The 1H NMR spectrum of ethylbenzene:

- Ethylbenzene is a compound with the formula C8H10. It consists of a benzene ring with an ethyl group attached to it.
- In the 1H NMR spectrum of ethylbenzene, the benzylic proton refers to the hydrogen atom directly attached to the carbon atom adjacent to the benzene ring.
- The benzylic proton appears as a quartet in the 1H NMR spectrum of ethylbenzene.

Explanation:

The appearance of the benzylic proton as a quartet in the 1H NMR spectrum of ethylbenzene can be explained by the concept of spin-spin coupling.

- Spin-spin coupling occurs when the magnetic field generated by one proton affects the resonance frequency of another nearby proton.
- In the case of the benzylic proton, it is coupled to the three neighboring protons on the benzene ring.
- The three neighboring protons on the benzene ring are equivalent, and they have a spin state that can be either aligned with or against the external magnetic field.
- When the benzylic proton is aligned with the magnetic field, it experiences a slightly higher resonance frequency due to the magnetic field generated by the neighboring protons.
- Conversely, when the benzylic proton is aligned against the magnetic field, it experiences a slightly lower resonance frequency due to the magnetic field generated by the neighboring protons.
- As a result, the benzylic proton splits into a quartet in the 1H NMR spectrum.
- The quartet pattern arises from the splitting of the benzylic proton signal into four peaks, with relative intensities of 1:3:3:1, due to the three equivalent neighboring protons.

Conclusion:

In the 1H NMR spectrum of ethylbenzene, the benzylic proton appears as a quartet due to spin-spin coupling with the three equivalent neighboring protons on the benzene ring. This quartet pattern arises from the splitting of the benzylic proton signal into four peaks, with relative intensities of 1:3:3:1.