SYNTHESIS OF HEXANAL

• In a suitably equipped 1 -litre three-necked flask prepare a Grignard reagent from 15 g (0.675 mol) of dry magnesium turnings, 225 ml of sodium-dried ether, and 94.5 g (79 ml, 0.625 mol) of 1-bromopentane. 

• When reagent formation is complete, cool the flask to about 5°C and add 74 g (83 ml, 0.5 mol) of triethylorthoformate during about 10 minutes. 

• Reflux the mixture for 6 hours; then arrange the condenser for distillation and remove the ether on a water bath. 

• Allow the reaction mixture to cool. 

• Add 375 ml of ice-cold 6 per cent hydrochloric acid with stirring; keep the contents of the flask cool by the occasional addition of a little crushed ice. 

• When all the white solid has passed into solution, transfer to a separatory funnel and remove the upper layer of hexanal diacetal. 

• Hydrolyse the acetal by distilling it with a solution of 50 g (27.5 ml) of concentrated sulphuric acid in 350 ml of water; collect the aldehyde, which distils over as an oil, in a solution of 50 g of sodium meta-bisulphite in 150 ml of water. 

• Remove the oily layer (largely pentan-1-ol) insoluble in the bisulphite solution and discard it. 

• Steam distil the bisulphite solution until 100 ml of the distillate have been collected: this will separate the remainder of the pentan-1-ol and other impurities. 

• Cool the residual bi-sulphite solution to about 45 °C, cautiously add a suspension of 40 g of sodium hydrogen carbonate in 100 ml of water and separate the resulting free aldehyde by steam distillation. 

• Remove the upper layer (crude aldehyde) of the distillate, wash it with three 25 ml portions of water and dry it with 10 g of calcium sulphate or magnesium sulphate. 

• Distil through a short fractionating column, and collect the hexanal at 127-129 °C. The yield is 25 g (50%).