Question

Five proton NMR spectra are given here, together with molecular formulas. In each case, propose a structure that is consistent with the spectrum.

Short answer

1. The formula has one element of unsaturation. The singlet at chemical shift 12.1 indicates a carboxylic acid. The multiplet and the doublet indicates an isopropyl group. The proposed structure from the spectrum data will be,
   
2. The formula has five elements of unsaturation. The pattern between chemical shift 7.2 and 7.4 indicates monosubstituted benzene. The peak at chemical shift 9.85 is an aldehyde trying to be a triplet because it is weakly coupled to an adjacent methylene group. The two triplets at chemical shift 2.7-3.0 are adjacent methylene groups. The proposed structure will be,
   
3. The formula has two elements of unsaturation. A singlet at chemical shift 2.3 is probably a methyl next to carbonyl. The quartet and a triplet are certain to be ethyl with the methylene at chemical shift 2.7 and this also appears next to carbonyl. The proposed structure will be,
   
4. The formula has one element of unsaturation. Signals from chemical shift 5 to 6 indicates a vinyl pattern. The complex quartet for at chemical shift value 4.3 is a bonded to an alcohol next to methyl group. The hydroxyl appears as a singlet at chemical shift 2.5 and the methyl next to methine carbon is a doublet at chemical shift 1.3. We have following proposed structure,
   
5. The formula is saturated. The oxygen must be an alcohol or an ether and the broad peak at chemical shift 1.2 is probably a hydroxyl group. The expansion of multiplet at chemical shift 1.7 shows seven peaks, a septet which indicates an isopropyl group. Six of the nine protons in the pattern at chemical shift 0.9 must be doublet from two methyls from the isopropyl. The quartet at chemical shift 1.5 must be part of an ethyl pattern from which the methyl triplet must be the other of the pattern at chemical shift 0.9. This leaves only a singlet at chemical shift 1.1 which must be methyl with no neighbouring hydrogens. We have the following fragments,
   
   These fragments can be assembled in a way, that the proposed structure will be,
   

Step-by-step solution

Step-1. Explanation of part (a):

NMR spectroscopy plays a major role in the determination of the structures and dynamics of proteins and other biological macromolecules. Chemical shifts are the most readily and accurately measurable NMR parameters.

In part (a), The formula has one element of unsaturation. The singlet at chemical shift 12.1 indicates a carboxylic acid. The multiplet and the doublet indicates an isopropyl group. The proposed structure from the spectrum data will be,

Proposed structure from spectrum (a)

Step-2. Explanation of part (b):

NMR spectroscopy plays a major role in the determination of the structures and dynamics of proteins and other biological macromolecules. Chemical shifts are the most readily and accurately measurable NMR parameters.

In part (b), the formula has five elements of unsaturation. The pattern between chemical shift 7.2 and 7.4 indicates monosubstituted benzene. The peak at chemical shift 9.85 is an aldehyde trying to be a triplet because it is weakly coupled to an adjacent methylene group. The two triplets at chemical shift 2.7-3.0 are adjacent methylene groups. The proposed structure will be,

Proposed structure from spectrum (b)

Step-3. Explanation of part (c):

NMR spectroscopy plays a major role in the determination of the structures and dynamics of proteins and other biological macromolecules. Chemical shifts are the most readily and accurately measurable NMR parameters.

In part (c), the formula has two elements of unsaturation. A singlet at chemical shift 2.3 is probably a methyl next to carbonyl. The quartet and a triplet are certain to be ethyl with the methylene at chemical shift 2.7 and this also appears next to carbonyl. The proposed structure will be,

Proposed structure from spectrum (c)

Step-4. Explanation of part (d):

NMR spectroscopy plays a major role in the determination of the structures and dynamics of proteins and other biological macromolecules. Chemical shifts are the most readily and accurately measurable NMR parameters.

In part (d), the formula has one element of unsaturation. Signals from chemical shift 5 to 6 indicates a vinyl pattern. The complex quartet for at chemical shift value 4.3 is a bonded to an alcohol next to methyl group. The hydroxyl appears as a singlet at chemical shift 2.5 and the methyl next to methine carbon is a doublet at chemical shift 1.3. We have following proposed structure,

Proposed structure from spectrum (d)

Step-5. Explanation of part (e):

NMR spectroscopy plays a major role in the determination of the structures and dynamics of proteins and other biological macromolecules. Chemical shifts are the most readily and accurately measurable NMR parameters.

In part (e), the formula is saturated. The oxygen must be an alcohol or an ether and the broad peak at chemical shift 1.2 is probably a hydroxyl group. The expansion of multiplet at chemical shift 1.7 shows seven peaks, a septet which indicates an isopropyl group. Six of the nine protons in the pattern at chemical shift 0.9 must be doublet from two methyls from the isopropyl. The quartet at chemical shift 1.5 must be part of an ethyl pattern from which the methyl triplet must be the other of the pattern at chemical shift 0.9. This leaves only a singlet at chemical shift 1.1 which must be methyl with no neighbouring hydrogens. We have the following fragments,

These fragments can be assembled in a way, that the proposed structure will be,

Proposed structure from spectrum (e)