Question

For each set of IR and NMR spectra, determine the structure of the unknown compound. Explain how your proposed structure fits the spectra.

(a) C₃H₅NO

(b) C₅H₈O₂

Short answer

(a)

(b)

Step-by-step solution

a. About the given molecular formula

The given molecular formula bears one nitrogen, one oxygen, three carbons, and five hydrogens. The NMR spectrum of this compound has three major peaks, and IR also has some major peaks. This compound has two elements of unsaturation.

About the NMR spectrum

The chemical shift values are 5.8 ppm, 6.3 ppm, and 4.8 ppm. The chemical shift value of 5.8 ppm has a 1H multiplet of vinyl hydrogen present next to the carbonyl group.

The chemical shift value of 6.3 ppm has a 2H multiplet of vinyl hydrogen pair on the third carbon. The chemical shift value of 4.8 has a 2H multiplet of amide hydrogens.

About the IR spectrum

There are two peaks between which confirm the presence of the N-H bonds. There exists a strong peak at which is of C=O and is of C=C. Thus, the structure of the compound is given below.

About the NMR spectrum

The chemical shift values of carbon NMR are 171 ppm of carbonyl carbon and 69 ppm of C-O carbon. The chemical shift values of proton NMR are 4.3 ppm and 2.5 ppm. Both the chemical shift values are of the group.

About the IR spectrum

There is one small peak at which is of C=O overtone. There is one strong peak at , which is of carbonyl ester. The C-O bond has a peak between and there is no C=C. Thus, the structure of the compound is as below.