grammar - Possessive pronouns vs possessive determiners

If my understanding is correct, the possessive personal pronouns (which are mine, thine, yours, his, hers, its, ours, and theirs) are used in place of nouns, whereas the possessive determiners (which are my, thy, your, his, her, its, our, and their) are used as adjectives.

If this is the case, then why is example 1 below correct, as opposed to example 2?

1. Whose book is this? It is mine.


2. Whose book is this? It is *my.