Effects of bond polarity

Large nucleophiles (such as phosphorus) often prefer the equally large (and sterically accessible) end of the C-X bond for X=C1 or Br. For example, attack of PPh at the halogen atoms of CCl and CBr initiates, respectively, the Appel and the Corey-Fuchs reaction.  

Halophilic reactions are also possible for other bond types. For example, they are quite common for N-Cl bonds since electronegativity of N and Cl is not drastically different but Cl is less sterically encumbered. As the result, either nitrogen or halogen can be transferred to the nucleophile in a R NH-Cl/Nu reaction. 

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Polar reactions occur because of the electrical attraction between positive and negative centers on functional groups in molecules. To see how these reactions take place, let s first recall the discussion of polar covalent bonds in Section 2.1 and then look more deeply into the effects of bond polarity on organic molecules. 

Before discussing the effect of bond polarity on bond lengths it will be convenient to introduce the concept of an ionic radius. 

The effect of bond polarity on the regiochemistry of insertion may be involved in the case of the Rh complex of equation (53) which adds to acrylonitrile as if it were Rh+—H, in contrast with the Ir complex of equation (54) which appears to add as Ir- —H+. 23 Relatively little is known about the pXa values of metal hydrides.124 In addition to the reactivity properties mentioned above, the M—H bond can also tend towards M+—H or M —H+ character in its ground state structure. This seems to have an influence on spectroscopic (Section 19.4.3), physical (Section 19.4.7) and structural (Section 19.4.2) properties. 

The effect of bond polarity is even more apparent in the 1,2-dichloroethenes, with their strongly polar carbon-chlorine bonds. The cis isomer has a large dipole moment (2.4 D), giving it a boiling point 12 degrees higher than the trans isomer, with zero dipole moment. 

Similar local anisotropic effects can mask the effect of bond polarization in diaryl acetylenes (tolanes). A number of ortho- and para-substituted tolanes have been examined to test the effect of alkyne polarization on the chemical shifts. " While bond polarization controls the relative chemical shifts in p-tolanes (more positive carbon displays a downfield shift), the trends in o-tolanes are complicated by the close proximity of the substiment to the alkyne (Figure 12.31). Local anisotropic contributions of orffto-substituents caused the alpha-afkyne carbon to display an upfield shift despite it-effects on alkyne polarization. 

The Combined Effect of Bond Polarity and Bond Strength We can see the combined effect of bond polarity and bond strength by examining the trends in acidity of the gronp 6A and 7A hydrides as shown in Figure 15.14 . The hydrides become more acidic from left to right as the H—Y bond becomes more polar. The hydrides also become more acidic from top to bottom as the H—Y bond becomes weaker. 

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