4- benzil

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). 

Method 1. Place 20 g. of crude benzoin (preceding Section) and 100 ml. of concentrated nitric acid in a 250 ml. round-bottomed flask. Heat on a boiling water bath (in the fume cupboard) with occasional shaking until the evolution of oxides of nitrogen has ceased (about 1 -5 hours). Pour the reaction mixture into 300-400 ml. of cold water contained in a beaker, stir well until the oil crystallises completely as a yellow solid. Filter the crude benzil at the pump, and wash it thoroughly with water to remove the nitric acid. RecrystaUise from alcohol or methylated spirit (about 2-5 ml. per gram). The yield of pure benzil, m.p. 94-96°, is 19 g. 

Method 2. Place 0-2 g. of cupric acetate, 10 g. of ammonium nitrate, 21 2 g. of benzoin and 70 ml. of an 80 per cent, by volume acetic acid -water solution in a 250 ml. flask fitted with a reflux condenser. Heat the mixture with occasional shaking (1). When solution occurs, a vigorous evolution of nitrogen is observed. Reflux for 90 minutes, cool the solution, seed the solution with a crystal of benzil (2), and allow to stand for 1 hour. Filter at the pump and keep the mother liquor (3) wash well with water and dry (preferably in an oven at 60°). The resulting benzil has m.p. 94-95° and the m.p. is unaffected by recrystallisation from alcohol or from carbon tetrachloride (2 ml. per gram). Dilution of the mother liquor with the aqueous washings gives a further 1 Og. of benzil (4). 

Method 2. Prepare a solution of 50 g. of sodium hydroxide and 11 5 g. of sodium bromate (or 12 -5 g. of potassium bromate) in 90 ml. of water 

In a 3-I. round-bottom flask are placed 410 g. of crude benzoin and 1 1. of nitric acid (sp. gr. 1.42), (2.5 cc. of nitric acid per gram of benzoin). The mixture is heated on a water bath for about one hour with occasional shaking, until the evolution of oxides of nitrogen ceases. The reaction mixture is then poured with stirring into four to five times its volume of cold water the benzil precipitates as small yellow lumps. The crude product is filtered and washed several times with water to remove the nitric acid. The yield after drying is 390 to 395 g. (96-97 per cent theory). Absolutely pure benzil may be obtained by crystallization from an alcohol solution. 390 g. of the crude benzil may be dissolved in about r 1. of boiling alcohol and since pure benzil is quite insoluble in cold alcohol, there is no difficulty in obtaining 370 g. of the pure product from the 390 g. used. 

A number of experiments upon the oxidation of benzoin to benzil with nitric acid were made in which the mixture was stirred continuously. An inferior grade of product and smaller yields (seldom over 75 per cent) were obtained. The probable reason for this is to be found in the fact that benzoin is readily soluble in nitric acid while the benzil is not, and the latter there- 

The copper sulfate-pyridine mixture is readily reoxIdized by passing a current of air through it for thirty-six hours (Note 4). To this resulting solution is now added 200 g. of pyridine and it is then used for oxidizing another batch of 1696 g. of benzoin. 

In checking this preparation, runs about 25 per cent of the size described were made. The yields were about 3 per cent less 

Copper hydroxide (or carbonate) does not dissolve in pyridine. 

In comparing the copper sulfate-pyridine method with the nitric acid method (Org. Syn. 1, 25) it should be pointed out that the constants on the samples are as follows  

Method m. p. crude m. p. recryst. Fehling s Test on recryst. 

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Usually prepared by the action of NaCN on benzaldehyde in dilute alcohol. It is oxidized by nitric acid to benzil, and reduced by sodium amalgam to hydrobenzoin PhCHOHCHOHPh by tin amalgam and hydrochloric acid to des-oxybenzoin, PhCH2COPh and by zinc amalgam to stilbene PhCH = CHPh. It gives an oxime, phenylhydrazone and ethanoyl derivative. The a-oxime is used under the name cupron for the estimation of copper and molybdenum. 

Colan The trivial name given to di-phenylethyne (diphenyl acetylene), CflHs C = C CeHs- Obtained as volatile colourless crystals, m.p. 6TC, by HgO oxidation of benzil bis-hydrazone. 

The first application of the time-resolved CIDNP method by Closs and co-workers involved tire Norrish 1 cleavage of benzyl phenyl ketone [24, 25]. Geminate RPs may recombine to regenerate the starting material while escaped RPs may fonn the starting ketone (12), bibenzyl (3), or benzil (4), as shown below. 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

The secondary alcohol group, -CH(OH) in benzoin is readily oxidised to a ketone group, and thus benzil, a diketone, is obtained. 

Benzil Osazone. CeH5C( N NHCeH5)C( N NHCeH5)CeH5. 

When benzii is heated with potassium hydroxide solution, it undergoes a molecular rearrangement with the formation of the potassium salt of benzilic acid, or diphenyl lycollic acid ... 

This reaction applies to many i,2 diketones, and is termed the Benzilic Acid Rearrangement. It provides a ready method for the preparation of disubstituted a4iydroxy-carboxylic acids. When applied to a cyclic 1,2-diketone, the ring system is necessarily reduced by one carbon atom for example, cyclohexan-i,2 ... 

To obtain the free acid, dissolve the potassium salt in 50 ml. of cold water, filter the solution if a small undissolved residue remains, and then boil the clear solution gently whilst dilute sulphuric acid is added until the separation of the acid is complete. Cool the solution and filter off the pale orange-coloured crystals of the benzilic acid wash the crystals on the filter with some hot distilled water, drain well, and then dry in a desiccator. Yield of crude acid, 4 g. Recrystallise from benzene (about 50 ml.) to which a small quantity of animal charcoal has been added, filtering the boiling solution through a preheated funnel fitted w ith a fluted filter-paper, as the benzilic acid readily crystallises as the solution cools alternatively, recrystallise from much hot water. The benzilic acid is obtained as colourless crystals, m.p. 150°. 

This rapid formation of the crystalline quinoxaline derivative can therefore be used to identify 1,2-diketones conversely, a nuclear-substituted o-phenyl-enediamine can be identified by the quinoxaline derivative which it forms with a known 1,2-diketone such as benzil. 

Oxidation of benzoin with concentrated nitric acid or by catalytic amounts of cupric salts in acetic acid solution, which are regenerated continuously by ammonium nitrate, yields the diketone benzil ... 

The latter procedure gives a purer product it is difficult to remove the last traces of benzoin from the benzil obtained by the nitric acid method. 

Benzil (and other a-diketones Ar—CO—CO—Ar) upon refluxing with aqueous-alcoholic potassium hydroxide undergo the beozilic acid rearrangement. Thus benzil is converted into a salt of benzilic acid ... 

Benzilic acid may be obtained in a high state of purity by the action of an alkaline bromate solution upon benzoin at 85-90° ... 

Moist and/or crude benzil (see Section I V,126) gives equally satisfactory resulta... 

Diphenylacetlc acid. The reduction of benzilic acid with red phosphorus and a little iodine in 98 per cent, acetic acid solution yields diphenylacetlc acid ... 

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