Thionyl chloride

Thionyl chloride is a chemical compound with the formula SOCl₂. It is related to sulfurous acid by replacing the −OH groups in its structure with chlorine (an acyl chloride). It is mostly used in the chemical industry and research to add chlorine atoms to other molecules (chlorination) or to remove water from mixtures or hydrates (dehydration).

Thionyl chloride is a reactive desiccant: it reacts strongly with water and will remove it from a solution or the air. Thionyl chloride reacts with water to make sulfur dioxide and hydrogen chloride:

SOCl₂ + H₂O → SO₂ + 2 HCl

Because both of the products are gases, it is easy to purify anything left after the reaction. This makes SOCl₂ useful for removing water from hydrated chlorine compounds like magnesium chloride and iron(III) chloride.

Thionyl chloride is very reactive with chemicals containing an −OH functional group. This includes not only water and hydroxides, but also organic chemicals like alcohols and carboxylic acids. These reactions almost always involve replacing the −OH with a chlorine atom, and the release of SO₂ and HCl.

With an alcohol like ethanol, it normally makes the matching organic chloride:^[1]

CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂ + HCl

If a lot of alcohol is used, and the reaction temperature is controlled, it can make sulfite esters instead:^[2]

2 ROH + SOCl₂ → (RO)₂SO + 2 HCl

With a carboxylic acid like acetic acid, it makes the matching acyl chloride:^[3]

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

Thionyl chloride also reacts with primary amides to make nitriles.^[4]

R−CONH₂ + SOCl₂ → R−C≡N + SO₂ + 2 HCl

It can also react with secondary amides in a name reaction called the von Braun amide degradation.^[5] This produces a nitrile and organic chloride:

RCONHR' + SOCl₂ → R−C≡N + R'Cl + SO₂ + HCl