[big]

Why would the protons in para-substituted benzene be a doublet of doublets? I can see how the vicinal proton to each of the 4 protons on the benzene would be a doublet, but I don't see where the extra doublet, making it a doublet of doublets instead of just a doublet, comes from.

Thanks so much!

[Nosterius]

It is because these aromatic protons are chemically equivalent, but not magnetically equivalent.

Do not forget that proton can couple farther than three bonds. For a proton " A " in your system, it would couple with the ortho proton " B ", but also with proton in para " B' ", although with a coupling constant much smaller. Common NMR spectrometer are now sufficiently powerful to allow seeing such coupling patterns... Otherwise, use an older machine and you will only see broadened doublet!