Why is there no coupling between the a and c protons or between the b and c protons in the cis and trans alkenes shown in Figure 14.20?

Verified step by step guidance

1. Identify the protons labeled as a, b, and c in both the cis and trans isomers of 3-bromopent-2-enoic acid.
2. Understand that coupling in NMR occurs between protons that are three bonds apart (vicinal coupling).
3. Examine the structure to determine the number of bonds between the a and c protons, and between the b and c protons.
4. Recognize that in both the cis and trans isomers, the a and c protons, as well as the b and c protons, are separated by more than three bonds, preventing vicinal coupling.
5. Conclude that the lack of coupling is due to the distance (more than three bonds) between these protons, which is beyond the typical range for J-coupling in NMR spectroscopy.

Key Concepts

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy is a powerful analytical technique used to determine the structure of organic compounds. It relies on the magnetic properties of certain nuclei, such as hydrogen, to provide information about the environment surrounding these nuclei. In NMR, the splitting patterns of signals indicate the number of neighboring protons, which is crucial for understanding coupling interactions.

Coupling Constants and Spin-Spin Splitting

Coupling constants arise from the interaction between neighboring nuclear spins, leading to spin-spin splitting in NMR spectra. The degree of splitting is influenced by the number of adjacent protons and their spatial arrangement. In the case of cis and trans alkenes, certain protons may not couple due to their geometric arrangement, resulting in distinct NMR signals.

Geometric Isomerism in Alkenes

Geometric isomerism occurs in alkenes due to restricted rotation around the double bond, leading to cis and trans configurations. In cis isomers, substituents are on the same side, while in trans isomers, they are on opposite sides. This spatial arrangement affects the coupling of protons in NMR, as protons that are not in close proximity or are in a different spatial orientation do not couple, leading to the absence of certain splitting patterns.