Molecular Structure of Caffeine
At some point in studying organic chemistry, you might have been taught that a nitrogen in an amide group is planar sp2, due to delocalization of its electrons into the carbonyl group. You also might have learned that nitrogen atoms in a pyrrole or imidazole ring are planar sp2 since they participate in the 6 electron aromatic pi bonding system. You would then conclude that all the nitrogen atoms in caffeine are coplanar. But without that essential background information, you might not have been expected to figure that out.