Carbonyl dibromide

The most remarkable feature of the chemistry of carbonyl dibromide is how little of it has been studied. Indeed, in a compendium of the uses and applications of chemicals published (perhaps significantly) in 1944, the only use of COBr to be listed (apart from the preparation of crystal violet. Section 14.7) was as a poison gas (similar in effects to phosgene) [826]. The only previous review to have been published [783] was in German. 

The physiological effects were judged (as a result of some rather amateur experiments on white mice) similar to those of phosgene [1822], but clearly a modern detailed evaluation is required if COBr is to be used more widely. Liver microsomes from phenobarbital 

The environmental problems associated with COBr are minimal when compared with phosgene, but there is a report that it may be a by-product from tribromomethane oxidation in waste water treatment [1702], and it has been identified as a minor impurity in commercial bromine produced by the electrolysis of brine [865], and a possible minor impurity in 

The first claim to have prepared COBrj was that of Schiel, in 1863, by the exposure of a mixture of CO and bromine vapour to sunlight (see also Section 14.4) [1795a]. Emmerling, in 1880, attempted to oxidize CHBr3 with a mixture of potassium dichromate(VI) and concentrated sulfuric acid, and obtained a highly impure product (b.pt. 12-30 C) [589a]. Later studies of the same reaction [2126] obtained trace amounts of a colourless liquid (b.pt. 60-68 C), identified as COBr by its reaction with Af,Af-dimethylaniline (see Section 10.2.1.4). 

Early attempts by Besson to prepare COBr by the reaction between CO and Br, by oxidation of CHBr3 or CCl Br with ozone, and by treating phosgene with aluminium(III) bromide were unsuccessful [184], but COBr, was claimed to be formed in the reaction 

---

In an incident in our own laboratories, where less than 2 cm of carbonyl dibromide were accidently released into a fume cupboard of volume sufficient to render the release safe, the local paper had the story as the main front page lead (see Fig. 1.32) - the number of errors are legion, both concerning the actual events and the science, in this short article - we suspect ICI would have been astounded to discover that it had been sponsoring experiments with carbonyl dibromide "to find new ways of welding metals together". 

Students were usii carbonyl dibromide in a fecial sealed cupboard when a controlled ex MosM>tt went wrong. 

Carbonyl dibromide is a dense, colourless, mobile liquid, fuming in moist air due to hydrolysis, with an odour similar to that of phosgene [2126]. Its relative molecular mass is 187.8184. It is much less stable to dissociation than phosgene even distillation at atmospheric pressure results In a slight yellow colouration due to liberated bromine. 

Carbonyl dibromide is much less stable than phosgene, being almost completely decomposed at weak red heat [2126] ... 

The diamagnetic susceptibility of carbonyl dibromide has never been measured. It has been estimated, however, from the computed value of the mean molecular polarizability (89.24 X 10"25 cm3) the mass diamagnetic susceptibility, Xm> estimated as... 

Reactions of carbonyl dibromide with other carbon halides... 

Carbonyl dibromide reacts with the other carbonyl halides, upon heating in an autoclave, according to [902,905] ... 

However, at 70-80 C, carbonyl dibromide does not react with sulfur(IV) oxide (although it does dissolve in it) [918a]. 

Similarly, the reaction with aniline itself proceeds differently. With phosgene, aniline gives the simple disubstituted urea (see Section 10.2.1.1), whereas it reacts with carbonyl dibromide in diethyl ether to yield a disubstituted urea, but with the phenyl rings 4-brominated [1937a] ... 

Treatment of pyridinium benzoate with carbonyl dibromide in toluene gives benzoyl bromide [1919] ... 

Similarly, the reactions between solid sodium iodide and either phosgene or carbonyl dibromide at -78 C produced only CO, Ij and NaX (X = Cl or Br) [1589b], and a reaction between tetrabutylammonium iodide and phosgene in dichloromethane at -78 C yielded only CO, Ij and [NBu ]Cl [1589b]. These results, particularly those obtained at low temperature, clearly point to COIj, even if it was formed as a transient intermediate, being extremely unstable. 

The electronic absorption spectrum of gaseous carbonyl dibromide has not been studied in detail, but the absorption onset occurs at 31250 cm and absorption is continuous up to 47620 cm" , the limit of the reported measurements [1821]. 

Carbonyl dibromide is not commercially available, and therefore must be prepared when required. It is conveniently prepared by the oxidation of tetrabromomethane with sulfuric acid ... 

---