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4.19 a) When an H is removed from the CH₂ group of the compound on the right, the electron pair of the conjugate base is stabilized by resonance with both the C=O and the C=N. Therefore this compound is the stronger acid. O: :O: b) The most acidic H in both compounds is the one bonded to the O. The compound on the left is a stronger acid because its conjugate base is stabilized by the electron withdrawing effect of the group. More importantly, the CH₃C=O group also stabilizes the conjugate base by resonance. c) This is similar to problem 4.18. The para isomer (left) is more acidic because the electron-withdrawing group is more effective in stabilizing the conjugate base as a result of additional resonance stabilization (see the last resonance structure of problem 4.19 b). The other isomer (right) has no resonance structure with the electrons delocalized onto the group. d) The H on the is the most acidic because the conjugate base formed by removal of this H is stabilized by resonance. : 53