SYNTHESIS OF 2,3-DIMETHYLBUTANE-2,3-DIOL (Pinacol)

CAUTION: This preparation should be conducted in an efficient fume cup­board.

Pinacol hydrate: 

• Place 20 g (0.83 mol) of dry magnesium turnings and 200 ml of anhydrous benzene (CAUTION) in a dry, 1-litre two-necked flask, fitted with a dropping funnel and an efficient double surface condenser and carrying calcium chloride guard-tubes. 

• Place a solution of 22.5 g of mercury(n) chloride (POISONOUS) in 100 g (127 ml, 1.72 mol) of dry AnalaR acetone in the funnel and run in about one-quarter of this solution; if the reaction does not commence in a few minutes, as indicated by a vigorous ebullition, warm the flask on a water bath and be ready to cool the flask in running water to moderate the reaction. 

• Once the reaction has started, no further heating is required. 

• Add the remainder of the solution at such a rate that the reaction is as vigorous as possible and yet under control. 

• When all the mercury(n) chloride solution has been run in and while the mixture is still refluxing, add a mixture of 50 g (63.5 ml, 0.86 mol) of dry AnalaR acetone and 50 ml of dry benzene. 

• When the reaction slows down, warm the flask on a water bath for 1-2 hours. 

• During this period the magnesium pinacolate_.swells up and nearly fills the flask. 

• Cool slightly, disconnect the flask from the con­denser and shake until the solid mass is well broken up: it may be necessary to use a stirrer. 

• Attach the condenser and reflux for about 1 hour, or until the magnesium has disappeared.

• Now add 50 ml of water through the dropping funnel and heat again on the water bath for 1 hour with occasional shaking. 

• This converts the magnesium pinacolate into pinacol (soluble in benzene) and a precipitate of magne­sium hydroxide. 

• Allow the reaction mixture to cool to 50 °C and filter at the pump. 

• Return the solid to the flask and reflux with a fresh 125 ml portion of benzene for 10 minutes in order to extract any remaining pinacol; filter and combine with the first filtrate. 

• Distil the combined extracts to one-half the original volume in order to remove the acetone: treat the residual benzene solution with 75 ml of water and cool in an ice bath, or to at least 10-15 °C, with good stirring. 

• After 30-60 minutes, filter the pinacol hydrate which has separated at the pump and wash it with benzene to remove small quantities of mercury compound present as impurities. 

• Dry the pinacol hydrate by expo­sure to air at the laboratory temperature. 

• The yield is 90g (48%), m.p. 45.5 °C. 

• This product is sufficiently pure for most purposes. 

• The crude pinacol hydrate may be purified by dissolving it in an equal weight of boiling water, treating with a little decolourising charcoal if necessary, filtering the hot solu­tion and cooling in ice; the recovery is over 95 per cent.

Pinacol: 

• Pinacol hydrate may be dehydrated in the following manner (com­pare 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.