Halogen compounds fragmentation

The Waste Management, Inc. (WMX), DeChlor/KGME process involves the ex situ dechlorination of liquid-phase halogenated compounds, particularly polychlorinated biphenyls (PCBs). KGME is the active species in a nucleophilic substitution reaction in which the chlorine atoms are replaced with fragments of the reagent. 

Hara et al. (1980) employed GC/MS for the analysis of trace amounts of mixed halogenated compounds in the blood and tissue of humans. Identification and quantitative analysis of various compounds was achieved by monitoring the mass fragments for selectively molecular, abundant or characteristic ions for each compound. Thus the monitoring ion (m/z) for quantification of 1,1 -dichloroethane was set at 83 [(M) -CH3]. A lower detection limit of 10 to 20 pg per sample was achieved. 

The types of ion fragments produced by the decomposition of organic halogen compounds in the mass spectrometer have been summarised by McLafferty - . Brief reviews have also been given by Beynon and by Budzikiewicz et al . These authors have also summarised the results of other workers, such as the early studies on the monohalides by Stevenson and Hippie and by Dibeler and Reese (ref. 146) and on the polyhalides by Bernstein et by McDowell et al and by Dibeler et Electron impact studies on aromatic halogen compounds have been reported by Majer and Patrick . The interpretations of the mass spectra of halogen compounds have been provided in some detail by McLaffierty , and the main features only of these spectra are discussed very briefly here. 

Tin-carbon bonds can be broken by reaction with electrophiles (e.g. protic acids, Lewis acids, halogens), nucleophiles (e.g. RLi), or free radicals (e.g. succimidyl, t-butoxyl), or with certain transition metal (particularly palladium) compounds. Fragmentation can also be induced through the radical cations which are formed by electron transfer. 

The effect of various additives on a styrene polymerization reinforces the tentative conclusion that vinyl polymerization is not taking place. Even though yields were increased by halogenated additives they were not decreased by additives expected to act as scavengers for free-radical species (benzophenone) or ionic species (butylamine or water). Under an assumed mechanism of fragmentation and rapid recombination to condensed products, halogenated compounds additives may serve to increase the efficiency of energy transfer from the electric field to the monomer. 

Unfortunately, it is not always possible to take advantage of these characteristic patterns to identify halogen compounds. Frequently the molecular ion peaks are too weak to permit accurate measurement of the ratio of the intensities of the molecular ion and isotopic peaks. However, it is often possible to make such a comparison on certain fragment ion peaks in the mass spectrum of a halogen compound. The mass spectrum of 1-bromohexane (Fig. 8.47) may be used to illustrate this method. The presence of bromine can be determined using the fragment ion peaks at m/e values of 135 and 137. 

Molecular ion Fragments m/z 69 [M-501+- or [Frag-50]+ For saturated aliphatic halogen compounds often weak, for polyhalogenated compounds often absent Characteristic isotope patterns for Cl and Br CF3 CF2 ... 

---