Sciencemadness Discussion Board

Stubborn benzophenone

Keras - 30-5-2024 at 22:29

Folks,

I've just made some benzophenone using a Friedels-Craft reaction between benzene and benzoyl chloride, as described in Vogel’s classic reference work. Distilled at 175 °C under reduced pressure, came as a clear, colourless liquid. Left in the flask was an orange compound (biphenyl?) not unlike what many people have obtained after distilling benzene from pyrolysis of sodium benzoate.

Problem is: albeit this benzophenone smells exactly as it should, with no overtones of any contaminant, it won’t crystallise. It is stubbornly liquid, and even a few hours at -18 °C won’t solidify it. Any idea why it displays such behaviour? Is benzophenone known to supercool in any way?

Thanks!

EF2000 - 31-5-2024 at 02:06

An article about benzophenone's glassy state (https://doi.org/10.1557/PROC-455-429) says that "In fact, it is difficult to freeze benzophenone on cooling."
You can try to introduce nucleation points, something like freezing a droplet and adding it to the rest of benzophenone.

[Edited on 31-5-2024 by EF2000]

Sir_Gawain - 31-5-2024 at 10:39

Try gently scratching the inside of the beaker with a glass rod. That usually does the trick.

Keras - 31-5-2024 at 11:50

Quote: Originally posted by Sir_Gawain  
Try gently scratching the inside of the beaker with a glass rod. That usually does the trick.


Yeah, I tried that. Not a beaker but a tiny bottle. No dice :/

bnull - 31-5-2024 at 12:02

Dissolve some drops of benzophenone in ethanol or ether, spread the solution on a dish to dry and then sprinkle the powder over the rest of the benzophenone. I wonder if it will crystallize suddenly or do it slowly.

Rainwater - 31-5-2024 at 13:25

When scratching the glass doesn't work, using glass frits usually does the trick.
Frits is easily made by heading the end of a glass rod to red heat, then submerging it into water.

Keras - 2-6-2024 at 11:31

Quote: Originally posted by bnull  
Dissolve some drops of benzophenone in ethanol or ether, spread the solution on a dish to dry and then sprinkle the powder over the rest of the benzophenone. I wonder if it will crystallize suddenly or do it slowly.


So far, neither. I've just ended up with droplets of liquid benzophenone all around.

Facing these repeated failures, I asked myself if there were traces of contaminants which could explain this stubbornness. I washed my benzophenone twice with boiling water (the rationale being that any leftover benzoyl chloride would hydrolyse into benzoic acid/hydrochloric acid and be dissolved in hot water, and any leftover benzene would float above the water layer while liquid benzophenone is denser), but apart of making wet benzophenone that I had to dry up with a significant amount of sodium sulphate, it is still liquid.

bnull - 2-6-2024 at 16:12

Did you distill it again?

Keras - 2-6-2024 at 22:38

Quote: Originally posted by bnull  
Did you distill it again?


No. I didn't get back to my lab since, and have only a reduced set of tools at home (my water aspirator is over there).

[EDIT] I did it without much hope using the cheap vacuum pump I use for vacuum filtration and which turned out to be sturdier than I thought. I was able to get the benzophenone boiling on my hob (stovetop) without even turning the smallest burner in full (see picture). There were definitely impurities since I was left with a nice benzophenone and some sort of carbonised compound in the RBF. Now I've put the distillate back in its bottle and in the fridge. Will it crystallise? Bets are open. :p

[Edited on 3-6-2024 by Keras]

Distilling_Benzophenone_on_the_Corner_of_a_Hob.jpeg - 2.7MBBenzophenone.jpeg - 1.6MBResidue.jpeg - 2.1MB

Keras - 3-6-2024 at 11:29

Well, I’m not sure what happened.

I stored the redistilled benzophenone in the same vial I used the first time. There were like small bubbles in it, I wonder if it’s a leftover from the pre-redistilled product (didn't wash the vial in-between, which was silly). The smell is slightly different, more ‘sooty’? Definitely stronger.

I put the vial in the fridge. Still liquid. Put it in the freezer. Very syrupy, but still liquid. Put it back in the fridge, then I was fed up, I grabbed it and simply placed it on the table. Nothing happened. Got to get somewhere else for two minutes, and when I came back it was all frozen solid! :/ And the vial was slightly warm.

Now, what's weird is that I even have crystals on the vial thread. It simply beats me. Now I’ll have to recrystallise it, obviously. I’ll scratch it to save some tiny crystals before… :p

IMG_2304.jpeg - 3.7MB

[Edited on 3-6-2024 by Keras]

bnull - 3-6-2024 at 13:14

I thought a re-distillation would work. It's good to see that it did.
Quote:
I put the vial in the fridge. Still liquid. Put it in the freezer. Very syrupy, but still liquid. Put it back in the fridge, then I was fed up, I grabbed it and simply placed it on the table. Nothing happened. Got to get somewhere else for two minutes, and when I came back it was all frozen solid! :/ And the vial was slightly warm.

Just like sodium acetate. Congratulations.

If you have more than enough benzophenone, you may want to try the synthesis of benzopinacol: benzophenone plus isopropanol (or ethanol) and light (http://www.orgsyn.org/demo.aspx?prep=CV2P0071).

Keras - 3-6-2024 at 22:03

Thanks! Also thanks for suggesting that second distillation. I’m always a bit wary, because of the inherent losses. Yet, with the microscale apparatus I use, very little product remain stuck to the glassware.

I can try benzopinacol. Couldn't find much information on it, though. Does it have a particular use or scent or w/e?

[EDIT] Speaking of acetate, I have no such problem with calcium acetate that I make as a byproduct of the haloform reaction between calcium hypochlorite and acetone. It crystallises quite well!


[Edited on 4-6-2024 by Keras]

bnull - 4-6-2024 at 06:52

You would have to do that second distillation anyway.

No, I just thought benzopinacol would be interesting because it is a photochemical synthesis, and you can suspend the reaction and continue later without problems. It resumes from where it had stopped. Benzopinacol is used in polymerizations by free radicals. As for the smell, no clue.

Keras - 4-6-2024 at 07:27

I also would like to try the making of tetrachloromethane from chloroform and chlorine in a seal quartz RBF left outside in the sun.