16.2:
Stability of Conjugated Dienes
The relative stabilities of nonconjugated and conjugated dienes can be assessed by comparing their heats of hydrogenation.
For example, the heat of hydrogenation of 1-butene is -127 kJ/mol. One would expect this value to increase by two-fold in 1,3-butadiene; however, it is 15 kJ/mol lower than expected.
So, what makes conjugated dienes more stable?
The carbon–carbon single bond in conjugated systems is formed by an sp2–sp2 overlap compared to an sp3–sp2 overlap in isolated systems. Since sp2 orbitals have a higher s character, the σ bond in conjugated dienes is shorter and stronger.
In addition, the delocalization of π electrons imparts extra stability to conjugated systems.
For electron delocalization to take effect, the p orbitals must be parallel, constraining the molecular geometry to a planar framework.
The s-cis and s-trans conformers of 1,3-butadiene satisfy the planarity criterion, with s-trans being more stable. The cis–trans activation energy is around 15 kJ/mol, equivalent to the stabilization energy of conjugated dienes.
16.2:
Stability of Conjugated Dienes
Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
The two main factors contributing to the enhanced stability of conjugated systems are the delocalization of π electrons and the sp2 hybridization of the carbons forming the single bonds.
Planar Conformers of Conjugated Dienes
Conjugated dienes adopt two planar configurations, s-cis and s-trans, where the unhybridized p orbitals are aligned parallel to each other, facilitating the delocalization of π electrons across the molecule. The s-cis and s-trans conformers of 1,3-butadiene are shown below.
The prefix “s” describes the orientation of the double bonds about the C–C single bond. The conformers undergo rapid internal conversion at room temperature via rotation around the central single bond. In the s-cis form, the two double bonds are on the same side of the single bond and have a dihedral angle of 0°. In s-trans, they are on the opposite sides with a dihedral angle of 180°. The s-cis conformer is less stable than the s-trans conformer due to the steric interaction between the two hydrogens as shown below.
