Benzil, preparation from benzoin

The only good method for the preparation of benzoin1 appearing in the literature is by the action of potassium cyanide upon an alcoholic solution of benzaldehyde, a method described first by Wohler and later modified slightly by Zinin and Zincke. The other methods are of theoretical importance only and have, therefore, not been studied. Thus, by the reduction of benzil,2 and by the oxidation of hydrobenzoin,3 benzoin can be formed it happens, however, that both benzil and hydrobenzoin are commonly made from benzoin. 

A further general route to the 1,2-dicarbonyl system involves the oxidation of a-ketols (acyloins) (cf. the preparation of benzil from benzoin, Expt 6.143). The acyloins may be prepared from carboxylate esters by a radical coupling reaction involving the use of finely divided sodium metal in anhydrous ether, benzene, or toluene.144... 

Hydroxy ketones of the type RCOCHjCHOHCHj are formed in 35-66% yields by partial catalytic hydrogenation of the corresponding /S-diketones over Raney nickel at KX) . Aromatic a-hydroxy ketones (benzoins) are prepared from the corresponding a-diketones (benzils) by catalytic reduction or by reduction with magnesium-magnesium iodide mixture. ... 

Chapter 55 Nitric Acid Oxidation. Preparation of Benzil from Benzoin. 

Enantiomerically pure hydrobenzoin could be prepared from benzil by employing a diamino-type ruthenium(ii) complex as depicted in Scheme 2.21. The success of this reaction was made possible due to the stepwise reduction of benzil to hydrobenzoin via the intermediate benzoin. This intermediate was itself configurationally labile and stereomutated rapidly. 

Recrystallise 5 0 g. from about 40 ml. of hot rectified (or methylated) spirit upon cooling, 4-5 g. of pure benzoin (a white, crystaUine solid, m.p, 137°) separate. Reserve the remainder of the preparation for benzil and benziUc acid (Sections IV.126 and IV.127 respectively). 

Benzil. Benzil [134-81-6] (diphenylethanedione) is a yellow solid that crystallizes from alcohol in hexagonal prisms. Benzil can be prepared by the oxidation of benzoin [579-44-2] (2-hydroxy-2-phenylacetophenone) (298,299), which is itself prepared by the self-condensation of benzaldehyde (300). Benzil is commercially produced in Japan and is used as a uv resin curing sensitizer (301). It has also been suggested as a chigger repellant (302). 

Diols may be prepared by reduction of a-diketones or a-hydroxy ketones such as biacetyl, benzoin, and benzil. Substituted benzoins containing methoxyl and p-dimethylamino groups have been reduced catalytically over platinum oxide and by sodium amalgam and alcohol. Levorotatory propylene glycol is made from acetol, CHjCOCHjOH, by an enzymatic reduction with yeast. ... 

Mix 10 grammes of benzaldehyde with 20 grammes of alcohol and treat the mixture with a solution of 2 grammes of potassium cyanide and 5 c.c. of water. Boil on the water-bath for one hour (reflux condenser). The hot solution is poured into a beaker and allowed to cool slowly the crystals separating out are filtered off, washed with alcohol, and dried on the water-bath. For conversion into benzil (see next preparation), they need not be recrystallised. In order to obtain perfectly pure benzoin, a small portion of the crude product is recrystallised from a little alcohol in a test-tube. Melting-point, 1340- Yield, about 90% of the theory. 

The equation representing the oxidation of benzoin to benzil has been given under the preceding preparation. The analogues of benzoin also give, on oxidation, compounds of the benzil series. Thus from anisoin and cuminoin, anisil and cuminil respectively are obtained ... 

Oxidation of benzoins to benzils. Benzoins can be converted to benzils by oxidation with cupric sulfate in HMPT with air (or O2) ebullition yields are 60-95%. This method was used for preparation of 4,4 -divinylbenzil from 4,4 -divinylbenzoin (73% yield). 

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