[furaniki18]

why don't aldoximes show beckmann rearrangement? usually we say that migratory aptitude of hydride is greater than that of an alkyl group, so comparing with the reaction mechanism of beckmann rearrangement aldoximes should be more reactive towards such a reaction. then where am i going wrong?

[Dan]

why don't aldoximes show beckmann rearrangement?

They basically do, some examples:

http://onlinelibrary.wiley.com/doi/10.1002/recl.19760950511/abstract
http://www.sciencedirect.com/science/article/pii/S1387181104004044
http://pubs.acs.org/doi/abs/10.1021/jo902461a

However, I would call this a dehydration/rehydration. You basically eliminate water across the C=N bond to form a nitrile. This can then be hydrated in the presence of water (to form an amide). If you heat benzaldehyde oxime in formic acid, for example, you will get benzonitrile. Personally, I wouldn't refer to these as Beckmann rearrangements - but I've not read the papers, so maybe they have mechanistic evidence for the H migrating to the N (as opposed to being abstracted bimolecularly).