[MasterChief]

Does anyone know if the DNP stain will also work on an acetal group?
It stains the ketone well, but I'm not sure if it will stain the corresponding acetal/ketal

[fledarmus]

Not usually. Acetals and ketals are used as protecting groups precisely to prevent this type of reaction from occurring.

However, silica gel plates tend to be slightly acidic and not all acetals and ketals are particularly stable on them. You may see some decomposition of the acetal back to the aldehyde on the silica gel which can react with the DNP. The longer you let your plate sit or the more you heat it, the more likely it is that you will see some deprotection occurring on your plate.

There are some acetals and ketals which are so easily hydrolyzed that stirring them in silica gel in an appropriate solvent is enough to deprotect them.