Question

Use the ${}^{\mathbf{1}}\mathbf{H}$NMR and IR spectra given below to identify the structures of two isomers (A and B) having molecular formula ${\mathbf{C}}_{\mathbf{4}}{\mathbf{H}}_{\mathbf{8}}{\mathbf{O}}_{\mathbf{2}}$.

Short answer

a. Based on the observations from the given data, the structure of the compound is as follows:

4-hydroxy-2-butanone

b. As per the given data, the structure of the compound is as follows:

2-methyl propanoic acid

Step-by-step solution

Indentification of the compounds using IR and H1 NMR spectra

IR spectroscopy works on the principle of the interaction of the infrared radiation with matter by the process of absorption, emission, or reflection.

It is used to identify the functional groups within a molecule. It can also determine the amount of the sample. The NMR spectroscopy determines the chemical structure of the sample.

Identification of the strucutures of the isomers of A

a. You can identify the compound A by the following steps:

• The molecular formula ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_2$of the compound suggests one degree of unsaturation.
• The IR absorption at 3600-3200 $c{m}^{-1}$confirms the O-H functional group. The absorption between 3000-2800 $c{m}^{-1}$ indicates the C-H bonds, and the absorption at 1700 $c{m}^{-1}$confirms the carbonyl group.
• One can determine the connectivity of the alkyl groups from the NMR data. The presence of one singlet at 2.2 ppm confirms the presence of three hydrogen atoms, indicating the presence of a methyl group.
• Another singlet at 2.55 ppm confirms one hydrogen atom next to an OH group.
• The triplet confirms the presence of two hydrogen atoms adjacent to two other hydrogen atoms.
• The triplet at 3.9 suggests the presence of a triplet with two hydrogen atoms next to two hydrogens.

Based on the above observations from the given data, the structure of the compound is as follows:

4-hydroxy-2-butanone

Identification of the strucutures of the isomers of B

b. You can identify compound B by the following steps:

• The molecular formula ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_2$of the given compound suggests one degree of unsaturation.
• The IR absorption at 3500-2500 $c{m}^{-1}$indicates the presence of a hydroxyl group, while the absorptionat 1700 $c{m}^{-1}$ confirms the carbonyl group.
• One can confirm the connectivity of the different groups from the NMR data. The presence of a doublet at 1.6 ppm confirms the presence of six hydrogens adjacent to one hydrogen atom.
• According to the data, one septet present at 2.3 ppm confirms the presence of one hydrogen atom, which is adjacent to six hydrogen atoms. One very broad singlet at 10.7 ppm suggests the presence of one hydrogen of the OH group.

As per the given data, the structure of the compound is as follows:

2-methyl propanoic acid