Why can we not compare the coupling constants to chemically equivalent (but not identical) protons to determine magnetic equivalence? I have been learning about magnetic inequivalence today, and I find it quite hard to undersand. See, for example:



In this case, according to the book I am using, $\ce{H_\mathrm{a}}$ and $\ce{H_\mathrm{b}}$ may be chemical equivalents, but they are not magnetic equivalents, due to the fact that $J_\mathrm{{\ce{a-c}}}$ and $J_\mathrm{{\ce{b-c}}}$ are different. Which I understand, but should we not consider that there is a $J_\mathrm{{\ce{b-d}}}$ which is equal to $J_\mathrm{{\ce{a-c}}}$?
I do not understand why they are magnetically different, but both have to chemical equivalent protons, one in ortho and one in para position.
The problem is that from the perspective of $\ce{H_{d}}$ and $\ce{H_{c}}$, $\ce{H_{a}}$ and $\ce{H_{b}}$ are different (you can tell which is which by their coupling constants). This difference means that there is a magnetic inequivalence between the two hydrogens $\ce{H_{a}}$ and $\ce{H_{b}}$, so there will be coupling between them.