Reaction with halogen molecules

Alkanes are rather inert chemically. Aside from burning in air or oxygen to produce carbon dioxide and water (or carbon monoxide and water), the most characteristic reaction they undergo is reaction with halogen molecules, initiated with light. For example. 

Chemi-ionization has been observed54 in the reactions of K2 with HgX2 and SnX4, but with much lower cross sections for ion formation, Q, 0T A2, than in the reactions with halogen molecules, Q-, 3-10 A2. Other polyhalide molecules studied did not yield any observable chemi-ionization giving an upper bound to their chemi-ionization cross sections, < 0001 A2. 

Halogens on alkyl side chains generally resemble halogens in alkyl halides. As with alkyl halides, preparation is typically carried out by (1) substitution of side-chain hydroxyl groups or (2) addition of HX or X2 to side-chain olefins. Free-radical reaction with halogen molecules, X2, yields a mixture of products and works best for chlorination or bromination. 

Platinum removes a halogen atom from the halide, causing homolytic fission of the C-halogen bond. The resulting Pt -XR radical pair can either react to form Ptn(R)X or separate, with subsequent reaction with RX leading to either PtX2 or PtRX species or reaction with solvent molecules. 

The electronic energy of the iodine atom in the 52Py2 state renders a number of atomic reactions with halogen and interhalogen molecules exothermic which would be endothermic for the analogous reactions of the ground state. 

Let us also consider briefly electron tunneling in gas-phase chemical reactions with atom transfer. As a classic example, we can take the reactions of alkaline metals with halogen molecules. At collision energies of several eV, the reaction proceeds via the following channels [68]... 

Harpoon reactions of alkaline metal atoms with halogen molecules in the gas phase seem to be the first instance of the observation of chemical electron transfer reactions at distances somewhat exceeding gas-kinetic diameters. Actually, as far back as 1932, Polanyi, while studying diffusion flames found for these reactions cross-sections of nR2, somewhat exceeding the gas-kinetic cross-sections [69]. Subsequently, more precise measurements which were carried out in the 1950s and 1960s with the help of the molecular beam method, confirmed the validity of this conclusion [70],... 

C. Reactions With Halogens and Other Small Molecules... 

Fig. 3. A schematic view of a crossed-molecular beam apparatus used for studying the reactions of chlorine atoms with halogen molecules. The mass spectrometer detector is rotatable about the scattering centre for measuring the angular distributions of the reaction products whose recoil velocities are determined by time-of-flight analysis. (Reproduced from ref. 558 by permission of the authors and the American Institute of Physics.)...

Despite an extensive study of the reactions of alkali dimers (see the reviews of Grice [208] and Herm [216]), there have been few direct measurements of energy disposal in these systems. The chemiluminescent reactions of oxygen and halogen atoms with alkali dimers have already been described [Sects. 3.1.7(c) and 3.1.8(b)], as have the alkali atom-alkali dimer exchange reactions [Sect. 3.1.2(b)], In this section we describe the chemiluminescent reactions of alkali diatomic molecules with halogen molecules. 

A wider range of possibilities of preparing cyclic derivatives is offered by the presence of carboxyl and a-amino groups in the molecule. Substituted 5-oxazolinone (Scheme 4.23) is prepared by refluxing with TFA anhydride, and substituted 5-oxazolidinone (Scheme 4.24)by reaction with (halogenated) acetone [117,118]. Thiohydantoins are formed by reaction with isothiocyanate (Scheme 4.25). If R is methyl or phenyl, then the corresponding methyl- or phenylthiohydantoin is produced. Thiohydantoins are usually not volatile enough and for the purposes of GC analyses must be further modified, e.g., by trimethylsilylation [119,120]. 

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... 

Comparison of Experimental Data on Energy Partitioning in the Reactions of H or D Atoms with Halogen Molecules... 

Alkynes undergo addition reactions with halogens (CI2, Br2) to produce tetrahalo alkanes. To saturate each allqme molecule, two halogen molecules are needed. Alkynes decolorize aqueous Br2 solution, as alkenes do. 

The reactions of alkaline earth metal atoms with halogen molecules, either forming alkaline earth metal monohalides or leading to chemi-ionization, represent an important case study in the reaction dynamics of divalent systems [91, 92, 95-100]. Let us take as an example the reaction... 

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