Formation of isomeric ethylenehalonium and α-haloethyl cations and the role of ion–neutral complexes in the decomposition of protonated β-haloethanols

✍ Scribed by Albert J. R. Heck; Leo J. de Koning; Nico M. M. Nibbering

• Publisher: John Wiley and Sons
• Year: 1993
• Tongue: English
• Weight: 891 KB
• Volume: 28
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/oms.1210280317

✦ Synopsis

Decomposition of protonated p-haloethanols was found to proceed predominantly via the loss of water. The mecbanism was studied in the decomposition of metastable and collisionally activated isotopically labelled protonated p-halwthanols. The study involved the characterization of the resulting fragment ions by collisionally activated decomposition and by low-pressure ion/molecule probe reactions. The results indicate that loss of water from protonated phaloethanols proceeds via two competing mechanisms. The competition between these mechanisms appears to be governed by the nature of the halogen and by the internal energy of the decomposing protonated phaloethanol. In the first mechanism, loss of water is Considered to be initiated by a concerted 1,2-elimination of H 3 0 + resulting in an ion-molecule complex between H,O+ and haloethene. Rapid and reversible proton transfers between the two species in the complex is followed by a relatively slow dissociation of the intermediate complex resulting in the formation of an water molecule and an a-haloethyl cation. In competition with this mechanism, loss of water can be initiated by carbon-oxygen bond cleavage, which is assisted by halogen-carbon bond formation resulting in the formation of a water molecule and cyclic ethylenehalonium ion.