Polarity of Bonds and Molecules

Bond polarities can range from nonpolar covalent, through polar covalent, to totally ionic. In the following examples, ethane has a nonpolar covalent C—C bond. Methylamine, methanol, and chloromethane have increasingly polar (C — N, C—O, and C—Cl) covalent bonds. Methylammonium chloride (CH3NH3 CF) has an ionic bond between the methylammonium ion and the chloride ion. 

In Section 1-6, we reviewed the concept of polar covalent bonds between atoms with different electronegativities. Now we are ready to combine this concept with molecular geometry to study the polarity of entire molecules. 

The polarity of an individual bond is measured as its bond dipole moment, /x, defined as 

Dipole moments are expressed in units of the debye (D), where 1 debye = 3.34 X 1CT30 coulomb meters. If a proton and an electron (charge 1.60 X 10-19 coulomb) were 1 A apart (distance 10-1 meter), the dipole moment would be 

A simple rule of thumb, using common units, is that 

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Chapter 10, Properties of Solids and Liquids, introduces electron-dot formulas for molecules and ions with single and multiple bonds as well as resonance structures. Electronegativity leads to a discussion of the polarity of bonds and molecules. Electron-dot formulas and VSEPR theory illustrate covalent bonding and the three-dimensional shapes of molecules and ions. The attractive forces between particles and their impact on states of matter and changes of state are described. Combining Ideas from Chapters 8, 9, and 10 follows as an interchapter problem set. 

The opening paragraph of this chapter emphasized that the connection between structure and properties is what chemistry is all about We have just seen one such con nection From the Lewis structure of a molecule we can use electronegativity to tell us about the polarity of bonds and combine that with VSEPR to predict whether the mol ecule has a dipole moment In the next several sections we 11 see a connection between structure and chemical reactivity as we review acids and bases... 

Since the parameters used in molecular mechanics contain all of the electronic interaction information to cause a molecule to behave in the way that it does, proper parameters are important for accurate results. MM3(2000), with the included calculation for induced dipole interactions, should model more accurately the polarization of bonds in molecules. Since the polarization of a molecular bond does not abruptly stop at the end of the bond, induced polarization models the pull of electrons throughout the molecule. This changes the calculation of the molecular dipole moment, by including more polarization within the molecule and allowing the effects of polarization to take place in multiple bonds. This should increase the accuracy with which MM3(2000) can reproduce the structures and energies of large molecules where polarization plays a role in structural conformation. 

The reason is that polarization of bonds and solvation of ions play an enormously important role in determining the reactivity of molecules. In Chapter 39 you will see that radicals are relatively unaffected by solvation and that their reactions follow bond strengths much more closely. 

Figure 1.16 Dipole moments of some molecules. Polarity of bonds and of molecules.

You can now predict the polarity of bonds within molecules, but it is also important to be able to predict the polar nature of the molecules themselves. The terms polar and nonpolar, when used to describe molecules, indicate their electrical charge symmetry (see > Figure 4.8). In polar molecules (often called dipoles), the charge distribution resulting from bond polarizations is nonsymmetric. In nonpolar molecules, any charges caused by bond polarization are symmetrically distributed through the molecule. > Table 4.5 illustrates polar and nonpolar molecules. The molecular shapes were determined by using the VSEPR theory. 

Raman spectroscopy is a complementary technique to IR. Both IR and Raman spectra arise from the vibrahonal energy levels of the molecules. The difference in the informahon content of the two vibrahonal methods arises from differences in selechon rules. In the simplest terms, IR absorphon arises from vibrahonal modes that give rise to changes in the dipole moments of the bonds and consequenhy is most sensihve to polar bonds. Raman absorphon arises from changes in the induced polarity of bonds and is most sensihve to nonpolar bonds. For polymers, IR absorphon is sensitive to subshtutents on the backbone of the chain, i.e. C— H, C=0, C—OH, etc., whereas Raman absorphon is sensihve to the C—C backbone... 

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... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

Bonds interact with one another in molecules. The bond interactions are accompanied by the delocahzation of electrons from bond to bond and the polarization of bonds. In this section, bond orbitals (bonding and antibonding orbitals of bonds) including non-bonding orbitals for lone pairs are shown to interact in a cychc manner even in non-cychc conjugation. Conditions are derived for effective cychc orbital interactions or for a continuous orbital phase. 

The electrostatic energy is calculated using the distributed multipolar expansion introduced by Stone [39,40], with the expansion carried out through octopoles. The expansion centers are taken to be the atom centers and the bond midpoints. So, for water, there are five expansion points (three at the atom centers and two at the O-H bond midpoints), while in benzene there are 24 expansion points. The induction or polarization term is represented by the interaction of the induced dipole on one fragment with the static multipolar field on another fragment, expressed in terms of the distributed localized molecular orbital (LMO) dipole polarizabilities. That is, the number of polarizability points is equal to the number of bonds and lone pairs in the molecule. One can opt to include inner shells as well, but this is usually not useful. The induced dipoles are iterated to self-consistency, so some many body effects are included. 

Lewis considered covalent and ionic bonds to be two extremes of the same general type of bond in which an electron pair is shared between two atoms contributing to the valence shell of both the bonded atoms. In other words, in writing his structures Lewis took no account of the polarity of bonds. As we will see much of the subsequent controversy concerning hypervalent molecules has arisen because of attempts to describe polar bonds in terms of Lewis structures. 

Potassium is a metal, and the polyatomic anion, C104 is a nonmetal therefore, the compound is an ionic solid at room temperature. When the compound is dissolved in water, the ionic bond between the cation, K+, and the polyatomic anion, Cl()4, is broken due to the polarity of the water molecule, resulting in the two aqueous ions, K+ and C104 . 

The molecules CH20 and BH3CO show a wider variety of electronic sub-structures than the simple hydrids discussed above in particular CH20 contains a (polar) ir-bond and sp2 hybrids and BH3CO has 7r bonds, sp hybrids and a dative bond. It is of some interest therefore to see how the GHOs behave in these situations. Table 2 contains the relevant orbital exponents. The pattern of orbital contraction for GHOs involved in a X—H bonds is repeated and reinforced by the C—0 a bond orbitals. 

These are 13 atoms of hexagons, the central atom of which has the coordination of 12 atoms. The process of rolling flat carbon systems into NT is, apparently, determined by polarizing effects of cation-anion interactions resulting in statistic polarization of bonds in a molecule and shifting of electron density of orbitals in the direction of more electronegative atoms. 

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