22. Acetoin

172

22.

Acetoin

Acetoin, also known as acetylmethylcarbinol, is the most important derivative of diacetyl. It is a valuable flavor commanding a higher price than diacetyl. As shown below, acetoin can be readily made from diacetyl, either by catalytic hydrogenation or by electrolytic hydrogenation. Acetoin is a substance of rather complicated behavior as summarized by the following features" (1) Liquid acetoin consists of two different molecules in equilibrium: A keto form (A), which is predominant, and an "oxide form" (B):

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Form (I), crystallized in the presence of zinc, has a melting point of 90 ~ whereas form (II), crystallized without zinc, has a melting point of 127.5 ~ Both (I) and (II) can be recrystallized from solution in acetone, chloroform, or

173

ethyl acetate, but this does not change their melting point as in these solvents the dimers are only partly depolymerized, so that the remaining dimer molecules in solution act as crystallization nuclei leading to the original form. By contrast, when the dimer is dissolved in water, diethyl ether, acetic acid, or paraldehyde, it is depolymerized completely, so that there are no crystallization nuclei left in the solution. Hence, it is difficult to recrystallize the dimer from such solutions. When crystallization finally does take place, it leads to form (II) unless a crystal of form (I) is used as a primer. (3) Liquid acetoin stored for a long period of time has a well-developed ketonic absorption band at 275 nm, indicating the prevalent form (A), whereas liquid acetoin freshly prepared by melting or dissolving the dimer shows no such absorption, which indicates that based on the dimer structure the latter liquid can be expected to consist of the oxide form (B), and that the equilibrium between the keto form and the oxide form is reached very slowly (in a matter of days). Using the rotatory power of acetoin, due to the asymmetric carbon atoms in the keto form (A) and the oxide form (B), particularly interesting observations on acetoin transformations were made by Dirscherl and Sch~llig [80]. Although these authors refrained from interpreting their seemingly strange findings, the knowledge of today permits a complete explanation of their results on the basis of the following facts: (a) The keto form (A), because of its intramolecular hydrogen bond, is more volatile than the oxide form (B), so that distillation produces a distillate rich in (A) and a sump fraction rich in (B). (b) Because of their structure, the dimers can form only from the oxide form (B). (c) The keto form has a higher rotatory power than the oxide form. Against this background, the seemingly weird findings of Dirscherl and SchOllig, reproduced in Figure 87, can be explained in all details" Observation 1: When acetoin produced by fermentation and then recovered from the fermentation broth by distillation was left standing for a day, the rotation first decreased. When it reached a minimum at point K, dimer crystals started precipitating, and the rotation started to increase. Explanation: As the acetoin had been recovered from the fermentation broth by distillation, its

174

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content of the keto form was greater than in equilibrium, so that it slowly reestablished equilibrium by converting keto form molecules to oxide form molecules, which lowered the rotation as the keto form has a higher rotation than the oxide form. Then, when at point K a critical concentration of oxide form molecules was reached, the latter started forming dimer molecules. As the consumption of oxide form molecules by crystallization was faster than the replenishment of oxide form molecules from keto form molecules, the rotation increased. Observation 2: When at point D the acetoin was submitted to a distillation, this abruptly

175

increased the rotation of the distillate, and the sump fraction rapidly solidified to dimer. Explanation: As the keto form molecules are more volatile than the oxide form molecules, the distillate was overly rich in keto form molecules, thus showing an increased rotation. At the same time, the sump fraction, depleted of keto form molecules, got overly rich in oxide form molecules, and for that reason it readily crystallized to dimer. Observation 3: When the distillate was left standing for 13 days, its rotation diminished, at first sharply and then more gradually until at point P it started increasing again. Explanation: In the distillate, the content of keto form molecules was greater than in equilibrium, so that it reestablished equilibrium by converting keto form molecules to oxide form molecules, which lowered the rotation. Then, when at point P a critical concentration of oxide form molecules was reached, the latter started forming dimer molecules, and as the consumption of oxide form molecules by crystallization was faster than the replenishment of oxide form molecules from keto form molecules, the rotation increased on account of an increasing concentration of keto molecules. Dirscherl and Sch611ig also discovered that the dimer formation of acetoin is catalyzed by glass containing sodium ions, whereas no such catalysis was observed in a quartz vessel. Thus, it is not surprising that acetoin readily dimerizes in normal glass bottles used for shipment.

22.1. Catalytic Hydrogenation of Diacetyl Acetoin can be readily made by a simple hydrogenation o f diacetyl, using palladium on carbon or alumina as the catalyst. The process is based on a publication by Skibina, Ioffe, and Artamonov [81]. A laboratory setup suitable for carrying out this reaction is shown in Figure 88. Hydrogen is passed through a diacetyl evaporator kept at 25 ~ by means of a thermostat, and the resulting gaseous mixture is fed through the catalyst bed heated by a tube furnace to 125 ~

The conversion to acetoin is in the order of 95 percent, the

rest being unconverted diacetyl and butanediol as a minor by-product. An industrial version of the process is shown in Figure 89. The final distillation must be carried out under nitrogen as

176

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distilling acetoin in the presence of air converts it to diacetyl [82].

22.2. Electrolytic Hydrogenation of Diacetyi A selective hydrogenation of diacetyl to acetoin can also be carried out in an electrolytic cell using a cathode with a high hydrogen overvoltage [83]. It is recommended to start with a solution consisting of 12.0 % by weight of diacetyl, 83.6 % by weight of water, and 4.4 % by weight of sulfuric acid. The cell characteristics should be as follows: Cathode: Nickel with nickel shot. The latter was found to greatly increase the rate of diacetyl conversion. Aplied potential difference: 2.2 volts per cell. Current density: 0.0665 amps/cm 2. From a reservoir, the solution is pumped through the cells and through a cooler. The latter must be designed to keep the temperature in the reservoir at 25 ~

Complete conversion of

the diacetyl is obtained after 24 hours. Although the electrolytic process is very simple, complications arise by the fact that (1) the acetoin must be extracted from the reaction mixture, (2) some acid is extracted as well, and (3) the extractant must be recovered by distillation. The most suitable extractant is believed to be dichloromethane. A respective plant is shown in Figure 90. As can be seen, due to the recovery procedures, the production of acetoin by an electrochemical hydrogenation of diacetyl is more complicated than the catalytic process.

22.3. Preferred Commercial Form On one hand, there is an equilibrium between keto form molecules and oxide form molecules, and on the other hand there is an equilibrium between oxide form molecules and dimer molecules. A superposition of these two equilibria means that from a liquid monomer mixture of keto form and oxide form molecules obeying the first equilibrium, some solid dimer crystals must form to satisfy the second equilibrium. However, this dimer formation

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consumes only oxide form molecules, thereby disturbing the first equilibrium, so that new oxide form molecules are generated from keto form molecules. This in turn permits the formation of more dimer molecules consuming oxide form molecules, and so forth, until very slowly, in long storage or shipment, an "equilibrium slurry" of dimer crystals in a liquid equilibrium mixture of keto form and oxide form molecules is reached. The slurry leads to a pasty sediment giving the product an ugly appearance. Although this sediment, being the dimer, is not an impurity, it is frequently misunderstood as such, and for this reason it has become customary to sell the dimer rather than a liquid monomer mixture inevitably converting into an unsightly sludge. To this end, the liquid monomer mixture originally obtained in the process is crystallized to the dimer in the presence of zinc, and these crystals are dissolved in acetone and recrystallized. This yields a beautiful white powder which is stable indefinitely. It can be readily converted to a liquid by mere heating above the melting point. The liquid obtained by melting the dimer consists at first of oxide form molecules which require days until the equilibrium mixture of keto form and oxide form molecules is reached. During this conversion, the smell changes as on account of the unlike structure the smell of oxide form molecules differs from that of keto form molecules. This must be considered when the liquid is submitted to olfactory tests.

References [80] W. Dirscherl and A. Sch611ig, Berichte der Deutschen Chemischen Gesellschaft 71 (1938) 418-423. [81] E. M. Skibina, Yu. M. Levin, I. I. Ioffe, and P. A. Artamonov, Zhur. Prikladnoi Khimii 19 (1976) 1554-1558. [82] H. von Pechmann and F. Dahl, Berichte der Deutschen Chemischen Gesellschaft 23 (1890) 2421-2427. [83] B. Mtiller, H. J. Dietz, H. Matschiner, S. Engelmann, and H. Herzberg, Patent of the German Democratic Republic No. DD 237 683 A1 (1986).