Gas phase Halogens

Gas-phase halogenation of benzenoid and heterocyclic compounds has been extensively studied by Wibaut and his co-workers, and a review of this work is available. ... 

Crutzen et al. (1992) suggested that the methylper-oxy radical formed in the CH4 oxidation may also play a role in the gas-phase halogen chemistry. (The role of... 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

Halogenations may also occur by a free-radical mechanism.121,218 Besides taking place in the gas phase, halogenation may follow a free-radical pathway in the liquid phase in nonpolar solvents. Radical halogenation is initiated by the alkene and favored by high alkene concentrations. It is usually retarded by oxygen and yields substitution products, mainly allylic halides. 

Substituent effects on the proton-losing step would then be responsible for the isomer-distribution pattern. A similar picture could be drawn for the high temperature gas-phase halogenation of pyridines. 

Figure 8.4 Schematic showing the chemical cycling of the gas-phase halogens (iodine highlighted in dark gray) in the atmosphere, and the links to aerosol formation and chemistry (adapted from Saiz-Lopez and Plane, 2004a).

Gillis EAL, Demireva M, Sarwar MG, Chudzinski MG, Taylor MS, Williams ER, Pridgen TD (2013) Structure and energetics of gas phase halogen-bonding in mono-, bi- and tri-dentate anion receptors as studied by BIRD. Phys Chem Chem Phys 15 7638... 

The detection of gas phase halogens by He MIP-MS has also been investigated by Brown and co-workers [98]. They utilized a commercially available ICP-MS instrument. The ICP torch box was removed and replaced with an internally tuned TMqio microwave cavity. The He plasma was formed using a demountable tangential flow torch. This was used in an effort to create a central region in the plasma, rich in analyte ions which allows for maximum sensitivity when sampling the plasma with the mass spectrometer. 

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