In simple terms, as grammar confuses me too easily, I am wondering if they are of the same nature, if they can be used interchangeably, regardless of the wish to be formal.
To me they are not as the example shows:
the illness of which he died
the illness whereof he died
the illness whose he died
But in a more usual sentence, I am not good enough in english to be able pin down the reason why we can say:
my work, the purpose of which is ....
my work, whose the purpose is ....
my work, whereof the purpose is ....
When to use whereof and not whose, or whose and not whereof ?
Well whereof is also possessive, from OED,
II.II rel. Of which.
6.II.6 Of which, in objective senses (of X).
7.II.7 Of which or whom, in partitive sense (of XIII). Also with ellipsis of antecedent as obj. of a vb.: = some or something of which (of 45).
8.II.8 Of which, in possessive and related senses (of XIV).
Form this, it would be all right to say,
the man whereof the dog...
the man whose (the?) dog