Dipole moment of chloromethane

It s relatively easy to measure dipole moments experimentally, and values for some common substances are given in Table 10.1. Once the dipole moment is known, it s then possible to get an idea of the amount of charge separation in a molecule. In chloromethane, for example, the experimentally measured dipole moment is /x = 1.87 D. If we assume that the contributions of the nonpolar C-H bonds are small, then most of the chloromethane dipole moment is due to the C-Cl bond. Since the C-Cl bond distance is 178 pm, we can calculate that the dipole moment of chloromethane would be 1.78 X 4.80 D = 8.54 D if the C-Cl bond were ionic (that is, if a full negative charge on chlorine were separated from a full positive charge on carbon by a distance of 178 pm). But because the measured dipole moment of chloromethane is only 1.87 D, we can conclude that the C-Cl bond is only about (1.87/8.54)(100%) = 22% ionic. Thus, the chlorine atom in chloromethane has an excess of about 0.2 electron, and the carbon atom has a deficiency of about 0.2 electron. 

The dipole moment of chloromethane (CH3CI) is greater (1.87 D) than the dipole moment of the C—Cl bond (1.5 D) because the C —H dipoles are oriented so that they reinforce the dipole of the C — Cl bond—they are all in the same relative direction. The dipole moment of water (1.85 D) is greater than the dipole moment of a single O— H bond (1.5 D) because the dipoles of the two O — H bonds reinforce each other. The lone-pair electrons also contribute to the dipole moment. Similarly, the dipole moment of ammonia (1.47 D) is greater than the dipole moment of a single N—H bond (1.3 D). 

FIGURE 2.4 (a) The dipole moment of chloromethane arises mainly from the highly polar carbon-chlorine bond. (t>) A map of electrostatic potential illustrates the polarity of chloromethane. 

Partial charges (6+ and 6—) are generally on the order of 10 °esu (electrostatic units) and the distances are generally on the order of 10 cm. Therefore, for a polar compound, the dipole moment (p) will generally have an order of magnitude of around 10 esu cm. The dipole moment of chloromethane, for example, is 1.87 x 10 esu cm. Since most compounds will have a dipole moment on this order of magnitude (10 ), it is more convenient to report dipole moments with a new unit, called a debye (D), where... 

Using these units, the dipole moment of chloromethane is reported as 1.87 D. The debye unit is named after Dutch scientist Peter Debye, whose contributions to the fields of chemistry and physics earned him a Nobel Prize in 1936. 

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. 

Figure 7.8 The reaction of ethynide (acetylide) anion and chloromethane. Electrostatic potential maps illustrate the complementary nucleophilic and electrophilic character of the alkynide anion and the alkyl halide. The dipole moment of chloromethane is shown by the red arrow.

The dipole moment of chloromethane (CH3CI) is greater (1.87 D) than the dipole moment of its C—Cl bond (1.5 D) because the C —H dipoles are oriented so that they reinforce the dipole of the C—Cl bond. In other words, all the electrons are pulled in the same general direction. 

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... 

Chloromethane. bond length of, 335 bond strength of, 335 dipole moment of, 335 dissociation enthalpy of, 195 electrostatic potential map of, 38, 143.335... 

Chlorohydrin. See Halohydrins Chloromethane, 148. See also Methyl chloride biosynthesis of, 713 boiling point of, 132 dipole moment of, 129 electrostatic potential map, 129... 

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. 

One liquid for which the value of seems suspect is 1-bromonaphthalene. It contains a polar carbon-bromine bond. I have not been able to find the dipole moment of 1-bromonaphthalene, but the values for bromobenzene, chlorobenzene and fluorobenzene which are chemically similar to 1-bromonaphthalene are 1.70 D, 1.69 D and 1.60 D, respectively. These molecules are almost as polar as chloromethane (1.97 D) and bromomethane (1.81 D), where the polarity of the... 

Before looking at the forces between molecules, it s first necessary to develop the ideas of bond dipoles and dipole moments. We saw in Section 7.4 that polar covalent bonds form between atoms of different electronegativity. Chlorine is more electronegative than carbon, for example, and the chlorine atom in chloromethane (CH3C1) thus attracts the electrons in the C C1 bond toward itself. The C-Cl bond is therefore polarized so that the chlorine atom is slightly electron-rich (8—) and the carbon atom is slightly electron-poor (<5+). 

Four examples of dipole moments are instructive. First, the dipoles for chloromethane and dichloromethane are 1.87D and 1.60D, respectively. Although two chlorine-carbon bonds are present in the latter, the dipole is not along either but rather bisects the angle between them. This is illustrated schematically using the stylized arrow with its positive end in the form of a cross. The orientation question is shown clearly in the rigid dichlorobenzene framework. The dipole is 2.13D for the ortho-isomer and 0D when the dipoles exactly oppose each other. 

For one-carbon halogenated aliphatics, the dipole moment decreases as the number of chlorines increases. The dataset consists of chloromethane, dichloromethane, chloroform, and carbon tetrachloride. The dipole moment represented 85.89% of the variance in the linear regression equation therefore, the probability of getting a correlation of -0.9268 for a sample size of three is between 5 and 10% ... 

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