# Oxalyl chloride

Oxalyl chloride is an organic chemical compound with the formula Cl−C(=O)−C(=O)−Cl. This colorless, sharp-smelling liquid, the diacyl chloride of oxalic acid, is a useful reagent in organic synthesis.

## Preparation

Oxalyl chloride was first prepared in 1892 by the French chemist Adrien Fauconnier, who treated diethyl oxalate with phosphorus pentachloride. It can also be prepared by treating oxalic acid with phosphorus pentachloride.
Oxalyl chloride is produced commercially from ethylene carbonate. Photochlorination gives the perchloroethylene carbonate C$_2$Cl$_4$O$_2$CO and hydrogen chloride HCl, which is subsequently degraded to oxalyl chloride and phosgene COCl$_2$:
    C$_2$H$_4$O$_2$CO + 4 Cl$_2$ → C$_2$Cl$_4$O$_2$CO + 4 HCl
    C$_2$Cl$_4$O$_2$CO → C$_2$O$_2$Cl$_2$ + COCl$_2$

## Reactions

As originally determined by Staudinger, oxalyl chloride reacts with water giving off gaseous products only: hydrogen chloride (HCl), carbon dioxide (CO$_2$), and carbon monoxide (CO).
    (COCl)$_2$ + H$_2$O → 2 HCl + CO$_2$ + CO
Other acyl chlorides hydrolyze with formation of hydrogen chloride and the original carboxylic acid.

## Applications in organic synthesis
### Oxidation of alcohols

Addition of a primary or secondary alcohol to a solution of oxalyl chloride in DMSO, followed by quenching with an organic base such as triethylamine converts alcohols to the corresponding aldehydes and ketones via the process known as the Swern oxidation.

### Synthesis of acyl chlorides

Oxalyl chloride is mainly used together with a N,N-dimethylformamide catalyst in organic synthesis for the preparation of acyl chlorides from the corresponding carboxylic acids. Like thionyl chloride, the reagent degrades into volatile side products in this application, which simplifies workup. One of the minor byproducts from the N,N-dimethylformamide catalyzed reaction, dimethylcarbamoyl chloride, is a potent carcinogen, stemming from the N,N-dimethylformamide decomposition. Relative to thionyl chloride, oxalyl chloride tends to be a milder, more selective reagent. It is also more expensive than thionyl chloride so it tends to be used on a smaller scale.
This reaction involves conversion of N,N-dimethylformamide to the imidoyl chloride derivative (chloromethylene(dimethyl)ammonium ion (CH$_3$)$_2$N$^+$=CHCl), akin to the first stage in the Vilsmeier–Haack reaction. The imidoyl chloride is the active chlorinating agent.

### Addition of carboxylic acid groups to arenes

Oxalyl chloride reacts with aromatic compounds in the presence of aluminium chloride to give the corresponding acyl chloride in a process known as a Friedel-Crafts acylation. The resulting acyl chloride can be hydrolysed to form the corresponding carboxylic acid.

### Preparation of oxalate diesters

Like other acyl chlorides, oxalyl chloride reacts with alcohols to give esters:
    2 R−CH$_2$−OH + Cl−C(=O)−C(=O)Cl → R−CH$_2$−O−C(=O)−C(=O)−O−CH$_2$−R + 2 HCl
Typically, such reactions are conducted in the presence of a base such as pyridine. The diester derived from phenol, phenyl oxalate ester, is Cyalume, the active ingredient in glow sticks.
A similar reaction occurs with amines to give substituted oxalamides:
    2 R−NH$_2$ + ClC(=O)−C(=O)Cl → R−NH−C(=O)−C(=O)−NH−R + 2 HCl

### Other

Oxalyl chloride was reportedly used in the first synthesis of dioxane tetraketone (C$_4$O$_6$), an oxide of carbon.

## Precautions

In March 2000, a Malaysia Airlines Airbus A330-300 was damaged beyond repair after a cargo of prohibited oxalyl chloride (falsely declared as hydroxyquinoline) leaked into the cargo bay. It is toxic by inhalation, although it is over an order of magnitude less acutely toxic than the related compound phosgene.