SYNTHESIS OF BUTYRYL CHLORIDE (Butanoyl chloride)

• Fit a 100-ml two-necked flask with a dropping funnel and a reflux condenser connected at the top to a gas absorption trap. 

• Place 36g (21.5 ml, 0.3 mol) of redistilled thionyl chloride in the flask and 22g (23 ml, 0.25 mol) of butyric acid in the separatory funnel. 

• Heat the flask gently on a water bath, and add the butyric acid during the course of 30-40 minutes (1). 

• When all the acid has been introduced, heat on a water bath for 30 minutes. 

• Rearrange the apparatus and distil: collect the crude acid chloride boiling between 70 and 110°C. 

• Finally, redistil from a flask provided with a short fractionating column and collect the butyryl chloride at 100-101 °C. The yield is 23g (86%).

Notes to keep in mind:

1.  Wrap a piece of absorbent cotton wool around the stem of the reflux condenser above the joint of the reaction flask to prevent condensed moisture seeping into the flask.

Cognate preparations: Hexanoyl chloride

• Place 58 g (62 ml, 0.5 mol) of hexanoic acid in the flask, heat on a water bath and add 72 g (43 ml, 0.6 mol) of redistilled thionyl chloride during 45 minutes; shake the flask from time to time to ensure mixing. 

• Reflux for 30 minutes and isolate the hexanoyl chloride by distillation, b.p. 150-155 °C. The yield is 56 g (83%).

Valeryl chloride (pentanoyl chloride)

• Use 51 g (0.5 mol) of valeric acid and 72 g (0.6 mol) of redistilled thionyl chloride. Proceed as for hexanoyl chloride; the yield of valeryl chloride is 42g (70%), b.p. 124-127 °C.

Isobutyryl chloride (2-methylpropanoyl chloride)

• Use 140 g (1.6 mol) of isobutyric acid and 236 g (2 mol) of redistilled thionyl chloride. Proceed as for hexanoyl chloride; the yield is 121 g (71%), b.p. 90-93 °C, after distillation through a Vigreux column (36 cm).

Isovaleryl chloride (3-methylbutanoyl chloride)

• Use 34 g (0.4 mol) of isovaleric acid and 47 g (0.5 mol) of thionyl chloride. Proceed as for hexanoyl chloride; the yield of isovaleryl chloride is 36 g (76%), b.p. 114-115°C, after distillation through a Vigreux column.

Cyclohexanecarbonyl chloride

• Use 91 g (0.7 mol) of cyclohexanecarboxylic acid and 166g (1.4 mol) of thionyl chloride. Proceed as for hexanoyl chloride but heat under reflux for 2 hours. The yield of cyclohexanecarbonyl chloride is 100 g (78%), b.p. 76-78°C/12mmHg, after distillation through a Vigreux column.