Final answer:
In 1-ethyl-3-methylbenzene, five different types of protons should be observed: the unique para proton, the ortho protons, the meta protons, the protons in the methyl group, and the protons in the ethyl group.
Step-by-step explanation:
To determine the number of different types of protons in 1-ethyl-3-methylbenzene, consider the symmetry and the chemical environment of each hydrogen atom in the molecule. On the benzene ring, the placement of the ethyl and methyl groups at positions 1 and 3 respectively creates a meta-disubstituted benzene ring, leading to distinct sets of chemically non-equivalent protons.
The protons on the ethyl group are split into two types: those on the CH2 group and those on the CH3 group. In the methyl group, all three protons are chemically equivalent. Due to the symmetry of the benzene ring, the protons in the ortho positions to the substituents (positions 2 and 6) are chemically equivalent, and the protons in the meta positions (positions 4 and 5) are also equivalent. The proton at the para position (position 1) is unique due to being adjacent to the ethyl substituent.
Thus, in 1-ethyl-3-methylbenzene, you should observe five different types of protons: the unique para proton, the two equivalent ortho protons, the two equivalent meta protons, the protons in the CH3 group, and the two types within the CH2CH3 (ethyl) group.