NMR Explanation for Proton Spectra of Benzene Ring

[uhstudent1138]

Hi everybody,

Homework Statement

I am working on the spectra from the following link:
http://www.chem.ucla.edu/cgi-bin/webspectra.cgi?Problem=bp22. Relevant factors and first attempt

For the most part, I understand how the spectra available contribute to the answer, but the left (upfield) side of the proton NMR is proving confusing. I see how the methyl group around 4ppm and the lone aldehyde hydrogen around 10.5 ppm produce their respective patterns, but the remaining 4 hydrogens on the left side of the benzene ring mystify me. To be more specific in my confusion:

3. The confusion

1. Why does the 7.0 ppm shift have a multiplicity of two? Given that each hydrogen is a different distance from the either and aldehyde, shouldn't each hydrogen have a different peak?

2. What could account for the two-multiplicty 4-peak split between 6.9-7.0 ppm? Since this is a benzene ring, each hydrogen should be adjacent to a maximum of two hydrogens, no? One on each carbon on each side of the bond?

I understand the other two shifts within the 7-8 ppm range, but not how they might fit in with the one above. What am I not seeing?

Thanks for your help.

 

[nate808]

Thank you for your question regarding the spectra from the provided link. I would like to offer some insights and explanations to help clarify your confusion.

Firstly, to address your question about the 7.0 ppm shift having a multiplicity of two, this can be attributed to the concept of coupling constants. Each hydrogen in a molecule can interact with its neighboring hydrogens through a process called spin-spin coupling. This interaction leads to the splitting of peaks in the NMR spectrum, resulting in a multiplicity of two. The distance between the peaks, known as the coupling constant, is determined by the number of neighboring hydrogens and their relative positions. Therefore, even though the hydrogens in the 7.0 ppm shift may be at different distances from the aldehyde, their coupling constants are similar, resulting in a doublet peak.

Regarding the two-multiplicity 4-peak split between 6.9-7.0 ppm, this can be explained by the concept of symmetry. In a benzene ring, the hydrogens are equivalent and therefore, the splitting pattern should be the same for all the hydrogens. However, due to the symmetry of the molecule, certain hydrogens may experience a slightly different environment, resulting in a slight difference in their coupling constants. This leads to the splitting pattern you observe in the 6.9-7.0 ppm range.

I hope this explanation helps to clarify your confusion. If you have any further questions or would like more information, please do not hesitate to ask. Science is all about curiosity and questioning, and I am happy to assist in any way I can.