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In most academic courses you'd learn that aldehydes are more acidic (lower $\text{p}K_\text{a}$) than ketones due to the lower electron donating effect of the proton compared to the alkyl group of the ketone.

However, when talking about condensations, for example, it's not a weird choice to form a ketone enol over an aldehyde enol on the basis of "thermodynamic stability due to a more substituted alkene under thermodynamic control". Yet, $\text{p}K_\text{a}$ is the measurement of such stability, and thermodynamically the aldehyde enol is more stable. Adding to this, a following argument is that "aldehydes react much quicker towards nucleophilic attack than ketones". I agree with this, but weren't we just discussing thermodynamic control?

The questions are then:

  1. Is it valid to use the "more substituted alkene" to argue a step on a mechanism under claims of thermodynamic control?
  2. Can this type of argument be coupled with a kinetic one like the example provided as well? I.e. is it valid to mix thermodynamic and kinetic arguments one in the same? (e.g. this intermediate forms because it's more thermodynamically favored, and the next one forms because it is kinetically favored).

(I realize question 2 may be broader than this, but I'm trying to keep this in the sense of mixed aldol condensations)

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  • $\begingroup$ I am probably wrong, but does the word "condensation" refer to simply aldol condensations (self and cross) or some other reactions as well? $\endgroup$ – Gaurang Tandon Mar 22 '18 at 8:21
  • $\begingroup$ Since the whole post is about aldehydes, ketones and their formation of enolates, I think it is safe to assume I'm talking about aldol condensations. $\endgroup$ – ralk912 Mar 22 '18 at 8:27

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