Sciencemadness Discussion Board

Drying acyl halides with molecular sieves?

SplendidAcylation - 24-6-2022 at 14:20

Hi,

Having recently prepared benzoyl chloride by the chlorination of benzaldehyde, I was about to add molecular sieves to the bottle, as I usually do, but then I began to doubt whether or not molecular sieves are compatible with acyl halides, having never dealt with the latter before.

I have Googled it, and I have found nothing to indicate any problems with molecular sieves and acid halides, but not very many examples of their being used for these compounds...

Can anyone confirm that it will not be a problem?


I usually don't like to post such dumb questions, but I really don't want to take the chance of ruining my benzoyl chloride!


Thanks in advance

PirateDocBrown - 24-6-2022 at 21:14

Sieves do not like acids. If you have some hydrolysis, you will have acids. Maybe try calcium chloride?

SplendidAcylation - 25-6-2022 at 02:16

Quote: Originally posted by PirateDocBrown  
Sieves do not like acids. If you have some hydrolysis, you will have acids. Maybe try calcium chloride?



Yes, that is one of the reasons I am unsure about it...
Indeed, one might argue that it isn't really necessary to dry acyl halides as they react with water and shouldn't contain any, but of course there will always be some that doesn't react straight away, besides, a drying agent could perhaps remove the water before it reacts, if there is a slight leak in the bottle or such like.

UC235 - 25-6-2022 at 08:31

There is no way there would be free water in acyl halides. Free carboxylic acid and dissolved HCl from reaction with moisture, yes.

Texium - 25-6-2022 at 13:01

If you want to neutralize any acids that may be present in your benzoyl chloride, perhaps treating it with some calcium carbonate would work. I haven't tried this myself, but I wouldn't foresee it being a problem. Benzoyl chloride itself shouldn't react with carbonates.

SplendidAcylation - 26-6-2022 at 02:08

Thanks for the responses.
Quote: Originally posted by UC235  
There is no way there would be free water in acyl halides. Free carboxylic acid and dissolved HCl from reaction with moisture, yes.


The reason I was initially worried about water was that there seems to be condensed droplets on the sides of the bottle; I thought this might have been aqueous, due to the dissolution of HCl vapours in atmospheric water vapour.

Since then, I have come to the conclusion that the droplets are probably just condensed benzoyl chloride.
Quote: Originally posted by Texium  
If you want to neutralize any acids that may be present in your benzoyl chloride, perhaps treating it with some calcium carbonate would work. I haven't tried this myself, but I wouldn't foresee it being a problem. Benzoyl chloride itself shouldn't react with carbonates.


I thought about this too!
If benzoic acid is insoluble in benzoyl chloride, then it would obviously precipitate out and cause no problem, if it does remain dissolved, then I suppose it might be neutralized by solid CaCO3; Either way, dissolved HCl would definitely react, so that sounds like a good idea.

I was surprised by how slowly benzoyl chloride seems to react with water; It only seems to fume in the air when it is humid, and droplets of benzoyl chloride seem to happily float atop water, even aqueous sodium hydroxide, for quite a while before dissolving.