Reactive halogen molecules

As with the other halogens, iodine is a diatomic molecule. It is always found in nature in a combined state, often as iodide salts where it has a -1 oxidation number. Compounds in which iodine is found to have oxidation numbers of 7, 3, 5, and 1 are also well known. Iodine is prepared commercially by treatment of natural salt solutions (seawater or brines) with chlorine (a more reactive halogen), according to the reaction ... 

The reactions of alkali dimers with halogen molecules also exhibit a multiplicity of reaction pathways with production of electronically excited atoms, electronically excited molecules, and ions all apparently possible [364, 367-370], In these reactions, however, the dominant pathway, with a reactive cross section >150 A2, leads to products in their electronic ground... 

The alkenes are more reactive than the alkanes. For example, alkene molecules undergo addition reactions in which the atoms from halogen molecules or from hydrogen halides bond to the carbon atoms involved in the double bond ... 

These reactions were pioneered by Grice and co-workers who first addressed the reactivity of alkali metal dimers with halogen molecules [108-111]. These reactions are four-center harpoon reactions. They are interesting because of the possibility of two sequential electron jumps [112. ... 

Exciplex lasers (also called excimer lasers) use reactive halogen atoms to form excited pseudo-molecules with noble-gas atoms. Molecules such as XeF are stable on ly in excited electronic states and quickly dissociate after transition to the ground state. This makes possible a large population inversion and produces laser action in the ultraviolet region. A simple prototype for such behavior is the Hcj excimer, which is an entry in Table 11.1. 

Much of the chemistry of alkanes involves free-radical chain reactions, which take place under vigorous conditions and usually yield mixtures of products. A reactive particle—typically an atom or free radical—is needed to begin the attack on an alkane molecule. It is the generation of this reactive particle that requires the vigorous conditions the dissociation of a halogen molecule into atoms, for example, or even (as in pyrolysis) dissociation of the alkane molecule itself. 

Fe(III) salts are known to oxidise electron-rich centres to foster the formation of radical species. They are particularly efficient in the oxidation of aromatic systems or a carbanion to the corresponding carbon-centred radical which undergoes C-C bond formation to yield the coupled products. For a successful synthesis, it is important to work in the absence of reactive synthetic molecules other than those which form the combination of radicals. Barton et al. used a simple water-soluble diselenide derivative that shows radical scavenger properties towards alkyl and hydroxyl radicals in Fenton-type chemistry (Fe2+-H202)4 The reaction rate between the produced alkyl radical and the diselenide overwhelms self-termination and halogen transfer reactions. The ability of diselenide to scavenge alkyl and hydroxyl radicals [ 3(0 °C) = 6.1 x 108 M-1 s-1] could be exploited as a new tool in both synthetic and mechanistic work conducted in aqueous media (Scheme 8.5).4... 

The halogens are electronegative and oxidizing elements, fluorine exceptionally so. They occur in nature as halides, and form highly reactive diatomic molecules. Molecular halides are formed with most nonmetals, ionic halides with metals. Some halides are good Lewis acids, and many halide complexes are known. 

The reactions44-45 of alkali dimers with halogen molecules offer an opportunity to study the dynamics of four-centre reactions. Three-centre reactions of an atom plus diatomic molecule have been extensively studied and also four-centre reactions46 of an atom plus triatomic molecule. However, there are few examples known47 of reactive four-centre diatomic plus diatomic systems. 

Angular distribution measurements of reactive scattering of alkaline earth atoms A by halogen molecules X2 have been reported80 for Ba + Cl2 and more extensively81-82 for Ba, Sr, Ca, Mg with Cl2, Br2, IC1, BrCN. The reactions form monohalide products AX, thus following primarily the reaction path... 

Angular distribution measurements107-109 of reactive scattering of halogen atoms Cl and Br by halogen molecules Br2,I2, IBr, IC1 marked the advent of successful molecular beam studies of non-metal reactions, and... 

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