Polarity C—F bonds

Figure 3.63 illustrates the gauche effect for vicinal lone pairs and polar C—F bonds with the examples of (a) hydrazine and (b) 1,2-difluoroethane, respectively. As seen in Fig. 3.63(a), the

lone pairs are anti to one another (thus squandering their powerful donor strength on vicinal moieties with no acceptor capacity) is disfavored by 3.2 kcal mol-1 relative to the preferred = 93.9° conformer in which each nN hyperconjugates effectively with... 

The fluorines are the most electronegative atoms present, and the bonds to them are polar covalent. The only nonpolar compound is the result of the polar C-F bonds pulling equally in opposite directions. 

Note carefully that this is an inductive effect there are no arrows to be drawn to show how fluorine withdraws electrons—It does it Just by polarizing C-F bonds towards itself. Contrast the electron-withdrawing effect of the nitro group, which works mainly by conjugation. 

FIGURE 12-10 (a) In methyl fluoride, the polar C—F bond is not canceled by the C—FI bonds, making the molecule polar, (b) In carbon tetrafluoride, the four polar C—F bonds cancel each other, making the molecule nonpolar. 

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