In the aldol/cross aldol reaction, of $α,β$-unsaturated carbonyl compound, under what circumstances does a 1,2-addition i.e. aldol condensation (carbanion formed directly attacking carbon of carbonyl group) and 1,4-addition i.e. Michael condensation (carbanion formed attacking $β$ carbon)?

These are the following points I considered:

  1. Stability of resulting product.

  2. The amount of conjugation in the compound. What I feel, extended conjugation ($γ,δ$-unsaturated double bond as well) would always lead to 1,4-addition.

  3. The group attached to the $β$ carbon. For example, an electronic withdrawing group would increase the probability of 1,4-addition and an electronic donating group would reduce the probability, favoring a 1,2-addition.

  4. The steric crowding around the carbonyl or the unsaturated carbon system.

Are these arguments valid?

aldol/cross aldol reaction, of α,β-unsaturated carbonyl compounds

I have added a few examples (I have just written the final product and not the mechanism as my question lies in the very step after generating carbanion). In examples 1, 2, and 4, aldol condensation (1,2) has taken place but in example 3, Michael condensation (1,4) has taken place.

Is there a way we can account for this?

  • 4
    $\begingroup$ Technical note: reactions 1, 2, 4 can be written with MathJax; reaction scheme 3 can be drawn with MarvinSketch (free, GUI, see licensing) or chemfig (free, $\mathrm\LaTeX$ package). $\endgroup$ – andselisk May 11 at 18:19
  • 2
    $\begingroup$ As for using a photo on ChemSE: if you do this as a resort (vide infra), the upload function here allows you to see how it will look like in your post -- have an eye on the dimension (as you see, it need not be huge) and orientation (now easier to read...). Better, though, to use a sketcher to export reaction schemes in formats accepted here. Invest some time to learn one sufficiently well, which will be helpful in your studies anyway (not only ChemSE). In addition to andselisk's comment, have a look at en.wikipedia.org/wiki/Molecule_editor and gunda.hu/dprogs. $\endgroup$ – Buttonwood May 11 at 18:24

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