Q 13-32E - Organic Chemistry (Mcmurry)

Question

Question: Why do you suppose accidental overlap of signals is much more common in ¹H NMR than in ¹³C NMR?

Step-by-Step Solution

Answer

¹³C NMR absorptions occur over a range of 250 ppm, while ¹H NMR absorptions generally occur over a range of only 10 ppm. The spread of peaks in ¹³C NMR is therefore much greater, so accidental overlap is less likely. In addition normal ¹³C NMR spectra are uncomplicated by spin-spin splitting, and the total number of lines is smaller.

Step 1: NMR spectroscopy

NMR stands for Nuclear Magnetic Resonance Spectroscopy. This is the technique used for determining the structure of the organic compounds. It is a non-destructive technique.

• · It exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of molecules in which they are contained.
• · Any proton or nucleus with odd mass number spin on its axis.
• · The nucleus or the proton behaves as a tiny spinning bar magnet due to both electric charge and mechanical spinning.

Step 2: Types of NMR spectroscopy

Nuclear Magnetic Resonance Spectroscopyis of two types ¹H NMR and ¹³C NMR. Proton nuclear magnetic resonance (¹H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within in the molecule of substance in order to determine the structure of its molecules.

Carbon-13 nuclear magnetic resonance is the application of nuclear magnetic resonance spectroscopy to carbon. It is analogous to proton NMR and allows the identification of carbon atom in an organic molecule just as proton NMR identifies Hydrogen atoms.

The possibility of carbon NMR appears surprising at first glance

The most abundant isotope of carbon ¹²C has no net nuclear spin, hence it is not detectable by NMR. The far less abundant isotope of carbon ¹³C has a nuclear spin of ½ and is detectable by NMR.

Step 3: Accidental overlap of signals

¹³C NMR absorptions occur over a range of 250 ppm, while ¹H NMR absorptions generally occur over a range of only 10 ppm as shown in figure given below. The spread of peaks in ¹³C NMR is therefore much greater, so accidental overlap is less likely. In addition normal ¹³C NMR spectra are uncomplicated by spin-spin splitting, and the total number of lines is smaller.

                                                             

                                                                      ¹H and ¹³C NMR spectra of the ethyl-2-chloropropanoate