Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year being produced during the early 1990s, but is occasionally also used as a solvent. It is toxic, reacts with water, and is also listed under the Chemical Weapons Convention as it may be used for the production of chemical weapons. Thionyl chloride is sometimes confused with sulfuryl chloride, , but the properties of these compounds differ significantly. Sulfuryl chloride is a source of chlorine whereas thionyl chloride is a source of chloride ions. Production The major industrial synthesis involves the reaction of sulfur trioxide and sulfur dichloride: This synthesis can be adapted to the laboratory by heating oleum to slowly distill the sulfur trioxide into a cooled flask of sulfur dichloride. SO3 + SCl2 -> SOCl2 + SO2 Other methods include syntheses from: Phosphorus pentachloride: SO2 + PCl5 -> SOCl2 + POCl3 Chlorine and sulfur dichloride: SO2 + Cl2 + SCl2 -> 2 SOCl2 SO3 + Cl2 + 2 SCl2 -> 3 SOCl2 Phosgene: SO2 + COCl2 -> SOCl2 + CO2 The first of the above four reactions also affords phosphorus oxychloride (phosphoryl chloride), which resembles thionyl chloride in many of its reactions. Properties and structure SOCl2 adopts a trigonal pyramidal molecular geometry with Cs molecular symmetry. This geometry is attributed to the effects of the lone pair on the central sulfur(IV) center. In the solid state SOCl2 forms monoclinic crystals with the space group P21/c. Stability Thionyl chloride has a long shelf life, however "aged" samples develop a yellow hue, possibly due to the formation of disulfur dichloride. It slowly decomposes to S2Cl2, SO2 and Cl2 at just above the boiling point. Thionyl chloride is susceptible to photolysis, which primarily proceeds via a radical mechanism. Samples showing signs of ageing can be purified by distillation under reduced pressure, to give a colourless liquid. Reactions Thionyl chloride is mainly used in the industrial production of organochlorine compounds, which are often intermediates in pharmaceuticals and agrichemicals. It usually is preferred over other reagents, such as phosphorus pentachloride, as its by-products (HCl and ) are gaseous, which simplifies purification of the product. Many of the products of thionyl chloride are themselves highly reactive and as such it is involved in a wide range of reactions. With