Bond, polar

In calculations of formal charge, we assume that a shared pair of electrons spends equal time around both of the atoms that form the bond. This is true only if the atoms are the same. More commonly, the bond is between two different atoms, and the electrons are not necessarily shared equally. The atom that has a greater share of the electrons can be determined from the electronegativities (electron-attracting abilities) of the atoms involved in the bonds. The electron density represented by a pair of shared electrons is greater around the atom with the greater electronegativity. 

When unequal electronegativities of two atoms involved in a bond result in charge separation as just described, we say that the bond is polar. Hydrogen chloride has a polar bond. The charge separation results in a dipole, that is, a positive and a negative pole in the molecule. The product of the amount of charge separation (e) times the distance of the charge separation (d) is called the dipole moment (p,). 

The dipole moment is a vector that is, it has direction and magnitude. The dipole moment is usually represented as an arrow pointing from the positive end to the negative end of the dipole, as shown earlier for hydrogen chloride. 

The concept of bond polarities is very important, because much of chemistry, both the physical properties of compounds and their chemical reactions, depends on the interaction of charges. For example, a reagent that is seeking positive charge will likely be attracted to the carbon of a carbon—oxygen bond. 

Note All energies are negative, representing average attractive potentials between the electrons and the nucleus for all terms of the specified orbitals. 

FIGURE 5-14 Molecular Orbitals and Photoelectron Spectrum of CO. Molecular orbitals la and Ict are from the Is orbitals and are not shown, The ej and 2 labels in the left-hand column are for the Cinfinityv symmetry labels the hj and 2 labels are for C2V symmetry. (Photoelectron spectrum reproduced with permission from J. L. Gardner and J. A. R. Samson, J. Chem. Phys., 1975, 62, 1447.) 

The molecular orbitals that will be of greatest interest for reactions between molecules are the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), collectively known as frontier orbitals because they lie at the occupied-unoccupied frontier. The MO diagram of CQ helps explain its 

The LUMOs are the 2tt orbitals and are concentrated on carbon, as expected. The frontier orbitals can contribute electrons (HOMO) or accept electrons (LUMO) in reactions. Both are important in metal carbonyl bonding, which will be discussed in Chapter 13. 

The atomic orbitals of the atoms that form homonuclear diatomic molecules have identical energies, and both atoms contribute equally to a given MO. Therefore, in the molecular orbital equations, the coefficients associated with the same atomic orbitals of 

FIGURE 5.13 Molecular Orbitals and Photoelectron Spectrum of CO. Molecular orbitals Icr and Icr are from the Is orbitals and are not shown. 

EXERCISE 5.3 Use a similar approach to the discussion of HF to explain the bonding in the hydroxide ion OH. (Exercise 5.3 by Kaitlin Hellie. Reprinted by permission.) 

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In stead, the electrostatic con tribn tion conies from definin g a set of bond dipole moments associated woth polar bonds. These bond moments are defined in the m m psir.LxL(dbf) file along with the bond stretching parameters and are given in units of Debyes. The cen ter of th e dipole Is defined to be th e m Idpoint of the bond an d two dipoles p. and pj. separated by Rjj. as shown beltnv ... 

Table 1 3 lists the dipole moments of various bond types For H—F H—Cl H—Br and H—I these bond dipoles are really molecular dipole moments A polar molecule has a dipole moment a nonpolar one does not Thus all of the hydrogen halides are polar molecules To be polar a molecule must have polar bonds but can t have a shape that causes all the individual bond dipoles to cancel We will have more to say about this m Section 1 11 after we have developed a feeling for the three dimensional shapes of molecules... 

We can combine our knowledge of molecular geometry with a feel for the polarity of chemical bonds to predict whether a molecule has a dipole moment or not The molec ular dipole moment is the resultant of all of the individual bond dipole moments of a substance Some molecules such as carbon dioxide have polar bonds but lack a dipole moment because their geometry causes the individual C=0 bond dipoles to cancel... 

Both water and carbon dioxide have polar bonds but water is a polar molecule and carbon dioxide is not... 

From the geometry of this triangular display, it follows immediately-if one overlooks the exceptions—that the more widely separated a pair of comonomers are in Fig. 7.2, the greater is their tendency toward alternation. Conversely the closer they are together, the greater their tendency toward randomness We recognize a parallel here to the notion that widely separated elements in the periodic table will produce more polar bonds than those which are closei together and vice versa. 

The polarity of covalent bonds between carbon and substituents is the basis of important structure-reactivity relationships in organic chemistry. The effects of polar bonds are generally considered to be transmitted in two ways. Successive polarization through bonds is called the inductive fect. It is expected that such an effect would diminish as the number of intervening bonds increases. 

The second component is called afield effect and is attributed to through-space interactions of the electric dipoles resulting from polar bonds. 

In Chapter 4, we will discuss the relative importance of inductive effects and field effects on reactivity. Generally, field effects appear to be the dominant mechanism for the transmission of electrostatic effects of polar bonds to other parts of a molecule. 

Several structural factors have been considered as possible causes of the anomeric effect. In localized valence bond terminology, it can be recognized that there will be a dipole-dipole repulsion between the polar bonds at the anomeric carbon in the equatorial conformation. This dipole-dipole interaction is reduced in the axial conformation, and this factor probably contributes to the solvent dependence of the anomeric effect. 

Even with mobile-phase modifiers, however, certain polymer types cannot be run due to their lack of solubility in organic solvents. In order to run aqueous or mixed aqueous/organic mobile phases, Jordi Associates has developed several polar-bonded phase versions of the PDVB gels as discussed earlier. Figures 13.60 thru 13.99 detail examples of some polar and ionic polymers that we have been able to run SEC analysis of using the newer bonded PDVB resins. 

What electronegativity difference, large or small, creates a more polar bond A more covalent bond ... 

Draw a Lewis structure for cyclohexenone that involves charge separation for the most polar bond. Then, draw a Lewis structure that will delocalize one or both charges. Next, examine the actual geometry of cyclohexenone. Are the bond distances consistent with the Lewis structure shown above, or have they altered in accord with your alternative (charge separated) Lewis structure (Structures for cyclohexene and cyclohexanone are available for reference.)... 

A five-membered heterocyclic ring packs a relatively large number of polarized bonds into a relatively small molecular space. This provides a convenient framework to which to attach necessary side chains. In some cases, the framework itself is believed to be part of the pharmacophore. 

The fact that a Lewis acid is able to accept an electron pair means that it must have either a vacant, low-energy orbital or a polar bond to hydrogen so that it can donate H+ (which has an empty7 Is orbital). Thus, the Lewis definition of acidity includes many species in addition to H+. For example, various metal cations, such as Mg2+, are Lewis acids because they accept a pair of electrons when they form a bond to a base. We ll also see in later chapters that certain metabolic reactions begin with an acid-base reaction between Mg2+ as a Lewis acid and an organic diphosphate or triphosphate ion as the Lewis base. 

Most organic compounds are electrically neutral they have no net charge, either positive or negative. We saw in Section 2.1, however, that certain bonds within a molecule, particularly the bonds in functional groups, are polar. Bond polarity is a consequence of an unsymmetrical electron distribution in a bond and is due to the difference in electronegativity of the bonded atoms. 

Electrophile (Section 5.4) An "electron-lover," or substance that accepts an electron pair from a nucleophile in a polar bond-forming reaction. 

In the HF molecule, the distribution of the bonding electrons is somewhat different from that found in H2 or F2. Here the density of the electron doud is greater about the fluorine atom. The bonding electrons, on the average, are shifted toward fluorine and away from the hydrogen (atom Y in Figure 7.9). Bonds in which the electron density is unsymmetrical are referred to as polar bonds. 

If a molecule is diatomic, it is easy to decide whether it is polar or nonpolar. A diatomic molecule has only one kind of bond hence the polarity of the molecule is the same as the polarity of the bond. Hydrogen and fluorine (H2, F2) are nonpolar because the bonded atoms are identical and the bond is nonpolar. Hydrogen fluoride, HF, on the other hand, has a polar bond, so the molecule is polar. The bonding electrons spend more time near the fluorine atom so that there is a negative pole at that end and a positive pole at the hydrogen end. This is sometimes indicated by writing... 

All molecules, except those of elements, have polar bonds. 

Ihe arrow points toward the negative end of the polar bond (F atom) the plus sign is at the positive end (H atom). Ihe HF molecule is called a dipole it contains positive and negative poles. 

Carbon tetrachloride, CCU, is another molecule that, like BeF is nonpolar despite the presence of polar bonds. Each of its four bonds is a dipole, C - — CL However because the four bonds are arranged symmetrically around the carbon atom, they canceL As a result, the molecule has no net dipole it is nonpolar. If one of the Cl atoms in CCI4 is replaced by hydrogen, the situation changes. In the CHCl3 molecule, the H - — C dipole does not cancel with the three C -)— Cl dipoles. Hence CHC13 is polar. 

Polar bond A chemical bond that has positive and negative ends characteristic of all bonds between unlike atoms, 182-183... 

Unfortunately, both lithium and the lithiated carbons used as the anode in lithium ion batteries (Li C, l>x>0) are thermodynamically unstable relative to solvent molecules containing polar bonds such as C-O, C-N, or C-S, and to many anions of lithium salts, solvent or salt impurities (such as water, carbon dioxide, or nitrogen), and intentionally added traces of reactive substances (additives). 

What Do We Need to Know Already This chapter uses atomic orbitals and electron configurations (Chapter 1). It also extends the concept of Lewis structures introduced in Chapter 2. The discussion of polar molecules develops the material on polar bonds described in Section 2.12. 

A diatomic molecule is polar if its bond is polar. A polyatomic molecule is polar if it has polar bonds arranged in space in such a way that the dipole moments associated with the bonds do not cancel. 

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