Why coupling constant of trans alkene is more than that in cis alkenes?
The Correct answer and Explanation is:
The coupling constant of trans alkenes is generally larger than that of cis alkenes due to the spatial orientation of the hydrogen atoms attached to the carbon atoms involved in the double bond.
Explanation:
In nuclear magnetic resonance (NMR) spectroscopy, the coupling constant (denoted as J) is a measure of the interaction between two nuclear spins, typically of hydrogen atoms (protons), and is influenced by the dihedral angle between the protons as well as the bonding framework. For alkenes, which have a carbon-carbon double bond, the coupling constant between two protons (H–C=C–H) is particularly dependent on their relative position.
- Spatial Orientation in Trans vs. Cis Alkenes:
In a trans alkene, the two hydrogen atoms attached to the double-bonded carbons are positioned on opposite sides of the molecule. This results in a larger dihedral angle between the protons (about 180 degrees). In a cis alkene, the hydrogens are on the same side, resulting in a smaller dihedral angle (around 0–30 degrees). - Karplus Relationship:
The Karplus equation, which describes the relationship between the dihedral angle and the coupling constant, shows that a larger dihedral angle corresponds to a larger coupling constant. When the dihedral angle approaches 180 degrees, as in the case of trans alkenes, the coupling constant is at its maximum. For trans alkenes, the J value is typically in the range of 11–18 Hz.For cis alkenes, where the dihedral angle is much smaller (closer to 0 degrees), the coupling constant is also smaller, typically in the range of 6–12 Hz. - Electronegativity and Bonding:
In trans alkenes, the hydrogens are farther apart, which allows for stronger spin-spin interactions through the π-system of the double bond. In cis alkenes, the closer proximity of the protons reduces this interaction, leading to a smaller coupling constant.
In summary, the trans configuration allows for a more effective interaction between the nuclear spins, leading to a larger coupling constant compared to the cis configuration, where the spatial arrangement reduces the interaction.