gardenvariety
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understanding Cannizzaro
When are Cannizzaro reactions expected and when are they not? I'm having a devil of a time understanding this one.
Most generally, Cannizzaro rxns are on aldehydes with no alpha hydrogen, turning aldehydes into 50% alcohol and 50% carboxylic acid in the presence of
a strong aqueous base. The classical example is benzaldehyde, disproportionating to benzyl alcohol and benzoic acid.
My confusion is this: most of the syntheses I've read on substituted benzaldehydes involve multiple reactions in strong base, eg, alkylation, ring
substitution, Ullman condensations, etc. These published reactions are often aqueous, at high temps, and/or have long reaction times, and frequently
report yields in the range of 80-90%+. What gives?
Are phenolic benzaldehydes resistant to Cannizzaro?
Am I missing something obvious here?
Any help would be greatly appreciated.
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Arrhenius
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Synthesis is always a matter of relative rates. One can always envision multiple reaction pathways given a certain combination of reagents, but
ultimately one will win out - be it for thermodynamic or kinetic reasons. From personal observation, benzaldehyde will disproportionate to benzoic
acid and benzyl alcohol as you said, but this process is exceptionally slow. Conversely, anisaldehyde is quite stable. Considering the mechanism of the Cannizzaro reaction it shouldn't be suprising that electronics play a significant factor in substrate reactivity. I've only hear
the Cannizzro discussed with benzaldehydes, and this is probably an electronic issue. Hydroxide is sufficiently nucleophilic to add reversibly to
probably any benzaldehyde, but the process of eliminating the benzylic hydrogen as a hydride ion is going to be highly dependent on the electronic
character of the system. Of course, one can force a reaction to take place, but in general, electron rich benzaldehydes such as anisaldehyde or
hydroxybenzaldehydes do not undergo facile disproportionation.
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gardenvariety
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Many thanks!
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