I learned that, by using NMR, you could tell how many different hydrogen environments an molecule has. For example, benzene has only one.
Since acetaminophen has an amide bond, and the bond has planarity, I thought the phenyl group of acetaminophen has four different hydrogen environments.
But the textbook said only two. What is wrong with my thinking?
The 'amide bond being planar' refers to restricted rotation about the C(O)–N bond, not the N-aryl bond.
The restricted rotation about the C(O)–N bond does lead to inequivalence in NMR, depending on the compound: a good example of this is N,N-dimethylformamide, where the two methyl groups have different chemical shifts.*
However, in your compound, this restricted rotation is irrelevant. It is rotation about the N–aryl bond which makes the pair of ring protons H-2 and H-6 chemically equivalent (and H-3 and H-5).†
* At room temperature, at least. If you heat it up enough the two peaks coalesce because there is enough energy to overcome the barrier to rotation.
† They are not magnetically equivalent, see e.g. NMR magnetically equivalent protons for a 1,4-disubstituted benzene ring, although that is a separate matter from what you're asking about.
