Benzoin, Benzil

Two derivatives of di-benzyl should be mentioned, viz., benzoin and benzil. 

Benzoin is formed by the condensation of benzaldehyde with itself when heated with potassium cyanide. This is analogous to the aldol condensation (p. 169). 

This compound is a mixed alcohol-ketone compound. When oxidized with nitric acid or chlorine the alcoholic group is converted into a ketone group and a de-ketone known as benzil is obtained. 

Benzil is also obtained when benzoyl chloride is treated with sodium amalgam. 

Benzoin being an alcohol-ketone similar to carbohydrates yields osa-zoneSf and both benzoin and benzil as ketones yield oximes. The dioxime of benzil exists in three stereo-isomeric forms. 

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Alkyl ethers of benzoin Benzil dimethyl ketal 2-Hydroxy-2-methylphenol-l-propanone 2,2-Diethoxyacetophenone 2-Benzyl-2-At, V-dimethylamino- l-(4-morpholinophenyl) butanone Halogenated acetophenone derivatives Sulfonyl chlorides of aromatic compounds Acylphosphine oxides and bis-acyl phosphine oxides Benzimidazoles... 

Benzoin.—Benzile, by reduction in dilute alcoholic sodium hydroxide and in alcoholic sulphuric acid, according to James,2 can inversely be converted into benzoin. Oxidation of benzoin in alkaline and sulphuric-acid solution gives a poor yield of benzoic acid. In alcoholic hydrochloric acid, especially at a high current density, benzile is formed. 

Benzoins Benzils Benzoins are oxidized to benzils in high yield (75-98%) by treatment with triphenylphosphine dibromide in acetonitrile. 

The oxide is a specific reagent for oxidation of acyloins to a-diketones, the oxide being reduced to the metal. A solution of the acyloin in acetic acid is treated with 1.2 times the theoretical amount of BijOj and the mixture is heated on the steam bath with stirring for about 15 min. Benzoin—> benzil (95% yield). 12-Hydroxy-11-ketosteroids —> 11,12-diketosteroids (70% yield)." Oxidation of cevine and isomers." Other examples are the oxidation of 2-hydroxypulegone (I) to diosphenolone (2) in 94%i yield (crude), of Ihe 2 -hydroxy-A -3-ketosteroid (3) to (4)," and of (5) to (6)."... 

Aldehyde Product Benzil Benzoin Benzil Benzoin... 

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

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. 

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

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). 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

Recently, Koitai et al. (17) have shown that 5,5-diphenyl-2,4-thiazolidinedithione (15) with aluminum chloride in refluxing toluene gives 4,5-diphenyl-A-4-thia2oline-2-thione (16) (Scheme 7). 3-Methyl-4,5-diphenyl (17) and 4,5-diphenyl-A-4-thia2oline-2-thiones (16) are obtained in very low yields (1 to 5%) as by-products of the reaction between deoxybenzoin. benzoin. l,2-diphenyl-1.2-ethanediol. 1.2-diphenylethanol, or benzil, and Sg in hexamethylphosphoamide (18), The transformation of A-4-thiazoline-2-ones to the corresponding thiones by P2S5 (19) is of little synthetic value since the latter are more easily prepared. 

The melting temperatures of mixtures of benzil and benzoin have been recorded by Vanstone, who showed that the maximum possible depression is about 10°. 

Reaction.—. dA Fehling s solution to benzoin dissolved in alcohol. Benzil is formed and cuprous oxide precipitated. Benzil is also formed on oxidation with nitric acid. 

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