Question

Each of the following compounds has a single NMR peak. Approximately where would you expect each compound to absorb?

Short answer

(a)The value of chemical shift is 0.6-1.3 ppm.

(b) The value of chemical shift is 2.0-2.4 ppm.

(c) The value of chemical shift is 6.5-8.0 ppm.

(d) The value of chemical shift is 9.7-10.0 ppm.

(e) The value of chemical shift is 5.0-5.5 ppm.

(f) The value of chemical shift is 2.1 ppm.

Step-by-step solution

Chemical shift

The area on the chart where a particular nucleus absorbs is termed the chemical shift. The chemical shift of TMS is considered as the zero point. The calibration of the NMR chart is performed using a scale termed the delta scale.

Converting the given frequency into δ units in option (a)

(a)In the given compound, the -CH₂protons absorb around 0.6-1.3 ppm as they are typical alkane protons. They do not have an electronegative atom near them to produce a higher chemical shift value.

(b)The -CO-CH₃ protons absorb around 2.0 to 2.4 ppm as they are carbon connected to a carbonyl group. The carbonyl group withdraws electron density from hydrogen causing deshielding.

(c)The aromatic protons absorb around 6.5 to 8.0 ppm. The hydrogen is connected to an carbon and the compound is aromatic making it to have a high value of 6.5 to 8.0 ppm.

(d)In the given compound, the aldehydic protons absorb around 9.7 to 10.00. The electron density on the hydrogen decreases due to the carbonyl group. So the value of chemical shift increases.

(e)The CH₂CI₂ protons absorb around 5.0 to 5.5 as they comprise carbon connected to two chlorine atoms. The chemical shift value increases due to deshielding by electronegative atom connected to the carbon atom.

(f)The (CH₃) absorption happens around 2.1 ppm. The nitrogen is more electronegative than carbon. So it withdraws electron density and electron density on hydrogen connected to carbon decreases leading to deshielding.