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322               anions. In a comparison of the acidity of acetic acid (pK 4.7) with that of alcohols (pK
                       16–18), the issue is the relative importance of the C=O bond dipole and resonance
     CHAPTER 3
                       in stabilizing the carboxylate anion. These approaches have led to estimates ranging
     Structural Effects on  from 50 to 80% of the stabilization being polar in origin.
     Stability and Reactivity
                                                 O                     ---  O –0.5
                                         CH 3  C        and     CH 3  C ---
                                                O –                    O –0.5

                                       polar (electrostatic)      resonance
                           Rablen 110  investigated carbonyl substituent effects by using a series of isodesmic
                       reactions designed to separate resonance and polar effects. He found the oxygen
                       contributions to be about 6 kcal/mol, as opposed to about 14 kcal/mol for nitrogen. The
                       order of the polar effect is F > O > N > C, whereas that for the resonance effect is N >
                       O > F. His analysis suggests that the stabilization in acetate (O donor) is about one-
                                                                          −
                       third resonance and two-thirds electrostatic. On the other hand, in amides and amidines,
                       the order is reversed, roughly two-thirds resonance and one-third electrostatic.
                                                                           +
                                         O                    O            NH 2

                                         C                    C            C
                                      R    O –              R   NH 2     R   NH 2
                                   electrostatic effects    resonance effects
                                   are dominant             are dominant


                           A physical property that reflects the electronic character of the carbonyl
                                     17
                       substituent is the O NMR chemical shift. Although the relation of the chemical shift
                       to electronic properties is complex, there is a correlation with the resonance electron-
                       donating ability of the substituent. The  -donor substituents cause large downfield
                       shifts in the order Cl 
 F < OCH < NH . Nonconjugating substituents, such as cyano,
                                                  3     2
                       trifluoromethyl, and methyl, have much smaller effects (Table 3.23).

                                               Table 3.23.  17 O Chemical
                                                  Shifts for CH COX a
                                                            3
                                                X                 17 O
                                               CN                 603 b
                                                                  592
                                               CF 3
                                               H                  592
                                                                  571
                                               CH 3
                                               Cl                 502
                                               F                  374
                                               CH 3 O             361
                                                                  313
                                               NH 2
                                               a. H. Dahm and P. A. Carrupt, Magn.
                                                Reson. Chem., 35, 577 (1997).
                                               b. J.-C. Zhuo, Molecules, 4, 320
                                                (1999).

                       110
                          P. R. Rablen, J. Am. Chem. Soc., 122, 357 (2000).
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