Molecular halogen

Atom abstraction occurs when a dissociation reaction occurs on a surface in which one of the dissociation products sticks to the surface, while another is emitted. If the chemisorption reaction is particularly exothennic, the excess energy generated by chemical bond fomiation can be chaimelled into the kinetic energy of the desorbed dissociation fragment. An example of atom abstraction involves the reaction of molecular halogens with Si surfaces [27, 28]. In this case, one halogen atom chemisorbs while the other atom is ejected from the surface. 

Valentin J J, Coggiola M J and Lee Y T 1977 Supersonic atomic and molecular halogen nozzle beam source Rev. Sc/. Instrum 48 58-63... 

Common name CAS Registry Molecular Halogen, Specific MP, °C Q Sources... 

More complex is the hydrolytic disproportionation of the molecular halogens themselves in aqueous alkali which is a commercial route to several alkali-metal halides ... 

There have been rather more kinetic studies of molecular halogenation than of positive halogenation and apart from the problems with iodination noted above, interpretation of the kinetics has presented few difficulties. 

Arriya PA, Khalizov A, Gidas A. 2002. Reactions of gaseous mercury with atomic and molecular halogens kinetics, product studies, and atmospheric implications. J Phys Chem A(106) 7310-7320. 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

Trialkyl phosphites undergo reaction with molecular halogen via a mechanism reminiscent of the Michaelis-Arbuzov reaction to form the dialkyl phosphorochloridate in good yield (Equation 4.4).7 With cyclic esters, the halogen performing the displacement reaction at carbon remains attached within the molecule. 

Halogenation of conjugated dienes proceeds chiefly by 1,4-addition with molecular halogens (equation 3). 1,2-Addition is favored in the presence of pyridine-halogen complexes and amine tribromide salts (equation 4)9. The stereochemistry of 1,4-bromine addition with 2,4-hexadienes and cyclopentadiene is primarily anti in the presence of amine, but syn with molecular halogen in the absence of amine. 

F. Molecular Halogen Emission in Flow Discharge Systems... 

H. Time-Resolved Molecular Halogen Emission Following Flash Photolysis... 

The emission from molecular halogens in the 3n0u+ state resulting from shock heating could, in principle, arise from both inverse predissociation and direct recombination on account of the large thermal populations of the excited atoms at these high temperatures. Emission from this state has been observed hitherto at high temperatures from iodine, bromine, and chlorine... 

Among the numerous mediator systems, halide ions show the broadest applicability. The internal electrochemical generation of the active species, i.e., molecular halogen, hypohalite ions, and halonium ions, is always possible by the in-cell method without... 

Alkylbenzenes and other activated aromatics, such as polynuclear compounds, may react without a catalyst. Halogenation, in this case, occurs with the involve-ment of molecular halogens. Polarization of chlorine and bromine molecules is brought about by their interaction with the aromatic ring. For example, bromination in the presence of acetic acid is suggested to take place through the transition state 5, leading to the formation of the a complex ... 

Halogenation of the double bond usually prevents the satisfactory preparation of vinylogous a-halo ketones by direct reaction of unsaturated ketones with molecular halogen however, exceptions have been noted ... 

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