Chapter 13.12, Problem 22P

Interpretation Introduction

Interpretation:

The structure of a given molecular formula C₁₁H₁₆ to be predicted using ¹³CNMR spectra.

Concept introduction:

The ¹³CNMR spectrum gives information on the different electronic environments of carbon. As like ¹HNMR, the number of signals generated in ¹³CNMR are predicted by performing symmetry operations (rotation or reflection symmetry). Only chemical shift values are reported in the spectrum but not the multiplicity and integration values because the coupling between two neighboring ¹³C-¹³C nuclei are weakly involved due to the low abundance of ¹³C isotopes of carbon atom.

To Identify:

The structure of an alcohol of given molecular formula C₁₁H₆.

Broadband-decoupled spectrum:

The spectrum shows seven signals whereas the given molecular formula also has seven carbon atoms. Thus all the seven carbons have chemically different electronic environments showing signals.

• The signal in the region of 150-220 ppm indicates the carbon atom of carbonyl group (C=O).
• The three signals in the region of 10-25 ppm indicate the sp³ hybridized carbon atoms which can be methyl / methylene or methine groups.

DEPT (Distortionless enhancement by polarization transfer):

a) DEPT-90: The spectrum exhibits signal only from CH group and no signals from CH₃, CH₂, CH and quaternary carbon (carbon with no protons).

b) DEPT-135: The spectrum exhibits CH₃ groups and CH groups as positive signals (pointing up); CH₂ groups appear as negative signals (pointing down) and quaternary carbon does not appear.

The signals appear in each type of spectrum: