Question

The number of aldol reaction(s) that occur in the given transformation is:

• A. 1
• B. 2
• C. 3
• D. 4

Answer 1

The correct option is C

3

First and foremost, formaldehyde cannot be enolated. So it can act only as an electrophile and be attacked by the enolated acetaldehyde. The rest is all unimportant. In this case, clearly, there can only be three aldol reactions because there are only 3 alpha hydrogens in acetaldehyde.

But why are there 4 molecules of HCHO? Therein lies the trick! Once three aldols have taken place, there are no alpha hydrogen atoms left!

But this makes a compelling case for crossed Cannizaro (using the fourth HCHO molecule). In crossed cannizaro, HCHO is always oxidized to formate.