Chloromethane molecular dipole moment

In chloromethane, the tetrahedral shape is clear, but there is only one polarized bond and the dipole for the molecule is easily predicted. In dichloromethane, however, there are two bond moments, and the dipole for the molecrde is the vector sum of these two bond moments (magnitude and direction). The dipole is shown. For trichloromethane (chloroform), the magnitude and direction of the three polarized C-Cl bonds lead to the molecular dipole moment shown. Carbon tetrachloride is interesting. There are four C-Cl bonds with equal bond polarization and dipole moments. Summing all four dipole moments for the bonds, which are directed to the corners of a regular tetrahedron, leads to a dipole moment of zero because the magnitudes of the individual bond moments cancel. 

Chloromethane was a simple example, because it has only one polar bond. When dealing with a compound that has more than one polar bond, it is necessary to take the vector sum of the individual dipole moments. The vector sum is called the molecular dipole moment, and it takes into account both the magnitude and the direction of each individual dipole moment. For example, consider the structure of dichloromethane (Figure 1.45). The individual dipole moments partially cancel, but not completely. The vector sum produces a dipole moment of 1.14 D, which is significandy smaller than the dipole moment of chloromethane because the two dipole moments here partially cancel each other. 

Molecular models for chloromethane, dichloromethane, and trichloromethane are given to show the direction of the dipole of molecule more clearly. Calculated dipoles for these three molecules are 2.87, 2.50, and 1.72 Debye, respectively, and it is clear that the directional nature of the individual bond dipoles plays a role in the overall magnitude of the dipole moment for the molecrde. With three chlorine atoms directed to different regions of space, chloroform is the least polar of the three molecules, despite the presence of three polarized bonds. 

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