Factors Influencing Chemical Shift in 1H NMR Spectroscopy: Determining the Farthest Downfield Proton

which of the indicated protons in the following compound would appear farthest downfield in the 1H NMR spectrum given

In order to determine the proton that would appear farthest downfield in the 1H NMR spectrum, we need to consider the various factors that influence chemical shifts

In order to determine the proton that would appear farthest downfield in the 1H NMR spectrum, we need to consider the various factors that influence chemical shifts.

Chemical shift is a measure of the difference in resonance frequency of a nucleus compared to a reference compound, typically tetramethylsilane (TMS), which is assigned a chemical shift of 0 ppm. The chemical shift is affected by the electronic environment around the proton, including nearby electronegative atoms, functional groups, and ring currents.

In the given compound, we need to assess the electronic environment of each indicated proton to determine their relative positions in the spectrum:

– Proton A is adjacent to a carbonyl group (C=O). The carbonyl group is highly electronegative, withdrawing electron density from the neighboring protons. This deshielding effect causes proton A to experience a higher effective magnetic field and, consequently, a higher chemical shift value. Therefore, proton A is expected to appear farther downfield in the 1H NMR spectrum.

– Proton B is adjacent to a benzene ring. The aromatic ring has a ring current, which creates a local magnetic field that opposes the external magnetic field, shielding the protons. This shielding effect causes the chemical shift of proton B to be lower compared to a similar proton in an aliphatic compound. Hence, proton B would appear at a lower chemical shift, relatively upfield, in the 1H NMR spectrum.

– Proton C is in close proximity to a nitrogen atom and a carbonyl group. Both the nitrogen and carbonyl group have electron-withdrawing properties, which would deshield proton C and result in a higher chemical shift.

– Proton D is farther away from any electronegative atoms or functional groups, and it is in an aliphatic (non-aromatic) environment. In general, protons in aliphatic regions tend to appear at lower chemical shifts, relatively upfield.

Based on the analysis of the electronic environments of the indicated protons, we expect proton A to appear farthest downfield in the 1H NMR spectrum, followed by proton C. Protons B and D would appear at lower chemical shifts, relatively upfield, due to the effects mentioned above.

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