Carbon-halogen bond dipole moment

The a-haloketone effect, as this phenomenon is known, is believed to be a result of dipolar interactions between the carbonyl group and the carbon-halogen bond dipole. The conformation with the smaller dipole moment is the one with the halogen axial, and is favored in solvents of low dielectric constant. 

Carbon-oxygen and carbon-halogen bonds are polar covalent bonds and carbon bears a partial positive charge in alcohols ( " C—0 ) and in alkyl halides ( " C—X ) Alcohols and alkyl halides are polar molecules The dipole moments of methanol and chloromethane are very similar to each other and to water... 

Table 6.1 Carbon-Halogen Bond Lengths, Bond Strength and Dipole Moment...

Since the size of halogen atom Increases as we go down the group in the periodic table, fluorine atom is the smallest and iodine atom, the largest. Consequently the carbon-halogen bond length also Increases from C—to C— I. Some typical bond lengths, bond enthalpies and dipole moments are given in Table 10.2. 

A molecular dipole moment is the vector sum of the individual bond dipole moments. Molecular dipole moments are not easy to predict because they depend on the bond angles and other factors that vary with the specific molecule. Table 6-1 lists the experimentally measured dipole moments of the halogenated methanes. Notice how the four symmetrically oriented polar bonds of the carbon tetrahalides cancel to give a molecular dipole moment of zero. 

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

Alkyl halides have large dipole moments since the carbon-halogen bond is polar. They also have reasonably high boiling points because of the polarity and the high molecular weights. 

FIGURE 6.3 A substantial dipole moment exists in the polar carbon—halogen bond, X = F, Cl, Br, or I. 

Table 10.2 Carbon-Halogen (C— X) Bond Lengths, Bond Enthalpies and Dipole Moments...

The polar C-X bond present in alkyl halides has a substantial dipole moment. Alkyl halides are poorly soluble in water, but are soluble in organic solvents. They have boiling points that are similar to alkanes of comparable molecular weight. Due to polarity, the carbon is an electrophilic centre and the halogen is a nucleophilic centre. Halogens are extremely weak nucleophilic centres and therefore, alkyl halides are more likely to react as electrophiles at the carbon centre. 

An apolar aprotic solvent is characterized by a low relative permittivity (sr < 15), a low dipole moment [ju < 8.3 10 Cm = 2.5 D), a low value ca. 0.0... 0.3) cf. Table A-1, Appendix), and the inability to act as a hydrogen-bond donor. Such solvents interact only slightly with the solute since only the non-specific directional, induction, and dispersion forces can operate. To this group belong aliphatic and aromatic hydrocarbons, their halogen derivatives, tertiary amines, and carbon disulfide. 

Table 1.3 Comparison of the characteristics of the carbon-halogen and carbon-carbon bonds (bond lengths in pm binding energies in kcal mol electronegativities from Ref [3] dipole moments in D Van derXXftials radii in pm from Ref [4] atom polarizabilities a in 10 cm from Ref [5]).

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