Question

Question: How many signals would you expect to see in the ¹³CNMR of the following compounds? In each case, show which carbon atoms are equivalent in the ¹³CNMR.

Short answer

Answer

Structure 1: There will be 1 signal of structure 1 that will be recorded.

Structure 2: There will be 3 signals of structure 2 that will be recorded. Structure 3: There will be 2 signals of structure 3 that will be recorded.

Structure 4: There will be 4 signals of structure 4 that will be recorded.

Step-by-step solution

Introduction to NMR

NMR spectroscopy is a technique used to determine the number of signals that can be shown by a particular compound. The protons that have the same chemical environment will show one signal. In proton NMR spectroscopy, the primary, secondary, and tertiary protons are detected in NMR. The quaternary carbon atom also gives one signal.

Determining the number of signals

a. Structure 1 is cyclopentane. There are 5 carbon atoms of the same chemical environment. So, only 1 signal will be expected from 10 protons of this structure.

b. Structure 2 is cyclopentanone. There are 5 carbon atoms in this structure. The first carbon atom is a quaternary carbon exhibiting one signal, the protons of the second and fifth carbon atom have same chemical environment so 4 protons will show the same signal. Similarly, the protons of the third and fourth carbon atom have same chemical environment so 4 protons will show the same signal. Structure 2 will show 3 signals.

c. Structure 3 is oxane. There are 4 carbon atoms and 1 oxygen atom in this structure. The second numbered carbon atom and fifth numbered carbon atom have same chemical environment so 4 protons will show the same signal. Similarly, the protons of the third and fourth carbon atom have the same chemical environment so 4 protons will show the same signal. Structure 3 will show 2 signals.

d. Structure 3 is a cyclic ester. There are 4 carbon atoms and 2 oxygen atoms in this structure. The first carbon atom is a quaternary carbon exhibiting one signal, the second numbered carbon atom has two protons showing one signal. Similarly, the protons of the third and fourth carbon atom have different chemical environment so 2 protons each will show two different signals. Structure 4 will show 4 signals.