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Predict chemical shift, integration, and splitting patterns of the molecule below. Write a table...

Question:

Predict chemical shift, integration, and splitting patterns of the molecule below. Write a table to compile data. Then draw the predicted {eq}^{1} {/eq}H spectrum. Label the peaks in the spectrum, the table, and the hydrogens on the molecule with a, b, c, etc. The peak labeled "a" should be farthest up field (closest to zero).

The Effect of Electronegativity on Chemical Shift:

Chemical shifts closer to 0 are referred to as upfield while those further away from 0 are referred to as downfield. While there are several factors that play into the shift of a particular proton in {eq}^{1} {/eq}H NMR, a critical factor is proximity to atoms with a high electronegativity. Protons that are more upfield are more shielded from the magnetic field of the instrument by local electrons. This shielding by local electrons is compromised when there are nearby atoms with high electronegativities because, by definition, those atoms will pull electron density towards them. As the electron density flows away from the proton in question, it becomes less shielded from the magnetic field from the instrument and the energy required to achieve resonance goes up (and the chemical shift goes downfield).

Answer and Explanation:

The table below lists the predicted signals alongside the other desired information:

{eq}\begin{array}{|C|C|C|C|C|C|C|} \hline Signal & \delta & Integration & Splitting \\ \hline a & 1.1 & 3 & t\\ \hline b & 2.4 & 2 & q\\ \hline c & 4.3 & 2 & s\\ \hline \end{array} {/eq}

The molecule, seen below, has been labeled according to the above table:

Signal (a) was certainly the most shielded as it is the furthest away from all three electron-withdrawing groups (the carbonyl oxygen, bridging oxygen, and the chlorine atom). Shifts for a terminal methyl reasonably range anywhere from 0.5ppm to 1.8ppm; because there are several EWGs present, even at a distance, the predicted signal is shifted downfield. As there are 2 adjacent protons, this signal would be split as a triplet.

Signal (b) has proximity to two EWGs; alpha-carbon protons are typically within the 2-3ppm region so an assignment of 2.4 shouldn't be unreasonable. As there are 3 adjacent protons, this signal is split as a quartet.

Signal (c) is certainly the most downfield as it has direct adjacency to two EWGs and proximity to a third. Protons with adjacency to an ester are usually within the 3.5-4.5ppm range so the proximity to the chlorine atom would most likely cause this signal to be shifted further downfield. As there are no nearby protons, this signal would show as a singlet.

A mock spectrum has been produced below in accordance with the above predictions:


Learn more about this topic:

How to Read NMR Spectra of Organic Compounds
How to Read NMR Spectra of Organic Compounds

from Organic Chemistry: Help & Review

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