Lionel Spanner
Hazard to Others
Posts: 163
Registered: 14-12-2021
Location: near Barnsley, UK
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Acid chlorides (and acetic anhydride) according to Venkataraman
The attached paper by Venkataraman and Wagle, from Tetrahedron Letters (1979), describes a way of preparing acid chlorides and their derivatives using
cyanuric chloride - which is much less toxic, less generally hazardous, and easier to find than thionyl or oxalyl chlorides. I bought some from
Laboratorium Discounter at €28 for 500 g.
Recently I tried the procedure myself by making acetyl chloride, and then acetic anhydride by reacting it with anhydrous sodium acetate, as per Rhodium. Below are my lab notes. I intend to scale this up at a later date, and will very likely make a video.
Experimental
Acetone (100 mL) was added a dry 250 mL round bottomed flask. Cyanuric chloride (9.2 g, 0.05 mol) was added with stirring and gentle heating until
dissolved. Glacial acetic acid (6 g, 0.1 mol) was added and mixed until uniform. At this point the flask was moved to an ice bath, and triethylamine
(10.1 g, 0.1 mol) was added dropwise in small portions. As soon as the triethylamine addition began, a head of very fine white particles (smoke)
developed above the surface of the reaction mixture - it even formed around the tip of the dropper itself before the first drop was added. Additions
of triethylamine produced more smoke, up to the neck of the flask, and further material was not added until this subsided. After all the triethylamine
was added (2 hours), the smoke thinned and eventually disappeared within 15 minutes. Mixing was continued for a further hour, then the reaction
mixture was filtered by gravity, yielding a clear filtrate, which was left to stand overnight, and a white precipitate that proved to be readily
soluble in water, which was discarded.
Note: The smoke is believed to be triethylaminium acetate, formed from the vapour-phase neutralisation of acetic acid and triethylamine, hence the
very fine particles. Around 30 minutes after the triethylamine addition was completed, some barely visible transparent needles 1-3 mm in length were
observed on the wall of the flask, just above the surface of the mixture.
The following day, it was observed that more material had precipitated from the filtrate in the form of coarse colourless crystals, which also proved
to be soluble in water - apparently, the reaction was incomplete when the mixture had been filtered. The mixture was filtered again, and anhydrous
sodium acetate (8.2 g, 0.1 mol) was added to the filtrate, with stirring, creating a suspension of fine white solids. Mixing continued for a further 2
hours, before the mixture was filtered by gravity, yielding a clear filtrate and a very fine white precipitate, which was washed with a few mL of
acetone and eventually compressed to remove as much entrained solvent as possible. (Going forward, it would be wise to perform this filtration under
light vacuum.)
Note: After one hour, a sample of suspended powder was mixed with conc. sulphuric acid, producing hydrogen chloride gas, which fumed in moist air,
suggesting the suspended powder contained a substantial amount of sodium chloride. The same result was observed when conc. sulphuric acid was added to
the residues of the reaction flask. Both sodium acetate and sodium chloride are insoluble in this solvent system, and are visually almost
indistinguishable, other than by particle size.
Acetone was removed from the mixture by distillation. Vapour started to condense at around 54-56 °C, and the boiling point increased as the solution
became more concentrated. The boiling point peaked around 114 °C and began to fall; at this point, approximately 100 mL of liquid had been obtained,
and the distillation was stopped. Once cooled to room temperature, the distilland took the form of a harsh-smelling pale yellow liquid with a large
amount of colourless crystals entrained in it. This was reheated to boiling point, and all the liquid that distilled below 130 °C (a few mL) was
discarded. The main product was a clear colourless liquid that distilled around 138-140 °C. As distillation neared completion, the temperature at the
still head fell and the distillation flask became filled with smoke. The distillation was therefore ceased.
From the distillation, acetic anhydride was obtained as a clear colourless liquid with an odour of vinegar with organic-solvent overtones (10.2 g, 0.1
mol; 100% wrt acetic acid.) As there was still a noticeable smell of acetic anhydride in the distillation flask, it is very likely the product was not
entirely pure, and still contained a little residual acetone.
Conclusion
The method in the paper was successfully reproduced, and is potentially a very useful general method for the preparation of acid chlorides. Going
forward, the distillation should be performed in one continuous run.
Attachment: acid chlorides, amides, esters etc. from cyanuric chloride.pdf (223kB)
This file has been downloaded 198 times
[Edited on 22-4-2023 by Lionel Spanner]
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