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JJay
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Trifluoroacetic mixed anhydrides can often substitute for acyl chlorides. Trifluoroacetic anhydride is a little exotic but has many uses and isn't out
of reach of the advanced amateur.
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Melgar
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I ended up taking a second look at phosphorus halides. If I had to choose between already-prepared phosphorus pentachloride, and phosphorus
tribromide, and I mainly wanted to prepare acid chlorides, which should I use and why? I'm leaning towards phosphorus pentachloride, since it's
cheaper, and turns into phosphoryl chloride when it loses two chlorine atoms. Apparently, phosphoryl chloride can turn some, but not all carboxylic
acids into their acid chlorides, depending on which chlorine atom is active and whether the carboxylic acid is stronger than the corresponding
phosphoric-chloride acid? Does phosphorus tribromide have a better atom efficiency? Are acid bromides that much easier to work with than chlorides,
assuming most of the acids I'll be working with will have three or more carbons?
Still, there's a lot more literature on acid chlorides, so I'm kind of biased towards that approach anyway.
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Kapitan
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Thionyl chloride is one of my favorite reagents. It always seems to always work really well, and the workup is a snap: Just distill off the excess
reagent. Plus, once I use SOCl2 I can't smell any of the other bad-smelling reagents that I use for the rest of the day 
Oxalyl chloride is nice because it's like the magical self-purifying reagent (all the decomposition products are gaseous), but in my experience the
results are more inconsistent than with SOCl2. I guess it's supposed to be like the "greener" SOCl2, but it just doesn't seem to work as well.
I try to avoid POCl3 and PCl5, both because they're super nasty reagents, and also because I was sprayed in the face with them once and can't stand
the smell.
[Edited on 7-3-2017 by Kapitan]
[Edited on 7-3-2017 by Kapitan]
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Elemental Phosphorus
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I think that for almost all uses, it does not matter whether an acid bromide or chloride is used. The chlorine atom will form a stronger bond with
alkali ions, like in the creation of anhydrides, but it is a worse leaving group owing to the increased strength of the carbon-chlorine bond. The
bromine atom forms a weaker bond with sodium/potassium ions and hydrogen but is a better leaving group.
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Melgar
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Quote: Originally posted by Elemental Phosphorus  | | I think that for almost all uses, it does not matter whether an acid bromide or chloride is used. The chlorine atom will form a stronger bond with
alkali ions, like in the creation of anhydrides, but it is a worse leaving group owing to the increased strength of the carbon-chlorine bond. The
bromine atom forms a weaker bond with sodium/potassium ions and hydrogen but is a better leaving group. |
Probably. Assume phosphorus is the limiting reagent though. Wouldn't PCl5 be better, if only because it has more Cl groups? After all, after it
gives up the first two, it's still the acid chloride of phosphoric acid, and can continue to react with carboxylic acids to produce acid chlorides.
Also, I've since learned that if Cl2 and SO2 are combined in the presence of phosphorus, thionyl chloride and phosphorus oxychloride are both formed
together, and can be separated by distillation. That seems like far less of a pain in the ass than screwing around with SO3, and you get two useful
reagents instead of just one. You can use POCl3 when possible then, and SOCl2 when it's not possible.
[Edited on 3/27/17 by Melgar]
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Elemental Phosphorus
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If the PCl5 is already prepared and cheaper it is probably the better way. Otherwise, preparation of PBr3 is easier to do. I would rather make the
thionyl chloride to be honest, since it is not too hard to prepare SO3 (think a thick steel retort with FeSO4 on a coal fire) and the steel will be
passivated by an FeSO4 layer. Mole for mole, gram for gram PCl5 is the better way (with phosphorus as limiting reagent), if you don't mind chlorine
gas, or just buy the PCl5.
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