Benzoins

The mixture of benzaldehyde, potassium cyanide and alcohol is heated on the water-bath with an upright condenser for about half an hour. On cooling the liquid, the benzoin separates out as a mass of small colourless crystals, which are filtered and u ashed with a little alcohol. Yield, about 20 grams. A portion of the substance may be purified by recrystalhsation from spirit. 

ProperAi s. — Co]omless prisms m. p. 137 slightly soluble in water soluble in alcohol and ether. 

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. 

In a 500 ml. round-bottomed flask place 65 ml. of rectified spirit, 50 g. (47 5 ml.) of pure benzaldehyde (1) and a solution of 5 g. of sodium cyanide (96-98 per cent.) CAUTION) in 50 ml, of water. Attach a reflux condenser (preferably of the double surface type) and boil the mixture gently for half an hour (2). Cool the contents of the flask (preferably in an ice bath). Filter the crude benzoin, wash it with cold water, drain well (3) and dry. The yield of crude benzoin, which is white or pale yellow in colour, is 45 g. 

In a 3-I. round-bottom flask fitted with a reflux condenser, 625 cc. of alcohol (95 per cent), 500 cc. of water, 500 g. of pure benzaldehyde and 50 g. of sodium cyanide (96-98 per cent) are placed. This mixture is then heated and kept boiling for 0.5 hour. In the course of about twenty minutes, crystals begin to separate from the hot solution. At the end of the thirty minutes, the solution is cooled, filtered with suction, and washed with a little water. The yield of dry crude benzoin, which is white or light yellow, amounts to 450 to 460 g. (90-92 per cent theory). In order to obtain it completely pure, the crude substance is recrystallized from alcohol, 90 g. of crude material being dissolved in about 700 cc. of boiling alcohol upon cooling, a yield of 83 g. of white, pure benzoin is obtained, m. p. 1290. 

It is absolutely necessary that pure benzaldehyde be used for this reaction. The crude benzaldehyde which is obtained on the market should be washed with sodium carbonate solution, dried, distilled, and a fraction collected at 178-180°. Crude material will never give much more than a 50 per cent yield of benzoin. 

The above directions require 50 per cent of the amount of dilute alcohol which is ordinarily recommended in the literature. 

This allows one to prepare just double the amount of material in the same sized flask. The product obtained in this way, however, is slightly yellower than that obtained when more alcohol is used, but upon recrystallization it gives just as pure a product as that obtained by recrystallization of crude material made in the presence of more solvent. The results of many experiments lead to the conclusion that if large amounts of benzoin are to be prepared, the method described above is the better one. If, however, only a small amount is needed and a good grade of crude material is satisfactory, the larger amount of solvent is perhaps more desirable. 

Several times when benzoin was being prepared, the condensation took place and was complete after boiling five to ten minutes or even less, the product separating out in the hot solution with evolution of heat. The yield under these conditions was always good and the color better than when the reaction took a longer time. The cause of this occasional rapid reaction was not discovered, but it was probably due to inoculation with a little benzoin in certain of the experiments. When it takes place as just described, the flask and efficient condenser used will be sufficient to condense the sudden, rapid evolution of alcohol vapors caused by the heat of crystallization of the benzoin. 

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It was first described in 1608 when it was sublimed out of gum benzoin. It also occurs in many other natural resins. Benzoic acid is manufactured by the air oxidation of toluene in the liquid phase at 150°C and 4-6 atm. in the presence of a cobalt catalyst by the partial decarboxylation of phthalic anhydride in either the liquid or vapour phase in the presence of water by the hydrolysis of benzotrichloride (from the chlorination of toluene) in the presence of zinc chloride at 100°C. 

The name is also given to a balsamic resin obtained from Styrax benzoin, which is carminative and mildly expectorant. 

Many aromatic aldehydes (having the -CHO group joined directly to the benzene ring) undergo polymerisation when heated with a solution of potassium cyanide in aqueous ethanol. Thus benzaldehyde gives benzoin, a compound of double function, since it contains both a secondary alcoholic and a ketonic... 

C6H5CH(0H)CN I HCOCgHi - C6H5CH(OH)COC H, - HCN unchanged benzaldehyde, giving benzoin and regenerating the hydrogen cyanide... 

Add 5 g. of powdered potassium cyanide to a mixture of 20 ml. of water and 50 ml. of ethanol contained in a 200 ml. conical flask, and then add 25 mi. (26 g.) of freshly distilled benzaldehyde. Fit the flask with a reflux water-condenser, and boil the mixture gently on a water-bath for 30 minutes, a clear solution being rapidly obtained. Then pour the solution into a beaker and cool the benzoin separates as a crystalline mass... 

Filter at the pump, and wash well with water. (Yield, about 20 g.) Recrystallise a portion from hot methylated spirit, reserving the remainder for the following preparation. Benzoin is a very pale yellow (almost colourless) crystalline substance, m.p. 137°. 

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

Benzoin condensation. Aromatic aldehydes when treated with an alkali cyanide, usually in aqueous solution, undergo condensation to the -hydroxyketone or benzoin. The best known example is the conversion of benzaldehyde to benzoin ... 

By the use of 1 mol each of two different aldehydes, an unsymmetrical or mixed benzoin is obtained, for example ... 

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). 

Furfural undergoes condensation to furoin under the catal3rtic influence of cyanide ions in aqueous alcohol solution (compare Benzoin, Section IV,125) ... 

Benzoin condenses with hydroxylamine hydrochloride in the presence of alkah to give a-benzoinoxime ( cupron ) as the main product ... 

In a 250 ml. bolt-head flask, fitted with a reflux condenser, place a mixture of 10 g. of benzoin (Section IV,125) and 20 g. (25 ml.) of rectified spirit together with an aqueous solution of 8 0 g. of hydroxylamine hydrochloride which has previously been neutralised with 4-4 g. of sodium hydroxide. Reflux for 60 minutes. Add water to precipitate the benzoinoxime, and cool in an ice bath. Filter the solid with suction at the pump, wash it with water, and recrystaUise from dilute alcohol. Alternatively, the dry sohd may be recrystalhsed from ether. The yield of pure a-benzoinoxime, m.p. 151°, is 5 g. 

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... 

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