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Stereoelectronic effects on the basicities of carbonyl compounds Granger, Maurice Roy

Abstract

The basicities of fifteen ortho-substituted benzoic acids and five benzophenone analogues have been measured with the aim of studying the effects of stereoelectronic factors on the strengths of very weak bases. The ortho effects of the substituents on the basicities of the benzoic acids have been estimated by two methods. The first method compares the pKβH⁺ values of the corresponding ortho- and para-substituted acids and uses the difference between them as a measure of the ortho effect on the assumption that the electrical effects of substituents are equal in the ortho and para positions. The second method is that of McDaniel and Brown (10). The ortho effects of alkyl substituents are base weakening and increase in the order methyl < ethyl < i-propyl < t-butyl. These effects are attributed to the bulk effect of the substituents, and in particular, to steric inhibition of resonance in the conjugate acid. The halogen substituents, on the other hand, appear to have base strengthening ortho effects in the order F > Cl > Br > I ≃ 0, which are attributed to stabilization of the protonated acid by intra-molecular hydrogen bonding. In a similar manner, oxygen containing substituents such as methoxyl and carboxyl are base strengthening. The basicities of these benzoic acids do not follow the linear relationships of either Taft (21) or Farthing and Nam (22) which have been developed to correlate the reactions of ortho-substituted benzene derivatives, a fact which serves to emphasize the high dependence of ortho effects upon the nature of the reaction. The basicities of benzophenone analogues in which the phenyl rings are rigidly held have also been studied. The basicities were found to be in the order 9-anthrone > 2,3;6,7-dibenzotropone > 2,3;6,7-dibenzosuberone > benzophenone > 9-fluorenone. The high basicity of anthrone is attributed to the planarity of the molecule which allows a high degree of resonance in both the unprotonated and protonated species. The low basicity of fluorenone is attributed to its similarity to cyclopentadienone in which the tendency of the carbonyl group to polarize in the usual direction, and so increase the electron density on oxygen, is greatly reduced through an effort to aromatize the ring.

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