LarryC
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Does a ketone always float on water? Need experimental help please.
Experimentally I began with a simple looking formula which I modified slightly because of expected higher yields.
| Quote: | | 350 ml of concentrated sulfuric acid is placed in a 3000ml flask and cooled to -10°C. The total first crop of moist alpha-phenylacetoacetonitrile
obtained according to the procedure above (corresponding to 188-206g or 1.2-1.3 moles of dry product) is added slowly, with shaking, the temp being
kept below 20°C (If pure dry alpha-phenylacetoacetonitrile is used, half its weight of water should be added to the sulfuric acid or charring will
take place on the steam bath). After all is added the flask is warmed on the steam bath until solution is complete and then for five minutes longer.
The solution is cooled to 0°C, 1750ml of water added rapidly, and the flask placed on a vigorously boiling water bath and heated for two hours, with
occasional shaking. The ketone forms a layer and, after cooling, is separated and the acid layer extracted with 600ml of ether. The oil and ether
layers are washed successively with 100ml of water, the ether combined with the oil and dried over 20g of anhydrous sodium sulfate. The sodium sulfate
is collected on a filter, washed with ether, and discarded. The ether is removed from the filtrates, and the residue distilled from a modified Claisen
flask with a 25 cm fractionating side arm. The fraction boiling at 110-112°C at 24 mmHg is collected; it weighs 125-150g (77-86% of the theoretical
amount). |
Rather than use dilute sulfuric acid I chose to prepare the ketone using 85% food grade phosphoric acid. 415ml to 200 grams of
alpha-phenylacetoacetonitrile. I boiled together for 3three and a half hours with shaking every 2 minutes or so. Yields looked excellent. I initially
separated the acid layer from the ketone and simply discarded the acid layer rather than bothering to extract it. I had roughly 175ml of slightly dark
but oily looking product. On the sides of the flask the oil appeared nice and yellowish as it ran down the sides so I was confident that I had the
correct product, though polluted and in need of purification. I added that product to my seperatory funnel and added 100 ml dH2O and shook vigorously
for 15 seconds. The ketone floated on the surface after it stood for a while. Repeating the process then with an additional 100ml dH2O the keton never
floated to the top. Rather oily balls similar to those you might see in a lava lamp formed and went together. The "ketone" layer if I can call it that
(as I have lost confidence) settled to the bottom of the separatory funnel. The water on the top as well as the suspected P2P is deep yellow in color
and highly terpid looking. The ketone also seems to have expanded to roughly 225ml as it is highly polluted with water now.
I seek advise on how to proceed. I had believed that P2P always floats but apparently it does not, or I have the wrong product. I plan on continuing
with fractional distillation. Specifically should I extract the P2P with ether before distilling or distill as is? Will it make a difference? And why
doesn't my ketone float?
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bbartlog
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Hrm. I don't like your substitution (phosphoric for sulfuric acid). Phosphoric acid is just *massively* less acid than sulfuric acid (many orders of
magnitude). And using it at 85% concentration, you're potentially driving the equilibrium for hydrolysis in the wrong direction (or at least not
helping it in the right direction), since at such concentration phosphoric acid is extremely hygroscopic. Most likely your reaction simply hasn't
proceeded very far.
The less you bet, the more you lose when you win.
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LarryC
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Quote: Originally posted by bbartlog  | | Hrm. I don't like your substitution (phosphoric for sulfuric acid). Phosphoric acid is just *massively* less acid than sulfuric acid (many orders of
magnitude). And using it at 85% concentration, you're potentially driving the equilibrium for hydrolysis in the wrong direction (or at least not
helping it in the right direction), since at such concentration phosphoric acid is extremely hygroscopic. Most likely your reaction simply hasn't
proceeded very far. |
Thank you for your rapid and succinct reply. I must now confess my utter confusion.
It was relayed to my that a Russian journal reported higher yields from the substitution of phosphoric for sulfuric acid in the above referenced
procedure at higher temperature of 140C. I initially began simply substituting phosphoric for sulfuric at the same dilution rate and ended up with
nothing but unchanged alpha-phenylacetoacetonitrile (cas# 4468-48-8 to be sure we're on the same page here). As a result of that initial failure I
used it at 85% concentration (food grade) which was the highest percentage easily available to me. When dried and powdered the phenylacetoacetonitrile
mixed into solution with the phosphoric acid and I boiled it together with occasional shaking. It seems to me and please explain why I'm in error,
that the higher % of the acid the better the hydrolysis. From all appearances of the product of the reaction it was indeed the ketone, although darker
than pure P2P should be. If I do not have the phenylacetone what have I produced? I am already loath to trust Russian science but it seemed to be
worth the experimental risk for what was reportedly higher yields. I can take and upload pictures if that would help.
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bbartlog
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OK, so at higher temperatures (140C versus 100C or so for the boiling water bath) I can imagine that the faster reaction rate would make up for the
lower acidity of the phosphoric acid. Fair enough.
The density of phenylacetone is really close to that of water, so if you want a clean separation in a separatory funnel I would use some salt to
increase the density of the aqueous layer. In the original Organic Syntheses preparation (which you could have linked to: http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0389 ) the fact that the 'aqueous' layer is actually about 25% sulfuric acid (density roundabout
1.18 gm/cc) allows for the ketone to float.
Shorter version, the floating/sinking behavior in pure water doesn't mean much and actually seems like grounds for cautious optimism. I would distill
and see what the boiling point looks like.
Also, if yield is really a concern for you, following the workup in the published synthesis more closely would probably help.
The less you bet, the more you lose when you win.
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LarryC
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Thank you for your reply. My mind is more at ease now. I won't have the opportunity to distill until Monday but I am optimistic. The water portion of
the two washings is saved in a glass jug. Assuming the ketone is what I have I'll build up a lot of the water portions to extract at a later time,
considering it a byproduct to process last after the phenylacetoacetonitrile has all been processed. I am sensitive to the difference in yield. It is
estimated at 60% when using Sulfuric and 75% with the use of phosphoric. Given the quantity I'm dealing with that's a 3kg difference in the amount of
p2p I can get for only a few euros more per gallon of phosphoric, which is also why my experiment is worthwhile to attempt. Unfortunately I don't read
Cyrillic so I had to take it as was relayed to me, to my annoyance. Thank you and best regards.
[Edited on 26-1-2013 by LarryC]
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