JULY, 1952
A SIMPLE PROCEDURE TOR PREPARING PINACOL HYDRATE JOSEPH E. WEBER and ARLO D. BOGGS Bawling Green State University, Bawling Green, Ohio
HOLLEMANW method for preparing pinacol hydrate by the reduction of dry acetone with magnesium, activated with mercuric chloride, in dry benzene appears in "Organic Synthese~."~An adaptation of this method also appears in Fieser's laboratory manuaLa We have found the following to be a shorter and more simple procedure for this method.
Magnesium pinaeolate (CHdnC-4
- 1.
,6H,O
+ MdOHh
(CH& --0--H Pinacol hydrate
3Mg(OHX
+ 2NasPOa
-
Mg.(PO&
+ 6NaOH
Three grams of magnesium turnings were placed in a 200-ml. round-bottomed ring-necked flask and 0.5 g. of mercuric chloride was added. To this was added 10 ml. of acetone from a dry graduated cylinder. The flask was shaken with a rotary motion to give the contents a swirling movement until the reaction started. It was then quickly connected to a watercooled reflux condenser and 40 ml. more of acetone was added in small portions with shaking. After the reaction had run for 1 5 . t o 20 minutes, another portion of 25 ml. of acetone was added and the flask shaken periodically to avoid the formation
' HOLLEMAN, F.,h e . l?au. ehim., 25, 206 (1906). ' "Orgsnio Syntheses," John Wiley & Sans, Inc.,
New York,
1932, Coll. Vol. I, p. 448.
FIESER,L. F., "Experiments in Organic Chemistry," 2nd ed., p. 97.
of a cake inside the flask. At the end of 30 to 45 minutes, heating over a steam bath was necessary t o maintain a rapid reflux rate. After one hour from the start the reaction had practically stopped and the flask was disconnected from the reflux condenser and connected by means of a one-hole rubber stopper and glass tube to a vacuum line (aspirator). While continuing to heat the flask on the steam bath, a vacuum was applied t o remove the unused acetone. By rotating the flask and heating simultaneously the contents of the flask (magnesium pinacolate, etc.) became powder-dry in 5 or 10 minutes. At this point the reflux condenser was reconnected to the flask and 100 ml. of water containing 5 g. of trisodium phosphate (technical grade) was added through the reflux condenser. The contents of the flask warmed up somewhat on the addition of the water. The mixture was refluxed over a low flame and wire gauze for 15 minutes. The hot mixture was then filtered using a small vacuum filter assembly (60-mm. Biichner funnel and 250-ml. filter flask). The filtrate was chilled in an ice-water bath, stirred vigorously, and seeded. When crystallization seemed to be complete the cold mixture was filtered, using the small vacuum filter assembly again. The crystals were washed in the funnel with 25 ml. of ice water. The vacuum was applied until the crystals were reasonably dry (15 t o 20 miqutes). They were then washed with 25 ml. of petroleum ether and allowed to dry once again. It was found that a satisfactory yield (6 to 8 g.) of pinacol hydrate of high purity could he prepared within two hours, using a minimum amount of equipment and ordinary and inexpensive reagents. I t will be observed that the use of benzene has been eliminated entirely from the preparation, the relative amount of mercuric chloride has been greatly reduced, and ordinary acetone has replaced the dried product. Trisodium phosphate mas added to improve the filtering characteristics of the magnesium residue.
