A colourless liquid acyl chloride (see acyl halides), CH₃COCl, with a pungent smell; r.d. 1.105; m.p. –112.15°C; b.p. 50.9°C. It is made by reacting ethanoic acid with a halogenating agent such as phosphorus(III) chloride, phosphorus(V) chloride, or sulphur dichloride oxide and is used to introduce ethanoyl groups into organic compounds containing –OH, –NH₂, and –SH groups. See acylation.