SYNTHESIS OF 2-ETHYLHEX-2-ENAL

• Place 100 ml of 1 m sodium hydroxide solution in a 500-ml three-necked flask fitted with a sealed stirrer unit and an efficient reflux condenser. 

• Heat the solution to 80 °C and attach to the flask a dropping funnel containing 216g (264 ml, 3.0 mol) of redistilled butyraldehyde. 

• With vigorous stirring, add the butyraldehyde as rapidly as the efficiency of the reflux condenser will allow and then boil the reaction mixture under reflux for 1 hour. 

• Cool, separate the organic layer and distil it without further treatment under reduced pressure through a fractionating column (e.g. of the Vigreux type). 

• Collect the pure 2-ethylhex-2-enal as a fraction of b.p. 66-67 °C/ 25 mmHg; the yield is 160g (85%).

Cognate preparation: 2-Methylpent-2-enal

• Add 174g (215ml, 3.0mol) of propionaldehyde with vigorous stirring to 100 ml of 1 m sodium hydroxide solution, but without initial heating, during 15 minutes. 

• Cool rapidly in an ice bath, isolate the organic product with the aid of a little ether and fractionally distil. The yield is 70 per cent; b.p. 38-39 °C/25 mmHg, 136-137 °C/ 760 mmHg.