Chlorinating acetic acid with red phosphorus... and a reptile lamp

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Ай бұрын

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He made chloroacetic acid with sulfur as the catalyst and gives a reaction mechanism that differs from the one I showed in one of the intermediates. I found a paper, revroum.lew.ro/wp-content/upl... that shows the intermediate in steps (5) and (6) of scheme 1, but the reaction conditions they used involved acetic anhydride as a catalyst and conc. H2SO4 + FeCl3 as catalyst promoters, so I don't know if it's applicable to either Chemiolis' or my reaction conditions. I didn't look further into it because the H-V-Z mechanism has nearly the same reaction conditions as mine and because of the radical mechanism.
Mechanism explanation:
I have two mechanisms, one involving chlorine radicals, one not. Up top, phosphorus reacts with chlorine to make phosphorus trichloride. I suppose it could react further with excess chlorine to make phosphorus pentachloride, but to simplify, let's assume it doesn't because it'll react faster with the acetic acid. Phosphorus trichloride reacts with acetic acid to make acetyl chloride. An electron pair of the acetic acid's hydroxy group attacks the phosphorus center of PCl3 and one of its chlorine atoms splits off to form a free chloride ion. The chloride ion attacks the carbonyl carbon, one of the carbonyl bonds attacks the hydroxy hydrogen, and the O-H bond returns to the positively charged oxygen. The newly formed hydroxy group's O-H bond moves to the C-O bond to remake the double bond, and the phosphorus adjacent C-O bond attacks the hydroxy hydrogen. What we're left with is acetyl chloride and this weird in between compound. I don't know if there's a common name for it, but ChemDraw generated the IUPAC name dichloro(hydroxy)phosphine. Doesn't really matter, it's probably too unstable to exist freely and only does as a fleeting intermediate. It can undergo the same reaction twice more to end up with one phosphorous acid molecule and 3 acetyl chloride molecules total.
Before going deeper into that, we'll look at the free radical chlorination mechanism. The half arrow heads signify single electron transfers. UV light splits chlorine into 2 chlorine radicals, half the C-H bond of acetic acid attacks a free chlorine radical and gives off HCl, the other half moves to the carbon. A radical results, which attacks another chlorine radical, forming chloroacetic acid. I'm not 100% confident in my depiction of this mechanism, if you have a better idea of how radical mechanisms work, let me know. All I did was apply the generic radical chlorination mechanism of alkanes to acetic acid.
Back to acetyl chloride. It tautomerizes to its ugly enol form, and its double bond reacts with chlorine to form chloroacetyl chloride and HCl. Chloroacetyl chloride reacts with acetic acid to form an intermediate that alternates between the two bracketed forms. The first form can kick off its chlorine to form free chloride, and oxygen's negative charge will move over to make a carbonyl group. The free chloride then attacks the other carbonyl carbon, and the C=O moves to the positively charged oxygen. The last intermediate undergoes a cool rearrangement, and what we're left with is chloroacetic acid and acetyl chloride. This reaction, i.e. everything that isn't the PCl3 equation or radical mechanism section, is known as the Hell-Volhard-Zelinsky halogenation.
Which of these two mechanisms predominates? I dunno. I think they both happen, but it could also be a lot more complex. Other videos where chloroacetic acid is produced use sulfur as a catalyst, one being Chemiolis' and depicting a slightly different mechanism, this, so it could also be that. I didn't have to quench the reaction with water, and nothing distilled under the boiling point of acetic acid, so it seems any acetyl chloride and chloroacetyl chloride produced were consumed during the reaction. This makes me think the radical mechanism is the main one.
The procedure I based this on reported a yield of 80-125g, but again, they exposed the reaction vessel to sunlight for one summer day or two winter days (in Edinburgh). I looked up the length of the day on the summer solstice in Edinburgh and it was 17h17m, whereas on the winter solstice, it's 6h57m, x2 equals about 14h. From this, what I'm calculating is obviously a comparitive approximation, as a "summer or winter day" isn't a precise unit of measurement. I think a yield of ~39% (of a 125g yield) to ~61% (of an 80g yield) with a difference of 11h40m-15h of UV light exposure and who knows how much less chlorine is quite good. The TCCA and HCl used to generate chlorine are cheap but generating chlorine for 14-17h straight sounds just awful.

Пікірлер: 13

@midwestchem368 Ай бұрын

I just read about doing this reaction and its so cool this popped up on my feed! Im excited for this!

@ChemDungeon Ай бұрын

Isn’t it cool? In every other video I saw, they use sulfur as a catalyst, which admittedly is much more attainable than red P for amateur chemists, so I thought this method would be interesting to try

@midwestchem368 Ай бұрын

@ChemDungeon so cool man! I'm actually planning on making chloroacetic acid from trichloroethylene. I'm glad to see more people using red p. It got a bad reputation for bad stuff we all know but it's so dang useful for a number of different uses.

@ChemDungeon Ай бұрын

@@midwestchem368 oh yeah? By the 75% H2SO4 high(ish) temp hydrolysis method? That’ll give you a much purer product for sure, no di- or trichlorinated side product formation. Red P definitely has gotten a bad rep, but it really is versatile and there are plenty of non-clandestine uses for it

@midwestchem368 Ай бұрын

@ChemDungeon yup that's the method! I just got a pressurized reaction vessel so I'm planning to try some small scale tests first. I'm trying it with some degreaser that is trichloroethylene and co2 and that way the method is essentially otc.

@ChemDungeon Ай бұрын

@@midwestchem368 is that how it’s done in whatever reference you’re using? 75% H2SO4 boils at ~180°C, the sealed system is used I’m guessing so the TCE doesn’t all just boil off at the reaction temp of 130-140°. I wonder if you could add the TCE drop wise after getting it up to temp, like how a typical ether from EtOH/H2SO4 synthesis is carried out

@SetTheCurve Ай бұрын

UV leds have a short lifetime and typically have poor efficiency, especially for uvc (I’ve heard). I work in agriculture and all sterilizing UV devices I’ve run into are HID style bulbs. The only UV leds I’ve seen that aren’t are low powered ones, mainly uva and uvb, intended to be used briefly and still be safe for plants.

@ChemDungeon Ай бұрын

That’s interesting, thanks for letting me know! I finally watched all of the extractions&ire cubane series so I know a little more about UV LEDs. Deep UV LEDs are still a work in progress as you probably know - I didn’t know they have a short lifetime/poor efficiency, but it makes sense given that they’re more or less in their developmental stages. I wonder how long it’ll be until high powered UV LEDs predominate over mercury lamps. If you can say, what exactly is UV sterilization used for in your agricultural occupation? I currently live in a state where ag is a main part of the economy but I didn’t grow up here so I’m not too keen on the specifics