Rotation around a carbon–carbon double bond is much less frequent than rotation…
CHEMISTRY · Multi-part · A Level · Chemistry · Introduction to organic chemistry · Introduction to organic chemistry
Problem
Rotation around a carbon–carbon double bond is much less frequent than rotation around a carbon–carbon single bond. The data below summarize average bond energies.
• Average σ component of a C–C single bond: 348 kJ mol⁻¹
• Additional π component present in a C=C double bond: 264 kJ mol⁻¹
• Experimentally measured rotational barrier about a typical C–C single bond: 12 kJ mol⁻¹
• Assume room-temperature thermal energy per mole is approximately 2.5 kJ mol⁻¹
(a) Using the information above, estimate the minimum energy (in kJ mol⁻¹) that must be supplied to achieve free rotation about a typical isolated C=C bond.
(b) Calculate the factor by which this energy exceeds the thermal energy available at room temperature. Show your work.
(c) Based on your calculations, explain why geometric isomers derived from C=C bonds are isolable at room temperature, whereas distinct conformers about C–C single bonds are usually not isolated.