night429
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A failed attempt at p-hydroxybenzoic acid
I was following the procedure per orgsyn.
I first measured out 12.2g of potassium hydroxide and added it to a mixture of 30.0g of salicylic acid and 33mL of water. It heated up rapidly as it
dissolved, and I ended up with a yellow solution. I then added an additional 1.5g of potassium hydroxide and 2.2g of sodium bicarbonate to give the
appropriate amount of potassium carbonate described in the procedure. I heated this until the surface started to crust over, then cooled it (pH around
10 at this point). Once everything solidified, I transferred it to a foil-lined oven rack and heated at 225°F for 2 hours. Once done, I let it sit
overnight.
I I then heated it for another hour in case of water absorption, since it was a rainy day. I then ground it up, then heated for another 2 hours at
225°F.
After this was done, I put it into a 50mL flask and heated in an vegetable oil bath, with the neck about an inch from the oil surface (I had covered
the pan of oil with foil—not the flask's neck). After 2 hours, a test (putting some of the solid into warm water, then acidifying with HCl) proved
to give a precipitate, so I figured the oil bath did nothing.
I then put the flask onto the hotplate directly, insulated with aluminum foil. I ensured the solid remained un-melted, and some amount of phenol was
produced throughout. The surface color turned brown over the course of the reaction. I also stirred it somewhat-often with a metal stir rod. After an
hour, the aformentioned test produced no precipitate. I waited an extra 15 minutes, then turned the heat down. After another 15 minutes, I dumped
everything into 200mL of hot water. I added an extra 100mL, then added 40mL of 31% HCl.
A brown precipitate formed. I heated until near-boiling (not everything dissolved), and I added 2.5g of activated carbon, then continued stirring for
10 minutes. I filtered everything off (including a lot of crystalline solid).
I let it filter over the course of 2 days, during which point a lot of needle-like crystals formed. I filtered off these crystals, and once mildly
dry, I scooped the crystals out into 100mL of water. I heated this until everything dissolved, then took out the stirbar and let everything
crystallize. I let these brown needle-like crystals dry, then did a melting point test. Extrapolated (since my thermometer only went up to 150°C),
the melting point was estimated to be around 173°C.
This leads me to believe that I probably only successfully made impure salicylic acid during this reaction. Temperature, rather than phenol production
or the aformentioned "test", seems to be more indicative of reaction progression, and one should have a way of measuring up to 240°C to successfully
do this reaction.
"Let's spin apart while racing through the atmosphere"
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bnull
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What follows is a speculation.
Quote: Originally posted by night429  | I was following the procedure per orgsyn.
I first measured out 12.2g of potassium hydroxide and added it to a mixture of 30.0g of salicylic acid and 33mL of water. It heated up rapidly as it
dissolved, and I ended up with a yellow solution. I then added an additional 1.5g of potassium hydroxide and 2.2g of sodium bicarbonate to
give the appropriate amount of potassium carbonate described in the procedure. I heated this until the surface started to crust over, then
cooled it (pH around 10 at this point). Once everything solidified, I transferred it to a foil-lined oven rack and heated at 225°F for 2 hours. Once
done, I let it sit overnight. |
You deviated substantially from the procedure. It says "(t)he finely powdered mixture of potassium salicylate and carbonate". You must dry and powder
the salicylate before adding the carbonate, rather than add carbonate to the salicylate solution.
Also, you used sodium carbonate and potassium hydroxide. Supposing that the appropriate amount of potassium carbonate was produced, you ended up with
some sodium hydroxide in the mix. As far as I remember, a mixture of sodium and potassium hydroxides is used in decarboxylations; that should explain
the presence of phenol.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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night429
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| Quote: Originally posted by bnull | What follows is a speculation.
You deviated substantially from the procedure. It says "(t)he finely powdered mixture of potassium salicylate and carbonate". You must dry and powder
the salicylate before adding the carbonate, rather than add carbonate to the salicylate solution.
Also, you used sodium carbonate and potassium hydroxide. Supposing that the appropriate amount of potassium carbonate was produced, you ended up with
some sodium hydroxide in the mix. As far as I remember, a mixture of sodium and potassium hydroxides is used in decarboxylations; that should explain
the presence of phenol. |
I disagree with your speculations. The procedure never described adding additional potassium carbonate to the dry neutralization mixture, only that
the mixture of salicylic acid and potassium carbonate was powdered, dried, then powdered again.
Also, because I used sodium bicarbonate, this should've theoretically reacted with the excess potassium hydroxide to form sodium
carbonate, potassium carbonate, and water.
Phenol is also described in the procedure as an equimolar side product, so it was expected to have been produced. I just don't think it was produced
it in sufficient quantities to drive the reaction to completion due to my poor temperature control.
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bnull
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Nevermind. It was just an speculation.
| Quote: Originally posted by night429 |
Also, because I used sodium bicarbonate, this should've theoretically reacted with the excess potassium hydroxide to form
sodium carbonate, potassium carbonate, and water. |
Ah, yes... Titivillus attacked again.
If you want a better thermometer, why not use a thermocouple?
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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clearly_not_atara
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Orgsyn says 230 C. Your procedure says 225 F. This discrepancy is alarming.
EDIT: Oh, I see. You didn't measure the temperature during the procedure. This usually results in a much worse result.
230 C is close to the maximum operating temperature of most vegetable oils. You shouldn't use a random vegetable oil like this unless it specifically
says it has a smoke point above 230 C. You absolutely need a thermometer when working at these high temperatures.
[Edited on 30-1-2024 by clearly_not_atara]
[Edited on 04-20-1969 by clearly_not_atara]
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night429
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| Quote: Originally posted by clearly_not_atara |
EDIT: Oh, I see. You didn't measure the temperature during the procedure. This usually results in a much worse result.
230 C is close to the maximum operating temperature of most vegetable oils. You shouldn't use a random vegetable oil like this unless it specifically
says it has a smoke point above 230 C. You absolutely need a thermometer when working at these high temperatures. |
I agree. My concern was primarily on overheating, but I was too conservative and it didn't react enough, if at all.
Initially, my thoughts were that, by having an oil with a smoke point below 240°C, I could visually see when it was getting to hot. After I ditched
this due to not getting up to the smoke point, I thought I could get away with it if I didn't let it melt (potassium salicylate's melting point is
reportedly about 250°C), but it again didn't heat enough. That's what I get for improvising.
I hope to redo it once I get a thermocouple and use a sand bath to get up to the appropriate temperatures.
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DraconicAcid
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I don't know anything about the synthesis of this stuff (I made mine from methyl paraben), but I hate it. I try to recrystallize it, and it comes out
of solution sooooo slowly. I'd filter it after it sat in the ice bath for an hour, and it would still crystallize in the filtrate the next day.
I may be exaggerating, but I well remember it being very frustrating.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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