While I was going through my course material of Aldehydes and Ketones, I came across Cannizzaro Reaction. It is of 3 types, namely: Self Cannizzaro, Cross Cannizzaro and Intramolecular Cannizzaro. I understood the Self one but was unable to understand the other two. In the Cross one,why is always Formaldehyde (HCHO) used and reduced? And in Intramolecular one, how do we know which part of the dicarbonyl group will get reduced and which will get oxidised? Or is Intramolecular Cannizzaro meant for only a few dicarbonyl compunds like: Phenyl glyoxal, Pthaladehyde, Glyoxal? Because I saw only these with which Intramolecular Cannizzaro was done and shown. What about other dicarbonyl compounds?