Trichloromethane, dipole moment

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). 

Inspection of Table 4-9 reveals that the axial cis isomer (34a), which is the con-former with the higher dipole moment, becomes more favoured as the solvent polarity increases. In the most polar solvent studied, acetonitrile, AG° is nearly zero. Benzene, toluene, trichloromethane, dichloromethane, and methanol are seen to behave as more polar solvents than their relative permittivities would lead one to predict. The deviation for trichloromethane was particularly dilficult to explain (for a full discussion, see reference [89]). In general, good correlations between values and other solvent-... 

According to its and NMR spectra, this vinylogous amide exists as the E-s-E form in polar solvents such as [D4]methanol, and as a mixture of Z-s-Z and E-s-E isomers in nonpolar solvents such as deuterio-trichloromethane (30 cmol/mol /36a) and 70 cmol/mol (36b)). As expected, the more dipolar E-s-E form is stabilized in polar solvents (dipole moment of the related E-s-E 3-dimethylaminopropenal 21 10 Cm). 

Trichloromethane (CHCI3, also called chloroform) has a larger dipole moment than CFCI3. PRACTICE PROBLEM 2.7 Use three-dimensional structures and bond moments to explain this fact. 

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. 

A Predict whether the dipole moment of each of the following molecules should be zero, nearly zero, or significantly larger than zero. Then use the Handbook of Chemistry and Physics to find their correct dipole moment values. Compare your predictions with the values found in the Handbook or at a suitable website, (a) acetylene (b) trichloromethane (chloroform) ... 

Determine if the following species have permanent dipole moments, (a) Methane, CH4 (b) Chloromethane, CH3CI (c) Dichloromethane, CHjClj (d) Trichloromethane (or chloroform), CHCI3 (e) Carbon tetrachloride, CCI4 (f) 2,2-Dimethylpropane, CH3C(CH3)2CH3... 

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