Question

3-Methyl-2-butanol has five signals in its ${}^{\mathbf{13}}\mathbf{C}$NMR spectrum at 17.90, 18.15, 20.00, 35.05, and 72.75 d. Why are the two methyl groups attached to C3 non- equivalent? Making a molecular model should be helpful.

Short answer

The Newman projection is helpful:

Newman Projection

Step-by-step solution

Newman projections

It shows the conformation of the molecule from front to back in which the front atom is represented by dot and the back atom is represented by the circle.

 13C NMR

Carbon-13 nuclear magnetic resonance is a technique similar to the NMR which is used to identify the carbon from the organic molecules or from the organometallic compounds like hydrogen is detected in NMR but it is only sensitive to carbon-13, not carbon-12.

Structure elucidation from 13C NMR

As per the given NMR peaks, it is clearly observed that the 2 methyl groups produce different peaks due to the different environments they are sharing at C3 because 1 methyl group is having OH in the environment and other methyl peaks are affected by the presence of the methyl group as well so that’s why they are non-equivalent and produce different peaks.

Molecular model of the 2-methyl 2-butanol

Staggered Newman projection is formed along C2-C3 because C2 is the chiral center of the molecule and the methyl group at the front projections is diastereotopic. Hence, the structure must be like this:

2-methyl,2-butanol