Pinacol hydrate

Submitted by Roger Adams and E. W. Adams. Checked by H. T. Clarke and Ross Phillips. 

The condenser should have an inner tube with a fairly large diameter (12-15 mm.) in order to take care of the acetone vapors during the vigorous reaction. 

The reaction mixture must be guarded carefully from moisture throughout the reaction if moisture is present the magnesium becomes coated with oxide or hydroxide and the reaction does not take place smoothly. 

If a larger amount of benzene is used, the reaction is too slow in starting. Commercial benzene is satisfactory if it has been distilled and the first portion containing the water rejected. 

The acetone used is the ordinary commercial grade dried over calcium chloride. The dryness of the acetone is important and determines the time required for the reaction to start if perfectly dry (two to three days over calcium chloride with occasional agitation) the reaction starts at once. 

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C6H,20. Colourless liquid with a camphorlike odour b.p. l03-J06 C/746mm. Prepared by heating pinacol hydrate with sulphuric acid and distilling the mixture. It is oxidized to tri-methylacetic acid. 

The pinacol hydrate may be used (i) for conversion to pina-colone (see below), and (ii) to illustrate the oxidation of 1,2-diols to aldehydes or ketones by periodic acid (p. 145),... 

Add cautiously 15 ml. of concentrated sulphuric acid to 50 ml. of water in a 100 ml. distilling-flask, and then add 10 g. of pinacol hydrate. Distil the solution slowly. When about 40 ml. of distillate (consisting of pinacolone and water) have been collected, and no more pinacolone comes over, extract the distillate with ether. Dry the extract over sodium sulphate. Distil the dry filtered extract carefully, with the normal precautions for ether distillation (p. 164). When the ether has been removed, continue the distillation slowly, rejecting any fraction coming over below 100 . Collect the pinacolone, b.p. 106 , as a colourless liquid having a peppermint odour. Yield, 4 5-5 o g. A small quantity of higher-boiling material remains in the flask. 

Pinacol (tetramethylethyleneglycol). Pinacol hydrate may be dehydrated in the following manner (compare Section 11,39). Mix 100 g. of pinacol hydrate with 200 ml. of benzene and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer... 

Pinacolone. In a 500 ml. round-bottomed flask carrying a dropping funnel and a connection to a condenser set for distillation, place 50 g. of pinacol hydrate and 130 ml. of QN sulphuric acid. Distil the mixture until the upper layer of the distillate no longer increases in volume (15-20 minutes). Separate the pinacolone layer from the water and return the latter to the reaction flask. Then add 12 ml. of concentrated sulphuric acid to the water, followed by a second 50 g. portion of pinacol hydrate. Repeat the distillation. Repeat the process twice more until 200 g. of pinacol hydrate have been used. 

From pinacol hydrate, using phosphoric acid as a catalyst. Dehn and Jackson, J. Am. Chem. Soc. 55, 4286 (1933). 

Pinacol hydrate (5, 87). The mixture should be chilled and kept cold for several hours instead of a few minutes. 

The only practical method for the preparation of pinacol hydrate is the reduction of acetone and the procedure described above is a modification of that of Holleman.1 The more common reducing agents that have been used are magnesium amalgam,2 aluminum amalgam,3 sodium,4 and sodium amalgam.5 Electrolytic reduction has also been used.6 1 Rec. trav. chim. 25, 206 (1906). 

In a 2-1. round-bottom flask, fitted with a stopper carrying a dropping funnel and a connection to a condenser set for distillation, are placed 750 g. of 6N sulfuric acid (Note 1) and 250 g. of pinacol hydrate (Note 2). The mixture is then distilled until the upper layer of distillate ceases to increase in volume (Note 3). This requires about fifteen to twenty minutes. The pinacolone layer in the distillate is separated from the water and the water is returned to the reaction flask. First, 60 cc. of concentrated sulfuric acid is added to the water, and then a second 250-g. portion of pinacol hydrate. The distillation is repeated. This process is repeated twice more until 1 kg. of pinacol hydrate has been used (Note 4). 

The pinacol hydrate is the unrecrystallized product obtained as described on page 87. The use of recrystallized pinacol hydrate increases the yield of pinacolone about 4 per cent. 

Pinacolone has been prepared by heating pinacol hydrate with dilute sulfuric acid or dilute hydrochloric acid 1 by treating anhydrous pinacol2 with concentrated sulfuric acid at o° by heating pinacol with 5 per cent oxalic acid 3 for twelve hours, or with 50 per cent tartaric, phosphoric, or oxalic acid 3 for three to four hours. 

Pinacol. Pinacol hydrate may be dehydrated in the following manner (compare Section 2.23, p. 168, Drying by distillation). Mix 100 g of pinacol hydrate with 200 ml of benzene (CAUTION) and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer of benzene to the distilling flask. Repeat the process until the benzene distillate is clear. Finally distil the anhydrous pinacol and collect the fraction boiling at 169-173 °C (50 g). The pure pinacol has m.p. 43 °C, but on exposure to moist air the m.p. gradually falls to 29-30 °C and then rises to 45-46 °C when hydration to the hexahydrate is complete. 

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