Pinacolization

Dimethylbutadiene is formed when the vapour of pinacol is passed over alumina at 400 C. 

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. 

Sodium me/aperiodate (NalO ) in cold aqueous solution readily oxidises 1,2-diols with splitting of the molecule and the consequent formation of aldehydes or ketones thus ethylene glycol gives formaldehyde and pinacol gives acetone. In the case of a 1,2,3-triol, the central carbon atom of the triol... 

In the following examples, pinacol is oxidised to acetone, which is identified as its semicarba2one and its 2,4 dinitrophenylhydra2one, and glycerol is oxidised to formaldehyde and formic acid. The formaldehy de is readily detected by the condensation product which it gives with dimedone, 5,5-dimethylcyclohexan-i,3-dione (p, 277). 

Dissolve I g. of pinacol (preparation, p. 148) in 20 ml. of water, and add 20 ml. of the 5% aqueous sodium periodate solution. After 15 minutes, distil the clear solution, collecting the first 5 ml. of distillate. Treat this distillate with 2,4-dinitro-phenylhydrazine solution A (p. 263). Acetone 2,4-dinitrophenyl-hydrazone rapidly separates from the solution when filtered off, washed with a small quantity of ethanol, and dried, it has m.p. 126-127°, and after recrystallisation from ethanol it has m.p. 128°. 

Acetone when treated in ethanol with sodium undergoes reduction mainly to isopropanol. By modifying the conditions, however, acetone may be induced to undergo a bimolecular reduction to pinacol. 

Work up the product by method (a) or (b) method (6) affords the anhydrous pinacol. 

Add about 60 ml. of petroleum (b.p, 60-80°) with stirring to the fraction of b,p, 150-180°, thus precipitating the pure anhydrous pinacol (2 5-3 8 ) Filter this off, and then shake the filtrate with ca. 4 ml, of water the remaining pinacol now separates as the hexahydrate (5 g.). The two crops may be united and recrystallised from ca. 10 ml, of water (total yield of hexahydrate, 8-9 g,). Dry and bottle the product rapidly as described in (a). 

As the reaction beings to subside, run in from the dropping-funnel without delay a mixture of 25 ml. of acetone and 20 ml. of benzene, in order to maintain a brisk and continuous reaction. When the reaction finally subsides, heat the mixture on a boiling water-bath for 45 minutes with occasional shaking. If the shaking does not break up the spongy mass of magnesium pinacolate,... 

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),... 

When pinacol is warmed with acids, it undergoes rearrangement to pinacolone, with an over-all loss of one molecule of water. 

Required Pinacol hexahydrate, 10 g. concentrated sulphuric acid, 15 ml. (28 g.). 

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. 

To meet the needs of the advanced students, preparations have now been included to illustrate, for example, reduction by lithium aluminium hydride and by the Meerwein-Ponndorf-Verley method, oxidation by selenium dioxide and by periodate, the Michael, Hoesch, Leuckart and Doebner-Miller Reactions, the Knorr pyrrole and the Hantzsch collidine syntheses, various Free Radical reactions, the Pinacol-Pinacolone, Beckmann and Arbusov Rearrangements, and the Bart and the Meyer Reactions, together with many others. 

Pinacol possesses the unusual property of forming a crystalline hexahydrate, m.p. 45°, and the pinacol is separated in this form from the unreacted acetone and the tsopropyl alcohol. The magnciaium is conveniently amalgamated by dissolving mercuric chloride in a portion of the acetone mercury is then liberated by the reaction ... 

The anhydrous compound, pinacol, is a liquid it may be prepared from the hydrate by azeotropic distilktion with benzene. 

Pinacol upon dehydration with acid catalysts e.g., by distillation from 6A sulphuric acid or upon refluxing for 3—4 hours with 50 per cent, phosphoric acid or hydrated oxalic acid) is transformed into methyl ter<.-butyr ketone or plnacolone ... 

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. 

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