Trichloromethane polar molecule

Trichloromethane, CHCI3, is a polar molecule. The three C—Cl dipoles point in a similar direction. Their combined effect is not cancelled out by the polarity of the C—H bond. This is because the C—H bond is virtually non-polar. The electron distribution is asymmetric. The molecule is polar, with the negative end towards the chlorine atoms. This is shown in Figure 4.33a. 

If the four atoms attached to the central atom in a tetrahedral molecule are the same, as in tetrachloromethane (carbon tetrachloride), CCI4 (30), the dipole moments cancel and the molecule is nonpolar. However, if one or more of the atoms are replaced by different atoms, as in trichloromethane (chloroform), Cl ICI, or by lone pairs, as in NH3, then the dipole moments associated with the bonds are not all the same, so they do not cancel. Thus, the CHCI, molecule is polar (31). 

Predict whether each of the following molecules is likely to be polar or nonpolar (a) C HjN (pyridine, a molecule like benzene except that one —CH— group is replaced by a nitrogen atom) (b) C2H6 (ethane) (c) CHC1, (trichloromethane, also known as chloroform, a common organic solvent and once used as an anesthetic). 

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. 

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. 

---