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A study of the protonation of carbonyl compounds Yates, Keith

Abstract

The protonation of very weak organic bases of the carbonyl type has been studied in order to determine both their absolute and relative magnitudes of basicity. These results have been used to establish the preferred modes of protonation in those cases where alternative sites of attachment of the proton are possible. The basicities of twenty m- and p-substituted acetophenones have been determined by a spectrophotometric method in sulphuric acid media. The simple Hammett relation does not hold for the ionization of these bases but a very good correlation exists between pK(BH+) and σ⁺. The basicities of seven m- and p-substituted benzaldehydes have been determined in the same way. The ionization of these bases exhibits a similar modified linear free energy relationship, and the results are compared with those for acetophenones. The basicity of o-tolualdehyde was determined in an attempt to estimate the steric effects of ortho substitution. The effects of symmetrical trisubstitution on the basicity of benzaldehyde and acetophenone are compared and discussed. The basicities of twenty-three similarly substituted benzoic acids and ten substituted benzamides have also been determined and the linear free energy relationships for these ionizations examined with respect to the model carbonyl compounds discussed above. The similarity of behaviour of the benzoic acids to that of the aldehydes and ketones indicates that protonation at the carbonyl oxygen is the preferred mode. On the other hand the ionization of benzamides follows a conventional linear free energy relationship, indicating absence of direct resonance interactions in the conjugate acids and thus that protonation on nitrogen is the preferred mode. The order of basicity of the various types of basic group has been established and the significance of the p-values for these ionizations is discussed. Correlations between observed ultraviolet spectral displacements produced by substitution in the various bases and conjugate acids provide supporting evidence for the preferred modes of protonation described above. A good correlation is found between pK(BH+) and the carbonyl stretching frequencies in the ketones but it appears that a similar correlation in the benzoic acid series cannot be used as an effective criterion of protonation site. The infrared spectra of amides and amide salts have been investigated and the changes in spectra produced on protonation support an N-protonated form for their conjugate acids. Other aspects of the problem of protonation of benzoic acids and benzamides including available n.m.r. evidence have been examined and discussed.

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