Methane bond polarities

In contrast with water, methanol, ammonia, and other substances in Table 2.1, carbon dioxide, methane, ethane, and benzene have zero dipole moments. Because of the symmetrical structures of these molecules, the individual bond polarities and lone-pair contributions exactly cancel. 

The molecular geometry of methane and of methyl fluoride is tetrahedral. In the case of methane, this symmetrical arrangement of polar covalent carbon-hydrogen bonds leads to a canceling of the bond polarities resulting in a nonpolar molecule. As a nonpolar molecule, the strongest intermolecular force in methane is a London force. In methyl fluoride, a fluorine atom replaces one of the hydrogen... 

Electron withdrawal in these molecules is the result of a bond polarization from an inductive effect (Chapter 5). The electrons in a o bond between carbon and a more electronegative element such as N, O, or F will be unevenly distributed with a greater electron density towards the more electronegative atom. This polarization is passed on more and more weakly throughout the carbon skeleton. The three fluorine atoms in CF3H reduce the p K"a to 26 from the 48 of methane, while the nine fluorines in (CFa CH reduce the pKTa still further to 10. 

The physical properties of the alkanes follow the pattern laid down by methane, and are consistent with the alkane structure. An alkane molecule is held together entirely by covalent bonds. These bonds either join two atoms of the same kind and hence are non-polar, or join two atoms that differ very little in electronegativity and hence are only slightly polar. Furthermore, these bonds are directed in a very symmetrical way, so that the slight bond polarities tend to cancel out. As a result an alkane molecule is either non-polar or very weakly polar. 

As the labels imply, the f s provide the geometric definition of the four half body-diagonals of a cube, which define a tetrahedron. When the definitions of 9 are compared with Equation (2) the symbols p and Py are seen to be the identification of Cartesian axes in polar coordinates. The sp linear combination is thereby identified as a geometrical, rather than a quantum-mechanical, definition of the four assumed tetrahedral bonds of methane in polar coordinates. 

Using the data in Table 6.2 we find that the sum of the bonding radii (rc + rp) is equal to 155.6 pm, while the third term which represents the contraction due to the bond polarity is -17.6 pm. The estimated C-F bond distance is 138.0 pm in good agreement with the experimental bond distance in CH3F. Is there any reason to believe that the polarity of the C-F bonds in fluorinated methanes increases with the number of F atoms ... 

The transfer of intensity parameters between molecules for quantitative intensity predictions encounters various problems that need care l consideration. As already emphasized in tius section, due to the very high sensitivity of intensities to structural changes transferability properties of intensity parameters are expected to be much less pronounced compared to other molecular quantities. Secondly, certain parameters will be dependent on the particular site symmetry of the chemical bonds or atoms considered. Additional complications can arise if rotational correction terms are to be calculated. Predictions by transfer of parameters should, therefore, only be attempted for closely related molecules, such as homologous series. Bond polar parameters have been used in predicting intensities in intiared spectra in fluorinated methanes [144], alltylacetylenes [145] and medium-size n-alkanes [143]. In Fig. 4.8 the predicted infrared spectra of different conformers of n-pentane using bond polar parameters from n-butane are presented [143]. In more quantitative terms the predicted intensities are compared with the experimental values in Table 4.12. As can be seen from Table 4.12, the agreement between calculated and observed intensities is quite satisfactory. 

Which of the following can be used to explain why all bond distances and angles in methane, CH, are the same (a) resonance (b) delocalization of electrons (c) bond polarities (d) electronegativity (e) orbital hybridization. 

The physical properties of hydrofluorocarbons reflect their polar character, and possibly the importance of intermolecular hydrogen bonding (3). Hydrofluorocarbons often bod higher than either their PFC or hydrocarbon counterparts. For example, l-C H F bods at 91.5°C compared with 58°C for n-Q and 69°C for Within the series of fluorinated methanes, the boiling point reaches a maximum for CH2F2, which contains an equal... 

Chloroform, CHCla, is an example of a polar molecule. It has the same bond angles as methane, CH4, and carbon tetrachloride, CCLi- Carbon, with sp3 bonding, forms four tetrahedrally oriented bonds (as in Figure 16-11). However, the cancellation of the electric dipoles of the four C—Cl bonds in CCL does not occur when one of the chlorine atoms is replaced by a hydrogen atom. There is, then, a molecular dipole remaining. The effects of such electric dipoles are important to chemists because they affect chemical properties. We shall examine one of these, solvent action. 

The rate is near first order in methane and zero order in oxygen for oxygen to methane ratios higher than 1. Also, the reaction kinetics remain unaffected upon polarization conditions. The kinetic data indicate weak bonding of methane and strong bonding of oxygen on the catalyst surface. 

The presence of -S02(OH) groups reduced the carbon dioxide permeability by a factor of three. This can be explained (15) by the decrease in local segmental mobility of the polymer chains due to the interactions arising from hydrogen bonding. However, the overall transport process for this polymer membrane is more complicated and involves a more pronounced discrimination against methane molecules due to the highly polar nature of the polymer. 

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