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Author: Subject: Cubane; Dimethyl Cubane-1,4-dicarboxylate synthesis
Tdep
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[*] posted on 29-6-2020 at 21:40


Oh that's because..... um...... hm I have no idea. Why did they use a medium pressure lamp? There's a small peak in the 300-350 range but its <1/10th the size of the main peak at 250nm.

Maybe you have to always target the first transition in a photochemical reaction? Ground/S0 -> S1 transition, not the S0 -> S2 or whatever? They must have used a medium pressure lamp for a reason, but I fail to see how the Supp Info figure shows that "300-350nm is a critical region"
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[*] posted on 30-6-2020 at 11:08


I also fail to see why one peak should be favored and would love an explanation.
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Tsjerk
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[*] posted on 30-6-2020 at 12:23


In the article they state their lamp is inefficient for this process, and 300-350nm is the optimum... So I'm leaning towards the 310nm lamp...

Edit: screw it, I just bought the 310nm lamp. If it doesn't work I can always buy the 254nm one, those are only 10 euro and fit in the same socket.

[Edited on 30-6-2020 by Tsjerk]
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Tsjerk
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[*] posted on 1-7-2020 at 02:23


I just realized they use a pyrex reaction for the photocyclization, which absorps anything below 275 and most below 300nm anyway... not sure what to make of that. Would that mean lower wavelengths do work but are not considered as they are not transmitted by the pyrex anyway, coming to the conclusion 300-350nm is the optimum because that is the lowest that can pass. Or did they use the pyrex reactor because 300-350nm is the optimum anyway?

pyrex_quartz_transmission.png - 100kB



Edit: this is the spectrum of a medium pressure Hg lamp after pyrex filtering the light.


emission_Hg_medium_pressure_pyrex_filtered.png - 14kB



[Edited on 1-7-2020 by Tsjerk]
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Tdep
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[*] posted on 1-7-2020 at 04:24


What's also strange, is that the medium pressure lamp seems to originate from this reference, where they did the photo-reaction in... benzene?? A solvent that would completely absorb all <300 nm light in the first centimetre of the reaction vessel.. completely impenetrable.

So did they use a medium pressure lamp because they had to use benzene for the reaction, which forced them to use ~350nm light? Or did they chose benzene because the <350nm light doesn't matter? Or was the medium pressure lamp just what they had and everyone just did it too? Does everyone hate quartz glassware? Is this a conspiracy????


medium pressure lamp.PNG - 170kB
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Tsjerk
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[*] posted on 1-7-2020 at 04:57


Well, the 85:5 methanol/water solvent system used by Falkiner et al. also absorbs significant amounts of everything <300nm, just as the benzene does. So I don't think conspiracies theories are needed here as an explanation (bummer), and pyrex is just cheaper than quartz.

methanol_water_UV-VIS.png - 24kB

[Edited on 1-7-2020 by Tsjerk]
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Tdep
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[*] posted on 1-7-2020 at 06:01


That's a weird diagram, because I know methanol is transparent to UV down to about 215nm, I've used it as a solvent for UV-Vis. Methanol/water mix won't absorb the 250nm low pressure mercury emission peak at all. But yes, maybe the critical region really is just where the substance absorbs but normal glass transmits fine
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Tsjerk
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[*] posted on 1-7-2020 at 07:16


The scale on that last graph is logarithmic, so absorption would be around 10% max indeed.
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Tsjerk
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[*] posted on 1-7-2020 at 11:29


I contacted the corresponding author of the Falkiner article and he told me that 300-350nm is the target. They use Pyrex on purpose because shorter wavelengths would induce unwanted side reactions. He said the mercury lamp produces about ten percent at the correct wavelengths, so with about 200 watt they can transform 0.25 gram/minute.

[Edited on 1-7-2020 by Tsjerk]
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Tdep
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[*] posted on 1-7-2020 at 19:04


Alright, I guess that's the answer then, good choice on buying the UV-B bulb. Excited to see how you go with it!
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Tsjerk
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[*] posted on 9-8-2020 at 06:18


Update: I'm pretty sure I finally got compound 10 (the dibromdicyclopentadiene-1,8-dione ketal)... The bromination and dehydrobromination were straightforward, just put the reactants together and stir/reflux. After adding water a brownish precipitate forms which settle to the bottom after some time.

In a separatory funnel I got rid of most of the supernatant, but then trouble started...

The brown sludge is impossible to filter, the stuff is so fine it clogs any filter in minutes. I managed to pull of a large part of the water over many hours over a high vacuum on a large Buchner filter. I then evaporated much water under vacuum over a 50 degrees water bath. The sludge was then left to stand for a couple of weeks after which most of the water evaporated.

The last 100 ml of water was pulled of again on a Buchner filter. After an hour the brown (partly black) product was extracted with ethyl acetate by boiling and hot filtering the mixture. A little black crud was left, but probably only 5-10% of the amount of product.

The solution in ethyl acetate is now evaporating, as in the freezer only water froze out. The solution has an orange color, but I saw some nice white/colorless crystals in the brine I used pull out the water from the ethyl acetate before leaving it to evaporate.

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[*] posted on 9-8-2020 at 13:57


I've been watching this very cool project with some interest. I'm tempted to buy some adipic acid and follow along.
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[*] posted on 9-8-2020 at 14:26


Cool! I can advice when making the ethylene diketal to run the Dean Stark as slow as you can. Ethylene glycol and toluene also form an azeotrope (110 degrees) which I'm sure screwed up my yield considerably by making me believe the reaction was done.

During the vacuum distillation of the ketal I'm pretty sure I also distilled some glycol as at the end a more viscous liquid came over. I wouldn't be surprised if my bromination/dehydrobromination would have been a lot less black if my ketal would have been cleaner.

I guesstimate I have about 30 gr of compound 10 now as really nice white crystals. They crystallize from the orange liquid, and there will probably be an orange crust when totally dry, but I won't try to clean that up as in the article they talk about a beige powder used without recrystallization.

Edit: I started with 40 gr of ketal.

[Edited on 9-8-2020 by Tsjerk]
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[*] posted on 9-8-2020 at 17:54


So I'm officially very invested in this project, here's my into video outlying the plan: https://youtu.be/ney-qTU3m4I
and my first attempt at making the ketal: https://youtu.be/sBA_UV15k0c

It'll be nothing new to you Tsjerk, or anyone following this thread closely. Looks like i've made a lot of mistakes too... aldol condensation... ethylene glycol-toluene azeotrope (I hadn't thought of that, that explains what I was seeing in the dean Stark I think!).... but isn't everything so much easier with hindsight! Hopefully the next attempt will be much more refined.
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Tsjerk
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[*] posted on 27-12-2020 at 07:46


Very nice video on YouTube Tdep! Good idea to not do the bromination and dehydrobromination in one pot. Your work-up looks a lot easier than my filtration did. Hopefully this won't happen after you do the dehydrobromination...

I do think the use of less than an equimolar amount of dioxane is what caused the off-gassing, not the use of DCM as a solvent. Bromine doesn't dissolve in DCM very well, while it forms a complex with dioxane. I think the combination of free bromine and HBr causes the gas to come off. I didn't notice any gas coming off.

Edit: I once dried sieves in a microwave/oven, thinking I turned on the oven part... Turned out I turned on the microwave part for 20 minutes which completely fused the sieves, the beaker and the glass plate in the microwave/oven combination. Apparently sieves happily absorb microwaves.

I was noticed by the smoke alarm things weren't going as I intended as the rubber lining below the glass plate was being turned to tar.

[Edited on 27-12-2020 by Tsjerk]
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[*] posted on 27-12-2020 at 07:55


Btw, I performed the de-ketalization. Straight forward process, I performed the reaction on a day the temperature in my lab was about 25 degrees during the day and didn't drop much at night. Good to know is that the reaction starts off quite exothermic, so cooling was necessary at the start.
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