suspend the dried 3-nitrophthalhydrazide in 100 mL of aqueous ethanol (50% ethanol in water) in a round-bottom flask. Add iron powder (excess, e.g., 15 g) to the suspension. Add concentrated hydrochloric acid (e.g., 20 mL) carefully to the mixture.

this, like previously, is not a real procedure. this is badly ripped from synthesis of aniline from nitrobenzene. while it is a real undergrad level preparation, i'd really wish it wasn't. why on god's green earth are you teaching this century old procedure when we have catalytic hydrogenation, they won't ever compete with bavarian coal tar dye industry from year 1904. it's annoying, it's not as high yielding as it could be, it's wet and dirty, rather harsh, and not green at all, but at least it's kinda real. the real procedure looks like this:

In two-necked round bottom flask with short reflux condenser put 18.5g (150mmol) of nitrobenzene and 30g (550mmol) of iron turnings. Add conc. hydrochloric acid in 2ml portions with shaking, 80ml total. Reaction mixture heats up strongly and starts boiling. If reaction is too vigorous, cool it down externally with water bath. After adding 30ml of acid bigger portions can be used. After adding all 80ml of acid, heat up flask in boiling water bath for 1h. When reduction is done, cool down flask and alkalinize reaction mixture with 45g of NaOH dissolved in 90ml of water. Aniline is then distilled off with water vapor.

then, aniline is extracted from distillate (two-phase) with ether, then ultimately distilled on its own. Yield 12g (84%), bp 182-183C.

the last bit is absolutely critical here. aniline can be distilled off like this, luminol can't. this is important because adding sodium hydroxide to mixture like this would result in miserable metal hydroxide yoghurt-like emulsion that under no pretext can be separated into anything useful. this is exactly what spicy autocomplete end up suggesting:

While still hot, filter the mixture to remove the iron salts (iron oxides). Wash the residue with hot water to extract any remaining product.

these iron salts are more precisely in form of iron chloride, entirely soluble in reaction mixture. there's nothing to filter. luminol hydrochloride could be probably precipitated, but only when cold and only if reaction mixture is concentrated enough in the first place

Combine the filtrates and cool them in an ice bath.

there luminol hydrochloride can drop out of solution. this is used in real synthesis where tin(II) chloride is used, precisely to avoid emulsion

Slowly add a solution of sodium hydroxide (10% NaOH) to the filtrate until the pH reaches around 8–9. Luminol will precipitate out of the solution.

yeah, maybe, along with multiple its weight of iron hydroxide that is now practically impossible to separate. this is footgun number 2.

none of this shit is workable or even real, unless the point is setting students up to fail

there are two incredible footguns in there, both of which can be trivially avoided