Dhristi JEE 2022-24

A Brief Study of Thionyl Chloride

SO is the thionyl group, which consists of a sulphur atom and an oxygen atom. SOF2 (thionyl fluoride) is an example of a chemical that contains it. In this topic, we will try to learn about this thionyl chloride briefly. In this topic, we will cover some basic properties like thionyl chloride density, thionyl chloride CAS number, and some important uses of Thionyl chloride.


What is Thionyl Chloride?

SOCl2, or thionyl chloride, is a popular reagent for converting carboxylic acids to acyl chlorides in chemical synthesis. The thionyl group, also known as a sulfoxide group or sulfinyl group in organic chemistry, has the general structure RS (= O)R.


SOCl2 is the chemical formula for inorganic compound named thionyl chloride. It's a colourless, fairly volatile liquid with a terrible acrid odour. Thionyl chloride is predominantly employed as a chlorinating reagent, with production peaking at over 45,000 tonnes (50,000 short tonnes) per year in the early 1990s, but it is also used as a solvent on occasion. It's poisonous, reacts with water, and is on the Chemical Weapons Convention's list of substances that could be used to make chemical weapons. Thionyl chloride is frequently assumed as sulfuryl chloride, SO2Cl2, but their characteristics are vastly different. Sulfuryl chloride is a chlorine source, whereas thionyl chloride is a chloride ion source.


The Reaction of Thionyl Chloride

Thionyl chloride is primarily utilised in the manufacture of organochlorine compounds, which are frequently used as intermediates in medicines and agrichemicals. It is frequently favoured over other reagents like phosphorus pentachloride because its by-products (HCl and SO2) are gaseous, making product purification easier. Because many of the thionyl chloride's products are very reactive, it is engaged in a wide range of reactions.


Properties and Structure of Thionyl Chloride

At room temperature and pressure, thionyl chloride is a colourless or pale yellow liquid with a distinct odour. It has a relative density of 1.676, a melting point of -104.5 degrees Celsius. Thionyl chloride boiling point is 78.8 degrees Celsius. When it comes into contact with water, it decomposes quickly into sulphur dioxide and hydrogen chloride. Benzene, chloroform, and carbon tetrachloride are all soluble in it. When heated to 150°C, it begins to degrade, and at 500°C, it is completely decomposed. Thionyl chloride is frequently mistaken for thionyl chlorine (SO2Cl2). If we talk about thionyl chloride pH value, it is acidic. Thionyl chloride density is 1.64 g/cm3.


However, the molecular constitution of these two molecules differs significantly, with the chlorine atom having a substantial replacement capacity on hydroxyl or sulphur groups. Thionyl chloride can be used to make corresponding chlorides by reacting with hydroxyl-containing phenol or hydroxyl-containing alcohol; it can also be used to make similar sulfoxide compounds by reacting with the Grignard reagent. The molecular structure of thionyl chloride is cone-shaped, with one lone pair of electrons in the sulphur (VI) centre. COCl2, on the other hand, has a planar structure. SOCl2 does not exist in nature because of the strong reaction between thionyl chloride and water.


With Cs molecular symmetry, SOCl2 has a trigonal pyramidal molecular geometry. The effects of the lone pair on the central sulfur(IV) centre are responsible for this shape. Although thionyl chloride has a lengthy shelf life, "old" samples develop a yellow tint, probably due to disulfur dichloride production. At just above the boiling point, it progressively decomposes into S2Cl2, SO2, and Cl2. Photolysis, which occurs predominantly through a radical process, is vulnerable to thionyl chloride. Distillation under lower pressure can be used to purify samples that exhibit symptoms of ageing, resulting in a colourless liquid.


Uses of Thionyl Chloride

Here are some important uses of thionyl chloride.


  • Thionyl Chloride is primarily utilised for the manufacturing of iso-carbophos, propargite tetramisole hydrochloride, indomethacin, and vitamin A in the pharmaceutical, pesticide, and dye industries. 

  • Chlorination of alcoholic hydroxyl groups, carboxylic acid chlorination, acid anhydride chlorination, and chlorine displacement of organic sulfonic acid or carboxylic acid are all examples of chlorinating agents for organic synthesis. 

  • It's also used to make acyl chloride and pharmaceutical intermediates like tetramisole hydrochloride and synthomycin palmitate. 

  • It can also be used as a solvent or a dehydrating agent.


The following procedures can benefit from thionyl chloride:


  • Dehydroxylation is used to enhance the production of indenones from 3-hydroxy-indanones.

  • To thio-acetalize aldehydes by functionalizing silica gel.

  • Acid chloride conversion of tertbutyl esters.

  • In the presence of triphenylphosphine, aliphatic and aromatic sulfoxides are converted to sulphides.


Environmental Issues With Thionyl Chloride

It's a good choice for distillation recycling. If you want to deal with it, slowly dissolve it in freezing water until it decomposes into SO2 and HCl, which may then be treated.


If the amount of thionyl chloride waste is significant, consider recycling or selling it for treatment outside. If the quantity is little, drop it into the alkaline water drop by drop for decomposition and neutralisation.

FAQs on Thionyl Chloride

1. What is the CAS number? What is the thionyl chloride CAS number?

A CAS Registry Number, also known as CAS RN or colloquially CAS Number, is a unique numerical identity provided to every chemical substance documented in the open scientific literature by the Chemical Abstracts Service (CAS). It now includes all chemicals described from 1957 to the present, as well as a few early 1800s molecules. Organic and inorganic compounds, minerals, isotopes, alloys, mixes, and non-structure-able materials all fall within this category (UVCBs, substances of unknown or variable composition, complex reaction products, or biological origin). CAS RNs are typically serial numbers (with a check digit), and unlike SMILES and InChI strings, they do not include any information about the structures themselves. Thionyl chloride CAS number is 7719-09-7.

2. What are the chemical properties of thionyl chloride?

Thionyl chloride is a colourless or pale yellow volatile liquid with a suffocating odour. Benzene, chloroform, and carbon tetrachloride are all miscible with it. Thionyl chloride is frequently used to convert alcohols into more reactive alkyl chloride, which can be transformed into a variety of chemicals that are not possible to make directly from alcohols. Nucleophilic substitution, which is often carried out under basic circumstances, is a key reaction of alkyl chloride.

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