16.2 : 공액디엔의 안정성

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.