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.