PAA + AA --> P2P impurities?

[hypo]

hi, bees

do we know the impurities produced in the phenylacetic acid + acetic anhydride
synthesis?

situation is this: a guy did this synthesis but on distillation he got _horrible_
separation! he collected a fraction from 100-110°C, one from 110-120°C and finally
one from 120-130°C. then he stopped, because going on seemed rather pointless. of
course all fractions were pale yellow and smelled like P2P   . the last fraction
additionally had a pungent acetic acid smell.

so what could be the culprit?

dibenzylketone? hardly, this one has a very high boiling point and shouldn't interfere.

P2P enol ether? is it's boiling point lower or higher than P2P? if it's higher,
would it be prudent to just continue with the 100-110°C fraction?

something completely different? what? a carbonyl compound? (say it ain't so!)

thx.

[Rhodium]

Differentiation of Illicit Phenyl-2-Propanone Synthesized from Phenylacetic Acid with Acetic Anhydride Versus Lead(II)Acetate
AC Allen, ML Stevensen, SM Nakamura, RA Ely

Journal of Forensic Sciences 37(1), 301-322 (1992)

(https://www.thevespiary.org/rhodium/Rhodium/chemistry/p2p.phenylacetic.html)

Abstract
The illicit synthesis of phenyl-2-propanone (P2P) in clandestine drug laboratories from phenylacetic acid and acetic anhydride in the presence of sodium acetate or pyridine, or from the dry distillation of phenylacetic acid and lead(II)acetate is examined. These two routes are investigated using capillary gas chromatography (GC) combined with vapor-phase Fourier transform infrared (FTIR) spectroscopy and electron impact mass spectrometry (EIMS) detection (GC-EMR-EIMS), and using nuclear magnetic resonance (NMR) spectroscopy to identify 21 reaction by-products. The mechanisms of the two reactions producing P2P are presented, along with the mechanisms giving rise to these by-products. This investigation has identified four reaction-specific compounds which can be used to differentiate the two synthetic methods.

[Xicori]

It is quite common in vacuum distillations to get the product over a wider temperature range. - did u use a column? This usually improves the sharpness!

Do a TLC on each fraction to estimate which fraction is contaminated and which is not. Then redistill the contaminated fractions!

good luck!
xicori

PS.: Swim did a HPLC on p2p that returned from an PAA/AAA syntheis. The raw P2P was distilled with a vigreux column, and the product was also collected over a wider temperature range.

The product contained about ~5% of total impurities of which 90% were a less polar substance (Dibenzylketone), 5% were a substance that has a polarity between P2P and Dibenzylketone, and the residual 5% were more polar than p2p.

[Rhodium]

Did the guy hydrolyze the reaction mixture with aqueous base before distilling to produce P2P from the P2P enol acetate, which is a principal product of the reaction?

P2P enol acetate has a bp of 100-103°C/3mmHg, while P2P itself boils at 70-75°C at the same pressure.

[hypo]

thanks, chief, that's what i call a shitload of byproducts.  
fortunately all carbonyl-compounds seem to have high molecular weight,
so the impurities should be eliminated by a/b. and the amine from
dibenzyl ketone can probably be removed by distillation.

> The product contained about ~5% of total impurities of which 90% were a less
> polar substance (Dibenzylketone)

so despite using a column there was about 5% of dibenzyl ketone? this
is surprising! i thought it was so high boiling that it would hardly
distill over...

[hypo]

reaction mixture was shaken with aq. NaOH multiple times.
anyway, imho a second distillation is in order. it's not hard,
and there is no scorching. i wonder if the people in this forum
realise what an easy substrate they have!!  

[Xicori]

> so despite using a column there was about 5% of dibenzyl > ketone? this is surprising! i thought it was so high
> boiling that it would hardly distill over...

swim is not sure if it´s dibenzyl ketone- but it is a less polar compound than p2p. The 5% are also a rough estimate, because swim has no possibility to integrate his peaks yet   


best wishes
xicori

[amalgum]

Sounds to me like your column was to hot, and the distillation was probably to hot from the start.  Did you distill slowly?  SWIM has had problems like this before when trying to distill something to fast (with extra heat).  Try gently increasing heat very slowly, as you move slowly through the fractions.  Your gonna want to distill at about three to four drops a second.  Nice and slow.

[hypo]

to conclude this thread: the first two fractions were redistilled taking
a 15°C cut, the rest (still strongly smelling like P2P) was dumped. 51%
yield despite dumping product. fair enough. amalgum: the distillation was
done slowly, some mixtures just aren't easy to distill, this seems to be
one of them.

the P2P was aminated with standard Al/Hg/MeNO2, A/B, distillation, addition
of equimolar amount of dil. HCl and drying 48h over silica in good yield.

resulting amine had a mp of 133-134°C (lit: 134°C) and was flushed down the toilet.
the mp was taken quickly, that might explain the missing degree.

ps: if you thought that distilling P2P is easy as pie, then distilling meth is
a friggen children picnic: it comes another 10°C lower.
pps: freebase should _really_ be distilled. lots of dirt cannot be removed by
A/B as shown by this before/after/residue series of images:


yumm..