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

Verified step by step guidance

1. Step 1: Analyze the symmetry of each structure to determine which carbon atoms are equivalent. Equivalent carbons will produce the same signal in the 13C NMR spectrum.
2. Step 2: For structure (i), cyclohexane, all six carbon atoms are equivalent due to the high symmetry of the molecule. This results in one signal in the 13C NMR spectrum.
3. Step 3: For structure (ii), thiacyclohexane, the sulfur atom breaks the symmetry of the ring. The carbons adjacent to the sulfur are equivalent to each other, and the remaining carbons are equivalent in pairs. This results in three distinct signals in the 13C NMR spectrum.
4. Step 4: For structure (iii), cyclohexanone, the carbonyl group breaks the symmetry of the ring. The carbonyl carbon is unique, and the carbons adjacent to the carbonyl are equivalent to each other. The remaining carbons are equivalent in pairs. This results in four distinct signals in the 13C NMR spectrum.
5. Step 5: For structure (iv), cyclohexane-1,4-dione, the two carbonyl groups further reduce the symmetry. The carbonyl carbons are equivalent to each other, and the carbons adjacent to each carbonyl are equivalent in pairs. The remaining carbons are also equivalent in pairs. This results in three distinct signals in the 13C NMR spectrum.

Key Concepts

Carbon NMR Spectroscopy

Carbon-13 NMR (Nuclear Magnetic Resonance) spectroscopy is a technique used to determine the structure of organic compounds by analyzing the magnetic environment of carbon atoms. Each unique carbon environment in a molecule produces a distinct signal in the NMR spectrum, allowing chemists to infer the number of different carbon types present.

Chemical Equivalence

In NMR spectroscopy, chemically equivalent atoms are those that are in identical environments and thus produce the same signal. For carbon atoms, this means they are bonded to the same atoms and have the same electronic environment. Identifying equivalent carbons is crucial for predicting the number of signals in the NMR spectrum.

Functional Groups and Their Effects

Functional groups, such as carbonyls and hydroxyls, significantly influence the chemical environment of adjacent carbon atoms, affecting their equivalence. For example, the presence of a carbonyl group can deshield nearby carbons, leading to shifts in their NMR signals. Understanding how functional groups interact with carbon atoms is essential for accurate NMR analysis.