Question

The following compounds all show a single line in their 1H NMR spectra. List them as they would appear in an NMR spectrum, e.g. by decreasing chemical shift with the lowest shift to the right. These compounds would appear in a spectrum in the order.

a. Benzene
b. CH4
c. Cyclohexane

These compounds would appear in a spectrum in the order: _

Answer 1

Benzene < Cyclohexane < CH4

Step-by-step explanation:

The location of a peak in an NMR spectrum is determined by chemical shift. In 1H NMR, a proton that is strongly shielded, It implies a proton that has a high electron density, is exposed to a low magnetic field, which causes the chemical shift to diminish, resulting in a signal towards the right direction of the spectrum.

However, a de-shielded proton with its electron density reduced is exposed to a high magnetic field, which causes and resulted in a chemical shift and causes it to shift to the left of the spectrum.

Electronegativity is one of the factors that influence the electron density because it decreases the proton's electron density attached to it as well as the ones attached to nearby atoms due to the inductive effect. The impact of an electronegative atom diminishes fast as the distance between them grows.

We can utilize the impact of electronegativity to calculate how much chemical shift will take place in each molecule in this question.

The chemical shift for benzene is = 7.26 ppm

Chemical shift for cyclohexane is = 1.44 ppm

Chemical shift for methane = 0.23 ppm

Thus, in decreasing order, the chemical shift with the lowest shift from left to right is:

Benzene < Cyclohexane < CH4