SYNTHESIS OF HEXANOIC ANHYDRIDE

• Place 116g (126 ml, 1 mol) of dry hexanoic acid in a 250-ml Drechsel bottle and cool in ice. 

• Pass in 21-23 g of keten(1).

• Carefully distil the reaction mixture through a highly efficient fractionating column (e.g. a well-lagged Widmer column) (2), using an oil bath for heating. 

• A fraction of low boiling point, containing acetone, keten, acetic acid and a little acetic anhydride, is thus removed at atmospheric pressure. 

• Raise the temperature of the bath to 220 °C over a period of 1 hour and maintain it at this temperature for 3 hours from the time distillation commences: this time is necessary to ensure that the conversion of the mixed anhydride to hexanoic anhydride and acetic acid is complete and that the acetic acid is completely removed. 

• Discontinue the distillation, allow to cool somewhat and distil the residue in the flask under reduced pressure (3-10 mmHg). 

• Discard the small fraction (20g) of low boiling point and collect the hexanoic anhydride at 118-121 °C/6mmHg(or 109-1 12 °C/3 mmHg). The yield is 90g (84%). Record the i.r. spectrum and compare it with that of acetic anhydride.

Notes to keep in mind:

1. Excess of keten over the calculated quantity does not increase the yield; it leads to more acetic anhydride being collected in the low boiling point fraction.

2. The best results are obtained with a fractionating column surrounded by an electrically heated jacket but this is not essential for hexanoic anhydride. For the preparation of propanoic or butanoic anhydride, a highly efficient fractionating column must be used in order to obtain satisfactory results.