SYNTHESIS OF BENZYL AMINE

CAUTION: Hydrogen gas is evolved in these reactions.

• To a stirred suspension of sodium borohydride (1.89g, 50mmol) and benzamide (1.21, 10mmol) in dioxane (20ml) is added acetic acid (3.0g, 50 mmol) in dioxane (10 ml) over a period of 10 minutes at 10 °C; the reaction mixture is stirred at reflux for 2 hours. 

• The reaction mixture is concentrated to dryness in vacuo, excess reagent is decomposed with water and the solution extracted with chloroform. 

• The extract is washed with water and dried over anhydrous sodium sulphate. 

• The chloroform layer is treated with dry hydrogen chloride, evaporated in vacuo and the residue recrystallised from methanol-ether to give benzylamine hydrochloride (1.09g, 76.2%).

Cognate preparation: General procedure for the reduction of amides with sodium borohydride I methanesulphonic acid in dimethyl sulphoxide

• To a vented 100-ml two-necked flask equipped with a magnetic stirrer, are added 16 mmol of amide, 1.61g (40 mmol) of sodium borohydride, and 20 ml of dimethyl sulphoxide. 

• Methanesulphonic acid (CAUTION) (freshly distilled prior to use, b.p. 112-114 °C/0.5mmHg) (3.4 ml, 55 mmol) and 20 ml of dimethyl sulphoxide are mixed and added dropwise to the reaction mixture by means of an addition funnel over a 30-minute period. 

• During this addition the reaction mixture is constantly stirred with the magnetic stirrer. 

• Gas is evolved during the addition of acid and a gelatinous substance is formed, which dissolves when the acid comes to exceed the sodium borohydride on a molar basis. 

• At the end of this time, if the reduction product is to be a primary or secondary amine, the reaction mixture is quenched by the addition of 20 ml of 10 per cent sodium hydroxide. 

• If the product is to be a tertiary amine, the reaction mixture is kept at 70 °C for 2 hours to liberate the product from what appears to be a complex with borane. 

• After this additional reaction period tertiary amines are isolated in the same way as primary and secondary amines. 

• After the reaction is quenched, the product is extracted from the reaction mixture with three 10-ml portions of dichloromethane. 

• The product solution is then washed with three 10-ml portions of 0.1 ᴍ sodium hydroxide to remove most of the residual dimethyl sulphoxide, and the product is extracted into three 10-ml portions of 10 per cent hydrochloric acid. 

• Neutralisation with 10 per cent sodium hydroxide, followed by extraction with three 10-ml portions of dichloromethane, drying over calcium sulphate and fractional distillation affords the amine. 

• In this way hexanamide gives hexylamine, b.p. 38-40 °C/25 mmHg, 74 per cent yield; acetanilide gives N-ethylaniline, b.p. 90-91 °C/1 5 mmHg, 77 per cent yield; N, N-dimethylbenz-amide gives N, N-dimethylbenzylamine, b.p. 73-77 °C/22 mmHg, 59 per cent yield.