They react with hydroxylamine and phenylhydrazine, with the formation of aldoximes and hydrazones.
The hydrazones are crystalline substances which are of value in the characterization of the aldehydes.
Both oximes and hydrazones, on boiling with dilute acid, regenerate the parent aldehyde.
The hydrazones are best prepared by mixing the aldehyde with phenylhydrazine in dilute acetic acid solution, in the absence of any free mineral acid.
Cane sugar has no reducing power and does not form an hydrazone or osazone; the other varieties, however, reduce Fehling's solution and form hydrazones and osazones, behaving as aldoses, i.e.