SYNTHESIS OF 3-HEXYLACRYLIC ACID (Non-2-enoic acid)

• Dissolve 57g (0.55 mol) of malonic acid in 92.5 ml of dry pyridine contained in a 500-ml round-bottomed flask, cool the solution in ice and add 57g (70 ml, 0.5 mol) of freshly distilled heptanal with stirring or vigorous shaking. 

• After a part of the aldehyde has been added, the mixture yields a semi-solid slurry of crystals. 

• Insert a calcium chloride tube into the mouth of the flask and allow the mixture to stand at room temperature for 60 hours with periodic shaking. 

• Finally, warm the mixture on a water bath until the evolution of carbon dioxide ceases (about 8 hours) and then pour into an equal volume of water. 

• Separate the oily layer and shake it with 150 ml of 25 per cent hydrochloric acid to remove pyridine. 

• Dissolve the product in benzene, wash with water, dry with anhydrous sodium sulphate and distil under reduced pressure. Collect the nonenoic acid at 130-132 °C/2mmHg. The yield is 62g (79%).

Cognate preparations: Crotonic acid [(E)-But-2-enoic acid] 

• Mix together in a 250-ml flask carrying a reflux condenser and a calcium chloride drying tube 25g (32 ml, 0.57 mol) of freshly distilled acetaldehyde with a solution of 59.5g (0.57 mol) of dry, powdered malonic acid in 67g (68.5 ml, 0.85 mol) of dry pyridine to which 0.5 ml of piperidine has been added. 

• Leave in an ice chest or refrigerator for 24 hours. 

• Warm the mixture on a steam bath until the evolution of carbon dioxide ceases. 

• Cool in ice, add 60 ml of 1:1 sulphuric acid (by volume) and leave in the ice bath for 3-4 hours. 

• Collect the crude crotonic acid (c. 27g) which has separated by suction filtration. 

• Extract the mother-liquor with three 25 ml portions of ether, dry the ethereal extract, and evaporate the ether; the residual crude acid weighs 6g. 

• Recrystallise from light petroleum, b.p. 60-80 °C; the yield of crude crotonic acid, m.p. 72 °C, is 20g (41%).

Sorbic acid (Hexa-2,4-dienoic acid):

• Place 40g (46.5 ml, 0.57 mol) of croton-aldehyde (b.p. 101-103 °C), 60g (0.575 mol) of malonic acid and 60g (61 ml, 0.76 mol) of dry pyridine (b.p. 113-115 °C) in a 500-ml round-bottomed flask, attach a reflux condenser and heat on a water bath for 3 hours. 

• At the end of this period the vigorous evolution of carbon dioxide will have ceased. 

• Cool the mixture in ice and cautiously acidify it by the addition of a solution of 21.3 ml of concentrated sulphuric acid in 50 ml of water with shaking. 

• Most of the sorbic acid separates out immediately; a more complete separation is obtained by cooling the solution in ice for 3-4 hours. 

• Filter the acid at the pump and wash it with a little ice-cold water. 

• Recrystallise from about 125 ml of boiling water; the maximum recovery of purified acid is achieved by leaving the solution in an ice chest or a refrigerator overnight and then filtering.The yield of sorbic acid, m.p. 134°C, is 20g (31%).