Hydrogen by halogen

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

The degree of reactivity to air or to water depends on the degree of substitution of hydrogen by halogen. This highly reactive group includes the individually indexed compounds ... 

Substituent Effects upon Replacement of Hydrogen by Halogen... 

However, if the molecules of 5 had R alkyl chains longer than Me, the steric hindrance prevented 100% substitution and IR examinations indicated a 50% less derivatization. Moreover, XPS analysis showed that the surface is partly modified by substitution of hydrogen by halogen . In the case of 5 with X = I and to some extent X = Br, the formation of X radicals (besides 12) in a secondary reaction was reported . They participate in reactions analogous to equations 21 and 22b, but with X instead of 12, and attach to the Si surface improving the electronic passivation of the surface at defect sites, sterically inaccessible to 12. A possibility that surface dangling bonds may also appear in the charged states was discussed as well . 

Analogous experiments with mixtures of identical combustion temperature containing various excess amounts of carbon monoxide showed that the flame velocity is proportional to [CO ]1/2, where [CO ] is the carbon monoxide concentration in the reaction zone. From this it follows that the chemical reaction in a flame is first order in carbon monoxide. The role of water in the combustion of carbon monoxide is well known. Analysis of available data shows that the flame velocity is proportional to [H20]1//2, i.e., the reaction is first order in water vapor content. The influence on combustion of such flegmatizers as CC14 may be ascribed to the binding of hydrogen by halogen with the formation of a molecule of HC1, which is dissociable only with difficulty. However, the latest experiments by Kokochashvili in our laboratory show that the influence of the... 

The MO models are also very helpful for explaining the effect of substituting hydrogen by halogen atoms yielding... 

While nucleophilic replacement of hydrogen by halogen is unknown in these compounds there are a number of examples of other groups which are displaced by halide and related nucleophiles. 

The values of C in Crafts s equation (1) are lower the greater the molecular complexity for similar substances an increase of molar weight without increased complexity either causes a similar change or has no influence. With isomers, C is lower for normal than for /.yc)-compounds. Replacement of hydrogen by halogen lowers C association leads to low values of C, and in hydroxy-compounds the influence of the OH group diminishes as the complexity of the organic radical increases, but the constant tends to fall as the complexity of the molecule increases. 

Replacement of the hydrogen by halogen in the square-planar plati-... 

I. Replacement of hydrogen by halogen or thiocyanate 1. Replacement of hydrogen by halogen in aliphatic hydrocarbons... 

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