Final answer:
The symmetrical structure of ethylbenzene, which includes both an ethyl group and a benzene ring, leads to four different NMR signals: one for the para hydrogens, one for the combined ortho and meta hydrogens, one for the methylene protons, and one for the methyl protons.
Step-by-step explanation:
When considering the number of different signals in the NMR spectrum of ethylbenzene, we have to account for symmetry and the unique environments of the hydrogen atoms. Ethylbenzene has a benzene ring with an ethyl group attached to it. Due to the symmetry of the benzene ring, there are fewer unique hydrogen environments than there are hydrogen atoms. The protons on the benzene ring can be divided into two groups due to symmetry: those on the ortho and meta positions relative to the ethyl group, and those on the para position. The ethyl group itself also provides two different types of environments for protons: the methylene (CH2) group and the methyl (CH3) group.
Taking these symmetries into account, we should expect to see a total of four different signals on the NMR spectrum for ethylbenzene: one for the para hydrogen atoms on the benzene ring, one combined signal for the ortho and meta hydrogen atoms on the benzene ring due to their symmetrical relationship, one for the methylene (CH2) protons, and one for the methyl (CH3) protons. This understanding helps to analyze the chemical shift and the NMR spectrum effectively.