Question

a. Which proton or set of protons in each of the following compounds is the least shielded?

b. Which proton or set of protons in each compound is the most shielded?

Short answer

Compound 1

Compound 2

Compound 3

Step-by-step solution

Step-by-Step Solution Step 1: Origin of signals in proton NMR

The nuclei of the protons have a magnetic moment as it spins about their axis. When an external magnetic field is applied to it, the randomly aligned magnetic moments align in the direction or opposite the applied magnetic field.

Therefore, two energy levels are formed with nuclei having magnetic moment aligned with the applied field having lower energy.

When radio waves are applied to this system, the nuclei can shift from one energy level. The frequency of radio waves absorbed by the nucleus depends on its chemical environment.

Shielding and de-shielding effect

When the magnetic field is applied, the electron density in the protons spins perpendicular to the applied field. This reduces the effective magnetic field that reaches the nucleus.

Therefore, protons with high electron density experience a lower magnetic field called shielded protons. They have low chemical shift values. Similarly, protons will lower electron densities are called de-shielded and show high chemical shift values.

Shielded and de-shielded protons of given compounds

Compound 1

(a) The chloride group in the compound is an electron-withdrawing group. Therefore, the protons closest to the group will have the lowest electron density. Therefore, the least shielded protons will be the protons closest to the chloride group.

(b) The inductive effect decreases with an increase in the number of carbon atoms. Therefore, the protons farthest from the electron-withdrawing group will have the highest electron density. Therefore, the most shielded protons will be farthest from the chloride group.

Compound 1

Compound 2

(a) The lowest electron density will be for the protons of the carbon that are directly attached to an electron-withdrawing oxygen atom. Therefore, the protons closest to the oxygen atom will be the least shielded.

(b) The protons farthest from the ester group will have the highest electron density and, therefore, be most shielded.

Compound 2

Compound 3

(a) The protons attached to the carbon containing two electron-withdrawing bromine groups will have the lowest electron density and be the least shielded.

(b) The protons attached to the carbon having no electron-withdrawing groups will have maximum electron density and, therefore, be most shielded.

Compound 3