Halomethanes, carbon-halogen bonds

Carbon-halogen bonds are polar, and halomethanes have dipole moments. 

Table 4.11 contains bond polar parameter values for carbon-halogen bonds determined from experimental infrared intensities in halomethanes and carbonyl halides. As emphasized before, bond polar parameters are not directly related to the equilibrium charge distribution and, therefore, any relationship between the magnitude or sign of diese quantities and the actual polarity of valence bonds may only be qualitative. It should also be pointed out that parameter values are dependent on the accuracy of experimental measurements and the force fields employed. 

The halonium structure 17 was also attributed107 to protonated bromo- and chloromethane, in agreement with theoretical predictions105,106. However, in these two ions an additional interaction (stronger for the bromo than for the chloro compound) is present between the hydrogen bonded to the halogen and the carbon atom107. Protonated iodomethane, on the contrary, structurally resembles protonated methane with the proton bonded to the carbon atom. These conclusions are supported by both MI and CID data for protonated and deuterated halomethanes and by thermochemical data. 

Thus, it is comprehensible that the electronic effects which modify the polarity of the bonds have an influence upon the chemical shifts. Table 15.3 illustrates the effect of electronegativity upon the position of the methyl signal in the halomethane series CHj-X where the chemical shift of the carbon atom signal increases with the electronegativity of the halogen X. 

Fluorine, chlorine, and bromine are all more electronegative than carbon (Table 1.5) as a result, C—bonds with these atoms are polarized with a partial negative charge on halogen and a partial positive charge on carbon. Table 8.1 shows that each of the halomethanes has a substantial dipole moment. The electrostatic potential map of fluo-romethane shows the large charge separation in this compound caused by the dipole. 

The C-X bond-dissociation energies in the halomethanes, CH3X, decrease along the series F, Cl, Br, I. At the same time, the C-X bond lengths increase (Table 6-1). The bond between carbon and halogen is made up mainly by the overlapping of an sp hybrid orbital on carbon... 

In Summary Diazomethane is a useful synthetic intermediate as a methylene source for forming cyclopropanes from alkenes. Halogenated carbenes, which are formed by dehydro-halogenation of halomethanes, and the Simmons-Smith reagent, a carbenoid arising from the reaction of diiodomethane with zinc, also convert alkenes into cyclopropanes. Additions of carbenes to alkenes differ from other addition processes because a single carbon atom becomes bonded to both alkene carbons. 

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