SYNTHESIS OF HEXANAL

Method A:

• Into a 500-ml three-necked flask, provided with a mechanical stirrer, a gas inlet tube and a reflux condenser, place 57 g of anhydrous tin(n) chloride and 200 ml of anhydrous ether. 

• Pass in dry hydrogen chloride gas until the mixture is saturated and separates into two layers; the lower viscous layer consists of tin(n) chloride dissolved in ethereal hydrogen chloride. 

• Set the stirrer in motion and add 19.4g (0.2 mol) of hexanenitrile through the separatory funnel. Separation of the crystalline aldimine hydrochloride commences after a few minutes; continue the stirring for 15 minutes. 

• Filter off the crystalline solid, suspend it in about 50 ml of water and heat under reflux until it is completely hydrolysed. 

• Allow to cool and extract with ether; dry the ethereal extract with anhydrous calcium sulphate or magnesium sulphate and remove the ether slowly by distillation through a short fractionating column. 

• Finally, distil the residue and collect the hexanal at 127-129 °C. The yield is 19 g (95%).

 

Method B:¹º⁴

• In a 1 -litre flask equipped with a condenser, a thermometer and a dropping funnel is placed 0.3 mol of lithium aluminium hydride in dry ether (300 ml). 

• A nitrogen atmosphere is maintained throughout the reaction. 

• To this stirred solution 0.45 mol of ethyl acetate is added over a period of 75 minutes maintaining the temperature at 3-7 °C. 

• The reaction is stirred for an additional 30 minutes. 

• To this solution at — 10 °C is added 29.1 g (0.3 mol) of hexanenitrile in 5 minutes. 

• The reaction temperature rises to 12°C in 10 minutes with the formation of a viscous solution. 

• It is allowed to stir for another 50 minutes at 3 °C. The reaction mixture is decomposed by the addition of 300 ml of 2.5 m sulphuric acid. 

• The ether layer is separated and the aqueous layer is extracted with ether. 

• The combined ether extracts are washed with sodium hydrogen carbonate solution and water and dried over anhydrous sodium sulphate (1). 

• Distillation through a 30.4-cm Vigreaux column gives 16.6 g (55.4%) of hexanal, b.p. 51-55 °C/53-55 mmHg and 2.5 g (8.5%) of hexanenitrile, b.p. 73 °C/53-55 mmHg.

 Notes to keep in mind:

1. Gas-liquid chromatography analysis indicates a 68 per cent yield of aldehyde and the presence of the nitrile.