Chapter 3.10, Problem 3.50P

For each compound below, predict the number of signals and the location of each signal in the expected ¹³C NMR spectrum.

Interpretation Introduction

Interpretation:

The number signals and the location of each signal in the expected ${}^{\text{13}}\text{C}$ NMR spectrum of the compound has to be predicted.

Concept Introduction:

${}^{13}C$ NMR spectroscopy:

In ${}^{\text{13}}\text{C}$ NMR spectroscopy, the chemical shift values typically range from $0to220ppm.$  A special technique that is called broadband decoupling is used in this type of spectroscopy that eliminates all ${}^{\text{13}}{\text{C-}}^{\text{1}}\text{H}$ splitting.  The use of this technique causes all the ${}^{\text{13}}\text{C}$ signals to collapse to singlets.  The number of signals in a ${}^{\text{13}}\text{C}$ NMR spectrum indicates the number of carbon atoms in different electronic environments.  Carbon atoms that are interchangeable by symmetry elements like rotation or reflection, they are equivalent and will produce one signal only.

Chemical shift values (in ppm) for several important types of carbon atoms are given below in a table.

Figure 1

• $0-50ppm$: Contains signals from $s{p}^3-$ hybridized carbon atoms.
• $50-100ppm$: Contains signals from $s{p}^2-$ hybridized carbon atoms that are deshielded by electronegative atoms, as well as $sp-$ hybridized carbon atoms.
• $100-150ppm$: Contains signals from $s{p}^2-$ hybridized carbon atoms.
• $150-220ppm$: Contains the carbon atoms of carbonyl groups.  These carbon atoms are highly deshielded.

Explanation of Solution

Determination of different types of carbon atoms:

The compound is symmetrical.  A mirror plane can pass through the middle of the molecule dividing it into two equal halves.  That means the left half of the molecule is the mirror image of the right half.  So, only six signals are expected in the ${}^{\text{13}}\text{C}$ NMR spectrum.

The expected chemical shift values are shown below, categorized according to the region of the spectrum in which each signal is expected to appear:

Figure 2