Polarization, of bonds

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... 

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... 

Polarity of bonds can lead to polarity of molecules, as shown in the case of the water molecule ... 

Polarizabilities of atoms, 75, 76, 125 Polarization of bonds, 207 Potential energy surfaces, see Potential surfaces... 

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 polarity of bonds indeed has a much more marked influence on their lengths. With increasing negative charge of a particle the repulsive forces gain importance. Examples ... 

Resonance structures have also been used to describe the polarity of bonds. For example, H—Cl can be described by the two resonance structures ... 

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. 

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. 

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. 

The direction of polarization of bonds between various elements may be predicted from Figure 10-11. For example, an O-Cl bond should be polarized so the oxygen is negative a C-N bond should be polarized so the nitrogen is negative ... 

The use of carbonyl groups to set the polarity of bond disconnections in ret-rosynthetic analysis is useful for the construction of rings as well. If a carbon electrophile and a carbon nucleophile are connected by a carbon chain, they can react with each other to form a carbon-carbon bond. This is an absolutely normal type of carbon-carbon bond-forming process, but the fact that the carbon nucleophile and carbon electrophile are connected by a chain means that the new carbon-carbon bond closes up the ends of the chain, forming a ring. 

The inductive effect of an inorganic, organic or organometallic substituent, Y, is characterized quantitatively by several widespread constants, such as a and others (a, a, F etc.)34,57. The relationship ai(Y) = ct (Y) = F(Y) = 0.45inductive constant a of a certain Y substituent (Table 5) reflects several electronic interactions. The most important independent mechanisms of polarization of bonds between the substituent Y and the reaction centre are the field and the inductive interactions. The contribution of each mechanism depends on the type of substituent Y29. 

We explained on p. 102 the origin of the polarization of bonds to electropositive elements. 

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. 

Ans. Not by oxidative addition of dihydrogen. Polarizations of bonds, including the C-H of methane by direct interaction between the substrate and metal ion. 

Illustration of nuclear spin-spin interaction transmitted via polarization of bonding electrons. The two electrons about each carbon will tend to be parallel, since this arrangement minimizes the electron-electron repulsion (Hund s rule for electrons in degenerate orbitals). 

The polarization of bonds leading to a chemical reaction occurs not only in molecules surrounding an ion, but in the remaining organic moiety of an ion as well. Thus, a relatively stable ion, the tert-butyl cation 26, readily loses a proton in the presence of even weak bases to form isobutene 27 (Scheme 2.14). In the course of this elimination reaction the electron pair of the C-H bond is completely shifted to the positive charge to form a carbon-carbon double bond. The shift is accompanied by a complete dissociation of the polarized C-H bond with the net loss of a proton. 

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