Question

A new chemist moved into an industrial lab where work was being done on oxygenated gasoline additives. Among the additives that had been tested, she found an old bottle containing a clear, pleasant-smelling liquid that was missing its label. She took the quick NMR spectrum shown and was able to determine the identity of the compound without any additional information. Propose a structure and assign the peaks. (Hint: This is a very pure sample.)

Short answer

The structure determined from NMR spectrum has four different kinds of protons and exhibit four proton-NMR signals. The values of chemical shifts for protons and splitting pattern of peaks are as follows,

a =3.6 (quartet,) c =1.5 (6 peaks,)

b =3.2 (triplet,) d =0.9 (triplet,)

Step-by-step solution

Determination of structure from given NMR spectrum:

_{From the given NMR spectrum, there is no evidence for vinyl hydrogens, so the compound must be small, saturated and oxygen-containing molecule. Starting upfield, the first signal is a triplet, this must be methyl next to methylene. The methylene signal at 1.5 but it has six peaks which means, it must be having five neighbouring protons, a methyl group on one side and methylene group on other side. The third carbon must be methylene, its signal is a quartet at 3.6 split by methylene and an hydroxyl group. Far downfield, the final methylene group must be bonded to oxygen. Remaining one proton signal must be from hydroxyl group. Thus, the compound must be propan-1-ol.}

Structure determined from given NMR spectrum

Values of chemical shifts and splitting pattern of protons in propan-1-ol:

The structure determined from NMR spectrum has four different kinds of protons and exhibit four proton-NMR signals. The values of chemical shifts for protons and splitting pattern of peaks are as follows,

a =3.6 (quartet,) c =1.5 (6 peaks,)

b =3.2 (triplet,) d =0.9 (triplet,)

Chemical shift values depend on electronegativity of nearby atoms and whether the electron density is more or less around that proton whose chemical shift value we are determining. In the given structure, oxygen is an electronegative atom, thus, protons-a and b will have higher chemical shift value than protons-c and d. Due to more electronegative atom present, region of low electron density or deshielded environment is created due to which chemical shift increases.

Different protons have different values of chemical shifts