Mesityl Oxide and Phorone Synthesis

Between my sophmore and junior years in chemistry, I was a lab assistant for the Columbus College Chemistry department. The supervisor for the lab assistants was a professor by the name of Al Osteen.

Mr. Osteen would often run odd experiments. One such experiment was to add phosphorus pentaoxide to acetone - which resulted in some thick brown sludge along with the remaining acetone. He pointed out that it also had the smell of some flower. A smell none of us could pin down.

Mr. Osteen left for greener pastures not long after that but the puzzle remained with me. I parlayed that mystery into a senior project several quarters later to find out what all was in the mixture.

Adding phosphorus pentaoxide to acetone triggers a series of Aldol condensations, producing a large number of products. I separated perhaps 6 individual compounds, identifying two: Diacetone alcohol (4-hydroxy,4-methyl,pentan-2-one) and Mesityl oxide, but I never ID'ed the floral smell. It was one of those mysteries that stuck with me.

Though I didn't go into chemistry as a profession, it always remained my first love, specifically organic chemistry with a strong emphasis on organic synthesis. Forty five years later, while perusing a book on organic synthesis I'd picked up from Amazon (Organic Syntheses - Vol I), the synthesis for Mesityl oxide caught my eye. As I read the procedure, I noticed a mention of a by-product called Phorone. In all my years in organic I'd never heard of phorone so I looked it up on wikipedia. I was dumbstruck by a single line, casually mentioned in the description: "is a yellow crystalline substance with a geranium odor".

I discovered the identity of my mystery compound.

I tried to track down Mr. Osteen, only to find out he'd died three years earlier. I wasn't able to let him know I'd solved the mystery. Not being able to inform him of my discovery, I decided to try a synthesis of phorone.

A search of the web brought me to a paper, Novel Aspects in the Preparation of Phorone by Maria Koieczny and George Sosnovsky at the University of Wisconsin-Milwaukee, published January 4, 1978. It provided a good procedure for the synthesis of phorone and mesityl oxide.

These are the syntheses for mesityl oxide and phorone (click pic for mechanism)

Procedure

1. Anydrous acetone (100g, 1.8 mol), containing 1%, by weight, of anydrous AlCl₃ is cooled in a ice/salt bath and saturated with anhydrous HCl gas. The HCl gas is produced by dropping conc (18 mol) H₂SO₄ into NaCl, then running the gas over CaCl₂ to ensure no moisture gets into the acetone mixture.
2. The acetone is a deep orange at the end of saturation.
3. 12 hours later, the mixture changes to a violet color.
4. The mixture is then allowed to stand for 21 days.
5. Ether (100 ml) is added to the reaction mixture, then washed with 10% aqueous NaOH. The ether layer should turn from a violet color to a yellow color. Washings should continue until the aqueous layer becomes basic.
6. The ethereal layer is then washed with distilled water (5 x 50 ml).
7. The ethereal layer is then dried over MgSO₄.
8. A step is skipped at this point. In the paper, they dried and rotovaped out the ether, analysed the product in a gas chromatograph, then added more ether. I'm not planning on running it through a gas chromatograph, so rather than dry/remove ether/add more ether, I will simply dry it over MgSO₄ and proceed to the step that followed addition of the ether.
9. To this mixture is added KOH saturated in absolute ethanol, until no more chloride salt is precipitated.
10. The etheral solution is washed with distilled water (10 x 50 ml) until neutral.
11. It is then dried over anhydrous MgSO₄. The drying agent is removed via filtration.
12. The products are separated from the ether by distillation.
13. The mesityl oxide can be distilled from the phorone.

1. 12/20/2018: Anhydrous acetone (100g, 1.8 mol) containing 1%, by weight, of anydrous AlCl₃ was cooled in a ice/salt bath and saturated with anhydrous HCl gas.
   
   The HCl gas was produced by slowly dropping concentrated sulfuric acid (80 ml of 18 molar H₂SO₄/1.5 mol) into granular NaCl (179 grams/3 mol, canning salt), running the gas over anhydrous CaCl₂ removing any moisture before bubbling through the acetone mixture. This process proceeded over an approximate 8 hour period, with a second addition of conc. sulfuric acid and salt.
   
   Video of this step.
   
   
   
   Video of 6 hours into saturation step.
   
   


2. The acetone mixture was supposed to be a deep orange by the end of the saturation with HCl, turning a deep purple 20 hours later. My solution was a light yellow at the end of the HCl saturation process.
3. Several hours later, it had turned a deep orange.
4. 12/21/2018: The color change proceeded to a purple orange color within 20 hours.
   
   The following shows the progression of the solution.
   
   

   		Several hours after saturation
   		24 hours after saturation
   		36 hours after saturation

5. I let it sit for 21 days
6. 01/10/2019: I washed the ether solution with 10% aqueous NaOH, multiple times, until the supernatant liquid was basic. It did not turn yellow as procedure predicted. It was still a dark orange brown by the time the NaOH wash remained alkaline.
7. The ether solution was washed with distilled water until supernatant liquid was a neutral pH.
   
   Separation after neutralizating with NaOH wash
   



8. The washed solution was dried over MgSO₄. The ether solution turned a dark black color, as did the MgSO₄.
   
   Drying after neutralization, along with EtOH/KOH
   



9. The solution was filtered.
   
   MgSO₄ Residue after drying
   



10. I prepared 100 ml of saturated ethanolic KOH.
11. I added alcoholic KOH. Heat was produced, bringing the ether to boiling point several times. A salt precipitated.
12. It did not seem completely reacted, such that the solution was still precipitating salt as the last of the ethanolic KOH was added, so another 100 ml of saturated ethanolic KOH was prepared.
13. I added a small amount fresh solution, salt quit being produced on this addition.
14. I filtered out salt.
15. Then it was washed with distilled water (5 x 50 ml) until dw eluate showed neutral pH. Four washes were made. Each is taking longer for the emulsion layer to separate.
    
    The last washing took over 30 hours for the emulsion layer to resolve into aqueous and ether layer. I suspect the amount of ether has fallen, such that there's a higher percent of mesityl oxide and phorone, plus any longer chain condensation products.
    
    These are three stages of emulsion separation
    
16. I dried the etheral solution over MgSO₄. This seem to accentuate the violet color moving to an almost black or black/brown solution.
17. A fractional distillation setup was constructed to remove ether. It left a very thick brown sludge/goo. The diethyl ether came over at about 34° C.
18. I kept the distillation going to seperate mesityl oxide. I got about 25 milliliters of mesityl oxide come over at about 125° C. I suspect my thermometer was off a little.
19. I kept the distillation going, without the reflux condenser, and at close to 200° C, I got a small amount of liquid come over that wasn't phorone. It had an unpleasant alkene type odor and didn't solidify at ambient temperatures (probably around 15° C). I couldn't get anything to go over at a higher temperature.
20. Repeated attempts to crystallize phorone out an ethanol solution of the remaining residue failed.
21. I repeated this entire synthesis several months later, getting similar results. I don't think I'll try a third synthesis without getting all ACS reagent grade chemicals.