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 Halogenation reagent, with approximately per year being produced during the early 1990s,
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.
Other methods include syntheses from:
-
Phosphorus pentachloride:
- :
- :
- :
- :
The second of the above five reactions also affords phosphorus oxychloride (phosphoryl chloride), which resembles thionyl chloride in many of its reactions. They may be separated by distillation, since thionyl chloride boils at a much lower temperature than phosphoryl chloride.
Properties and structure
SOCl
2 adopts a trigonal pyramidal molecular geometry with C
s 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 S
2Cl
2,
sulphur dioxide and
chlorine 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 water and alcohols
Thionyl chloride reacts exothermically with water to form
sulfur dioxide and hydrochloric acid:
By a similar process it also reacts with alcohols to form haloalkane. If the alcohol is chiral the reaction generally proceeds via an SNi mechanism with retention of stereochemistry;
Reactions with an excess of alcohol produce , which can be powerful methylation, alkylation and hydroxyalkylation reagents.
For example, the addition of to in methanol selectively yields the corresponding methyl esters.
With carboxylic acids
Classically, it converts
to
:
The reaction mechanism has been investigated:
With nitrogen species
With primary amines, thionyl chloride gives
sulfinylamine derivatives (RNSO), one example being
N-
sulfinylaniline. Thionyl chloride reacts with primary
to form
and with secondary formamides to give chloro
iminium ions; as such a reaction with dimethylformamide will form the Vilsmeier reagent.
By an analogous process, primary will react with thionyl chloride to form , with secondary amides also giving chloroiminium ions. These species are highly reactive and can be used to catalyse the conversion of carboxylic acids to acyl chlorides;
Primary amides will continue on to form if heated (Von Braun amide degradation).
Thionyl chloride has also been used to promote the Beckmann rearrangement of .
With sulfur species
-
Thionyl chloride will transform into sulfinyl chlorides
-
react with thionyl chloride to produce sulfonyl chlorides.
-
Thionyl chloride can be used in variations of the Pummerer rearrangement.
- :
With phosphorus species
Thionyl chloride converts
and
into phosphoryl chlorides. It is for this type of reaction that thionyl chloride is listed as a Schedule 3 compound, as it can be used in the "di-di" method of producing G-series
. For example, thionyl chloride converts dimethyl methylphosphonate into methylphosphonic acid dichloride, which can be used in the production of
sarin and
soman.
With metals
As reacts with water it can be used to dehydrate various metal chloride hydrates, such magnesium chloride (), aluminium chloride (), and iron(III) chloride ().
This conversion involves treatment with refluxing thionyl chloride and follows the following general equation:
If an exces SOCl2 is used to dehydrate aluminium trichloride, it will form an adduct (1 molecule of thionyl chloride for each molecule of the aluminium trichloride dimer).
Other reactions
-
Thionyl chloride can engage in a range of different electrophilic addition reactions. It adds to alkenes in the presence of to form an aluminium complex which can be hydrolysed to form a sulfinic acid. Both aryl sulfinyl chlorides and diaryl sulfoxides can be prepared from arenes through reaction with thionyl chloride in triflic acid
-
In the laboratory, a reaction between thionyl chloride and an excess of anhydrous alcohol can be used to produce anhydrous alcoholic solutions of HCl.
-
Thionyl chloride undergoes halogen exchange reactions to give other thionyl species.
- Reactions with fluorinating agents such as antimony trifluoride give thionyl fluoride:
- :
- A reaction with hydrogen bromide gives thionyl bromide:
- :
- Thionyl iodide can likewise be prepared by a reaction with potassium iodide, but is reported to be highly unstable.
Batteries
Thionyl chloride is a component of lithium–thionyl chloride
lithium battery,
where it acts as the positive electrode (in batteries:
cathode) with
lithium forming the negative electrode (
anode); the
electrolyte is typically lithium tetrachloroaluminate. The overall discharge reaction is as follows:
These non-rechargeable batteries have advantages over other forms of lithium batteries such as a high energy density, a wide operational temperature range, and long storage and operational lifespans. However, their high cost, non-rechargeability, and safety concerns have limited their use. The contents of the batteries are highly toxic and require special disposal procedures; additionally, they may explode if shorted. The technology was used on the 1997 Sojourner Mars rover.
Safety
SOCl
2 is highly reactive and can violently release hydrochloric acid upon contact with water and alcohols. It is also a controlled substance under the Chemical Weapons Convention, where it is listed as a Schedule 3 substance, since it is used in the manufacture of G-series
and the Meyer and Meyer–Clarke methods of producing
mustard gas.
History
In 1849, the French chemists Jean-François Persoz and Bloch, and the German chemist Peter Kremers (1827–?), independently first synthesized thionyl chloride by reacting phosphorus pentachloride with
sulfur dioxide.
[See:
] However, their products were impure: both Persoz and Kremers claimed that thionyl chloride contained phosphorus,
[The German chemist Georg Ludwig Carius noted that, when the reaction mixture that produced thionyl chloride was distilled, the crude mixture initially released substantial quantities of gas, so that phosphoryl chloride (POCl3) was carried into the receiver. From p. 94: " … dabei ist jedoch die Vorsicht zu gebrauchen, … und nie reines Chlorthionyl erhalten wird." ( … however, during that i.e.,, caution must be used, so that one carefully avoids a concentration of hydrogen chloride or excess sulfurous acid in the liquid that is to be distilled, as otherwise, by the evolution of gas that occurs at the start of the distillation, much phosphoryl chloride is transferred and pure thionyl chloride is never obtained.)] and Kremers recorded its boiling point as 100 °C (instead of 74.6 °C). In 1857, the German-Italian chemist
Hugo Schiff subjected crude thionyl chloride to repeated fractional distillations and obtained a liquid which boiled at 82 °C and which he called
Thionylchlorid.
[ The boiling point of thionyl chloride which Schiff observed, appears on p. 112. The name Thionylchlorid is coined on p. 113.] In 1859, the German chemist Georg Ludwig Carius noted that thionyl chloride could be used to make acid anhydrides and
from
and to make
Organochloride from alcohols.
[
]On p. 94, Carius notes that thionyl chloride can be ''" … mit Vortheil zur Darstellung wasserfreier Säuren verwenden."'' ( … used advantageously for the preparation of acid anhydrides.) Also on p. 94, Carius shows chemical equations in which thionyl chloride is used to transform [[benzoic acid]] (OC7H5OH) into [[benzoyl chloride]] (ClC7H5O) and to transform [[sodium benzoate]] into benzoic anhydride. On p. 96, he mentions that thionyl chloride will transform [[methanol]] into [[methyl chloride|Chloromethane]] (''Chlormethyl''). Thionyl chloride behaves like phosphoryl chloride: from pp. 94-95: ''"Die Einwirkung des Chlorthionyls … die Reaction des Chlorthionyls weit heftiger statt."'' (The reaction of thionyl chloride with [organic] substances containing oxygen proceeds in general parallel to that of phosphoryl chloride; where the latter exerts an effect, thionyl chloride usually does so also, only in nearly all cases the reaction occurs far more vigorously.)
See also
