Question

Which 1H NMR peaks will be key for discriminating (E)-stilbene from mesostilbene dibromide?

Answer 1

For both (E)-stilbene and mesostilbene dibromide, you're going to have 12 hydrogens. The difference in structure between the two is the absence/presence of two bromine atoms in the ethene bridging the two benzene molecules, although both still have two hydrogen atoms, one for each carbon atom, in the "bridge". You're going to only have 6 peaks for both, since it is symmetrical. However, in the (E)-stilbene, you're going to have a peak with higher shift around 5 ppm, and a peak anywhere from around 3-4 ppm for the mesostilbene dibromide, since the hydrogen atoms in the "bridge" are much more shielded in the molecule with the halogen atoms (halogens are rather large, especially compared to the double bond present in the first molecule), leading to a much smaller shift. (Remember, the more "shielded" an atom, the less of a shift it shows)
Everything else should pretty much be around the same. You will get some shift, of course, but the most significant ones will be those.