Oxalyl chloride

Oxalyl chloride is a chemical compound with the formula (COCl)2. This colourless liquid, the diacid chloride of oxalic acid, is a useful reagent in organic synthesis. It can be prepared by treating oxalic acid with phosphorus pentachloride.

Synthesis of acid chlorides
It is mainly used in organic synthesis for the preparation of acid chlorides from the corresponding carboxylic acids. Like thionyl chloride, the reagent produces volatile side products in this application:
 * RCO2H +  (COCl)2  →  RC(O)Cl  +  HCl  +  CO  +  CO2

Oxalyl chloride tends to be a milder, more selective reagent.

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

Preparation of diesters
Like other acid chlorides, oxalyl chloride reacts with alcohols to give esters:
 * 2 RCH2OH +  (COCl)2  →  RCH2OC(O)C(O)OCH2R  +  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.

Oxidation of alcohols
The combination of DMSO and oxalyl chloride converts alcohols to the corresponding aldehydes and ketones via the process known as the Swern oxidation.

Precautions
As with all acyl chlorides, oxalyl chloride reacts with water liberating HCl gas. Overall, its effects are comparable to those of phosgene.

In March 2000 a Malaysia Airlines Airbus A330 was damaged beyond repair after a falsely declared cargo of oxalyl chloride leaked into the cargo bay.